PIC en la práctica Un enfoque basado en proyectos por D. W. Smith.pdf

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PIC in Practice
A Project-Based Approach
D. W. Smith
AMSTERDAM BOSTON HEIDELBERG LONDON
NEW YORK OXFORD PARIS SAN DIEGO
SAN FRANCISCO SINGAPORE SYDNEY TOKYO
Newnes is an imprint of Elsevier

Newnes is an imprint of Elsevier
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First published 2002
Reprinted 2003 (twice), 2005
Second edition 2006
Copyright ß 2006, Dave Smith. All rights reserved
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Contents
Introduction ix
1 Introduction to the PIC microcontroller 1
The aim of the book 1
Program memory 2
Microcontroller clock 3
The microcontroller system 3
Types of microcontroller 4
Microcontroller specification 5
Using the microcontroller 6
1 Microcontroller hardware 6
2 Programming the microcontroller 9
2 Programming the 16F84 microcontroller 11
Microcontroller inputs and output (I/O) 12
Timing with the microcontroller 12
Programming the microcontroller 12
Entering data 13
The header for the 16F84 14
Program example 16
Saving and assembling the code 19
PICSTART PLUS programmer 23
Programming flowchart 26
Problem: flashing two LEDs 26
Solution to problem, flashing two LEDs 27
3 Introductory projects 29
LED_Flasher2 29
SOS 30
Code for SOS circuit 30
Flashing 8 LEDs 33
Chasing 8 LEDs 35
Traffic lights 39
More than 8 outputs 45
4 Headers, porting code – which micro? 47
Factors affecting the choice of the microcontroller 47
Choosing the microcontroller 48
Headers 49

5 Using inputs 64
Switch flowchart 66
Program development 67
Scanning (using multiple inputs) 73
Switch scanning 73
Control application – a hot air blower 77
6 Understanding the headers 82
The 16F84 82
16F84 memory map 87
The 16F818 88
7 Keypad scanning 93
Programming example for the keypad 94
8 Program examples 110
Counting events 110
Look up table 115
7-Segment display 115
Numbers larger than 255 126
Long time intervals 133
One hour delay 136
9 The 16C54 microcontroller 139
Header for the 16C54 139
16C54 memory map 142
10 Alpha numeric displays 143
Display pin identification 144
Configuring the display 145
Writing to the display 146
Program example 146
Program operation 160
Display configuration 161
Writing to the display 162
Displaying a number 163
11 Analogue to digital conversion 166
Making an A/D reading 167
Configuring the A/D device 168
Analogue header for the 16F818 171
A/D conversion – example, a temperature sensitive
switch 174
Program code 176
Another example – a voltage indicator 178
vi Contents

12 Radio transmitters and receivers 186
Measuring the received pulse width 189
13 EEPROM data memory 199
Example using the EEPROM 200
14 Interrupts 207
Interrupt sources 208
Interrupt control register 208
Program using an interrupt 209
15 The 12 series 8 pin microcontroller 216
Pin diagram of the 12C508/509 216
Pin diagram of the 12F629 and 12F675 216
Features of these 12 series 217
The memory map of the 12C508 217
Oscillator calibration 218
I/O PORT, GPIO 219
Delays with the 12 series 220
Header for 12C508/9 220
Program application for 12C508 222
Program application using the 12F629/675 225
16 The 16F87X Microcontroller 229
16F87X family specification 229
The 16F872 microcontroller 230
16F87X memory map 232
The 16F872 header 233
16F872 application – a greenhouse control 236
Programming the 16F872 microcontroller
using PICSTART PLUS 242
Reconfiguring the 16F872 header 243
17 The 16F62X Microcontroller 245
16F62X oscillator modes 245
16F62X and 16F84 Pinouts 247
16F62X port configuration 247
16F62X memory map 248
The 16F62X headers 248
HEAD62RC.ASM 250
A 16F627 application – flashing an LED on and off 252
The 16F627 LED flasher code 253
Configuration settings for the 16F627 255
Other features of the 16F62X 255
Contents vii

18 Projects 257
Project 1 Electronic dice 257
Project 2 Reaction timer 266
Project 3 Burglar alarm 272
Fault finding 282
Development kits 285
19 Instruction set, files and registers 287
The PIC microcontroller instruction set 287
Registers 289
Instruction set summary 292
Appendix A Microcontroller data 299
Appendix B Electrical characteristics 301
Appendix C Decimal, binary and hexadecimal numbers 303
Appendix D Useful contacts 306
Index 307
viii Contents

Introduction
The microcontroller is an exciting new device in the field of electronics
control. It is a complete computer control system on a single chip.
microcontrollers include EPROM program memory, user RAM for storing
program data, timer circuits, an instruction set, special function registers,
power on reset, interrupts, low power consumption and a security bit for
software protection. Some microcontrollers like the 16F818/9 devices include
on board A to D converters.
The microcontroller is used as a single chip control unit for example in a
washing machine, the inputs to the controller would be from a door catch,
water level switch, temperature sensor. The outputs would then be fed to a
water inlet valve, heater, motor and pump. The controller would monitor the
inputs and decide which outputs to switch on i.e. close the door – water inlet
valve open – monitor water level, close valve when water level reached. Check
temperature, turn on heater, switch off heater when the correct temperature
is reached. Turn the motor slowly clockwise for 5 seconds, anticlockwise
for 5 seconds, repeat 20 times, etc. If you are not that maternal maybe you
prefer discos to washing – then you can build your own disco lights.
The microcontroller because of its versatility, ease of use and cost will change
the way electronic circuits are designed and will now enable projects to be
designed which previously were too complex. Additional components such as
versatile interface adapters (VIA), RAM, ROM, EPROM and address
decoders are no longer required.
One of the most difficult hurdles to overcome when using any new technology
is the first one – getting started! It was my aim when writing this book to
explain as simply as possible how to program and use the PIC microcon-
trollers. I hope I have succeeded.
Code examples in this book are available to download from:
http://books.elsevier.com/uk//newnes/uk/subindex.asp?maintarget¼companions/
defaultindividual.aspisbn¼0750648120
Dave Smith, B.Sc., M.Sc.
Senior Lecturer in Electronics
Manchester Metropolitan University

1
Introduction to the PIC
microcontroller
A microcontroller is a computer control system on a single chip. It has
many electronic circuits built into it, which can decode written instructions and
convert them to electrical signals. The microcontroller will then step through
these instructions and execute them one by one. As an example of this a
microcontroller could be instructed to measure the temperature of a room and
turn on a heater if it goes cold.
Microcontrollers are now changing electronic designs. Instead of hard wiring
a number of logic gates together to perform some function we now use
instructions to wire the gates electronically. The list of these instructions given
to the microcontroller is called a program.
The aim of the book
The aim of the book is to teach you how to build control circuits using
devices such as switches, keypads, analogue sensors, LEDs, buzzers, 7 segment
displays, alpha-numeric displays, radio transmitters etc. This is done by intro-
ducing graded examples, starting off with only a few instructions and gradually
increasing the number of instructions as the complexity of the examples
increases.
Each chapter clearly identifies the new instructions added to your vocabulary.
The programs use building blocks of code that can be reused in many different
program applications.
Complete programs are provided so that an application can be seen working.
The reader is then encouraged to modify the code to alter the program in order
to enhance their understanding.
Throughout this book the programs are written in a language called assembly
language which uses a vocabulary of 35 words called an instruction set.
In order to write a program we need to understand what these words mean and
how we can combine them.

The complete instruction set is shown in Chapter 19 Instruction Set, Files and
Registers.
All of the programs illustrated in the book are available from:
http://books.elsevier.com/uk//newnes/uk/subindex.asp?maintarget¼
companions/defaultindividual.aspisbn¼0750648120
You will of course need a programmer to program the instructions into the
chip. The assembler software, MPASM, which converts your text to the
machine code is available from Microchip on www.microchip.com this website
is a must for PIC programmers.
Program memory
Inside the microcontroller the program we write is stored in an area
called EPROM (Electrically Programmable Read Only Memory), this
memory is non-volatile and is remembered when the power is switched off.
The memory is electrically programmed by a piece of hardware called
a programmer.
The instructions we program into our microcontroller work by moving
and manipulating data in memory locations known as user files and registers.
This memory is called RAM, Random Access Memory. For example in
the room heater we would measure the room temperature by instructing the
microcontroller via its Analogue to Digital Control Register (ADCON0)
the measurement would then be compared with our data stored in one of
the user files. A STATUS Register would indicate if the temperature was
above or below the required value and a PORT Register would turn the
heater on or off accordingly. The memory map of the 16F84 chip is shown in
Chapter 6.
PIC Microcontrollers are 8 bit micros, which means that the memory locations,
the user files and registers are made up of 8 binary digits shown in Figure 1.1.
Bit 0 is the Least Significant Bit (LSB) and Bit 7 is the Most Significant
Bit (MSB).
bit 7 bit 6 bit 5 bit 4 bit 3 bit 2 bit 1 bit 0
1 0 1 1 0 0 1 0
MSB LSB
Figure 1.1 User file and register layout
2 Introduction to the PIC microcontroller

The use of these binary digits is explained in Appendix C.
When you make an analogue measurement, the digital number, which results,
will be stored in a register called ADRES. If you are counting the number
of times a light has been turned on and off, the result would be stored as an
8 bit binary number in a user file called, say, COUNT.
Microcontroller clock
In order to step through the instructions the microcontroller needs a clock
frequency to orchestrate the movement of the data around its electronic
circuits. This can be provided by 2 capacitors and a crystal or by an internal
oscillator circuit.
In the 16F84 microcontroller there are 4 oscillator options.
An RC (Resistor/Capacitor) oscillator which provides a low cost solution.
An LP oscillator, i.e. 32kHz crystal, which minimises power consumption.
XT which uses a standard crystal configuration.
HS is the high-speed oscillator option.
Common crystal frequencies would be 32kHz, 1MHz, 4MHz, 10MHz
and 20MHz.
Newer microcontrollers, such as the 16F818 and 12F629, have an oscillator
built on the chip so we do not need to add a crystal to them.
Inside the Microcontroller there is an area where the processing (the clever
work), such as mathematical and logical operations are performed, this is
known as the central processing unit or CPU. There is also a region where
event timing is performed and another for interfacing to the outside world
through ports.
The microcontroller system
The block diagram of the microcontroller system is shown in Figure 1.2.
INPUT OUTPUT
CONTROL
Figure 1.2 The basic microcontroller system
Introduction to the PIC microcontroller 3

The input components would consist of digital devices such as, switches,
push buttons, pressure mats, float switches, keypads, radio receivers etc. and
analogue sensors such as light dependant resistors, thermistors, gas sensors,
pressure sensors, etc.
The control unit is of course the microcontroller. The microcontroller will
monitor the inputs and as a result the program would turn outputs on and
off. The microcontroller stores the program in its memory, and executes the
instructions under the control of the clock circuit.
The output devices would be made up from LEDs, buzzers, motors, alpha
numeric displays, radio transmitters, 7 segment displays, heaters, fans etc.
The most obvious choice then for the microcontroller is how many
digital inputs, analogue inputs and outputs does the system require.
This would then specify the minimum number of inputs and outputs (I/O)
that the microcontroller must have. If analogue inputs are used then the
microcontroller must have an Analogue to Digital (A/D) module inside.
The next consideration would be what size of program memory storage
is required. This should not be too much of a problem when starting out,
as most programs would be relatively small. All programs in this book fit into
a 1k program memory space.
The clock frequency determines the speed at which the instructions are
executed. This is important if any lengthy calculations are being undertaken.
The higher the clock frequency the quicker the micro will finish one task and
start another.
Other considerations are the number of interrupts and timer circuits required,
how much data EEPROM if any is needed. These more complex operations are
considered later in the text.
In this book the programs requiring analogue inputs have been implemented
on the 16F818 and 16F872 micros. Programs requiring only digital
inputs have used the 16F84 and 16F818. The 16F818 and 16F84 devices
have 1k of program memory and have been run using a 32.768kHz clock
frequency or the internal oscillator on the 16F818. There are over 100 PIC
microcontrollers, the problem of which one to use need not be considered until
you have understood a few applications.
Types of microcontroller
The list of PIC Microcontrollers is growing almost daily. They include devices
for all kinds of applications, for example the 18F8722 has 64k of EPROM
memory, 3938 bytes of RAM (User files), 1024 bytes of EEPROM, 16 10-bit

A/D channels, a voltage reference, 72 inputs and outputs (I/O), 3–16 bit and
2–8 bit timers.
There are basically two types of microcontrollers, Flash devices and One
Time Programmable Devices (OTP).
The flash devices can be reprogrammed in the programmer whereas OTP
devices once programmed cannot be reprogrammed. All OTP devices however
do have a windowed variety, which enables them to be erased under ultra violet
light in about 15 minutes, so that they can be reprogrammed. The windowed
devices have a suffix JW to distinguish them from the others.
The OTP devices are specified for a particular oscillator configuration R-C,
LP, XT or HS. See Appendix A Microcontroller Data.
16C54 configurations are:
16C54JW Windowed device
16C54RC OTP, R-C oscillator
16C54LP OTP, LP oscillator, 32kHz
16C54XT OTP, XT oscillator, 4MHz
16C54HS OTP, HS oscillator, 20Mhz
In this book the two main devices investigated are the 16F84 and the 16F818
flash devices. The 16F84 at present is the main choice for beginners, but
should be replaced in popularity by the better and cheaper 16F818. They
have their program memory made using Flash technology. They can be
programmed, tested in a circuit and reprogrammed if required without the need
for an ultra violet eraser.
Microcontroller specification
You specify a device with its Product Identification Code.
This code specifies:
The device number.
If it is a Windowed, an OTP, or flash device. The windowed device is
specified by a JW suffix. OTP devices are specified by Oscillator Frequency,
and the Flash devices are specified with an F such as 16F84.
The oscillation frequency, usually 04 for devices working up to 4MHz.,
10 up to 10MHz or 20 up to 20MHz. 20MHz devices are of course more
expensive than 4MHz devices.
Temperature range, for general applications 08C to þ708C is usually
specified.

The Product Identification System for the PIC Micro is shown in Figure 1.3.
Using the microcontroller
In order to use the microcontroller in a circuit there are basically two areas
you need to understand:
1. How to connect the microcontroller to the hardware.
2. How to write and program the code into the microcontroller.
1 Microcontroller hardware
The hardware that the microcontroller needs to function is shown in
Figure 1.4. The crystal and capacitors connected to pins 15 and 16 of the
16F84 produce the clock pulses that are required to step the microcontroller
through the program and provide the timing pulses. (The crystal and capacitor
can be omitted if using an on board oscillator in e.g. 16F818). The 0.1mF
capacitor is placed as close to the chip as possible between 5v and 0v. Its role is
to divert (filter) any electrical noise on the 5v power supply line to 0v, thus
bypassing the microcontroller. This capacitor must always be connected to
stop any noise affecting the normal running of the microcontroller.
Microcontroller power supply
The power supply for the microcontroller needs to be between 2v and 6v. This
can easily be provided from a 6v battery as shown in Figure 1.5.
PART No. -XX X /XX
Package L= PLCC
P = PDIP (standard plastic package)
SO = SOIC small outline IC
PQ = MQFP
JW = Windowed device (CERDIP)
Temperature range − = 0°C to +70°C
I = −40°C to +85°C
E = −40°C to +125°C
04 = 4MHz
04 = 10MHz
10 = 20MHz
Device i.e. 16C711
Frequency range
Figure 1.3 Product identification system

The diode in the circuit drops 0.7v across it reducing the applied voltage to
5.3v. It provides protection for the microcontroller if the battery is acciden-
tally connected the wrong way round. In that case the diode would be reversed
biased and no current would flow.
7805, Voltage regulator circuit
Probably the most common power supply connection for the microcontroller
is a 3 terminal voltage regulator, I.C., the 7805. The connection for this is
shown in Figure 1.6.
The supply voltage, Vin, to the 7805 can be anything from 7v to 30v.
The output voltage will be a fixed 5v and can supply currents up to 1amp.
So battery supplies such as 24v, 12v, 9v etc. can be accommodated.
16F84
6v
V+
0v
Figure 1.5 Microcontroller power supply
68p
68p
0v
32kHz 16
15
V+
MCLR
0v
14
4
5v
0v
0.1
16F84
5
Figure 1.4 The microcontroller circuit

Power dissipation in the 7805
Care must be taken when using a high value for Vin. For example if Vin ¼ 24v
the output of the 7805 will be 5v, so the 7805 has 24 5 ¼ 19v across it. If
it is supplying a current of 0.5amp to the circuit then the power dissipated
(volts current) is 19 0.5 ¼ 9.5watts. The regulator will get hot! and will
need a heat sink to dissipate this heat.
If a supply of 9v is connected to the regulator it will have 4v across it and
would dissipate 4 0.5 ¼ 2watts.
In the circuits used in this book the microcontroller only requires a current
of 15mA so most of the current drawn will be from the outputs. If the output
current is not too large say 5100mA (0.1A) then with a 9v supply the
power dissipated would be 4 0.1 ¼ 0.4watts and the regulator will stay cool
without a heatsink.
Connecting switches to the microcontroller
The most common way of connecting a switch to a microcontroller is via
a pull-up resistor to 5v as shown in Figure 1.7.
When the switch is open, 5v, a logic 1 is connected to the micro.
When the switch is closed, 0v, a logic 0 is connected to the micro.
5v
0v
10k
Micro
Figure 1.7 Connecting a switch to the microcontroller
7805
Vin 5v
Figure 1.6 The voltage regulator circuit

Some Microcontrollers such as the 16F84 and 16F818 have internal pull ups
connected to some of their I/O pins. PORTB in the above devices.
Figure 1.8 shows how the switch is connected using the internal pull up.
Connecting outputs to the microcontroller
The microcontroller is capable of supplying approximately 20–25mA to an
output pin. So loads such as LEDs or small relays can be driven directly.
Larger loads require interfacing via a transistor, for dc or a triac, for ac.
Opto-coupled devices provide an isolated interface between the microcontroller
and the load.
The LED connection to the Micro is shown in Figure 1.9.
2 Programming the microcontroller
In order to have the microcontroller perform some controlling action you
need to communicate with it and tell it what those instructions are to be.
When we communicate with one another we use a spoken language, when
we communicate with a microcontroller we use a program language. The
program language for the PIC Microcontroller uses 35 words (instructions)
0v
Micro
Figure 1.8 Connecting a switch using an internal pull up
0v
Micro
680R
Figure 1.9 Connecting an LED to the microcontroller

in its vocabulary. A few more instructions are used in the bigger
microcontrollers.
In order to communicate with the microcontroller we need to know what
these 35 instructions are and how to use them. Not all 35 instructions are
used in this book. In fact you can write meaningful programs using only 5 or
6 instructions.

2
Programming the 16F84
microcontroller
Microcontrollers are now providing us with a new way of designing circuits.
Designs, which at one time required many Digital ICs and lengthy Boolean
Algebra calculations, can now be programmed simply into one Micro-
controller. For example a set of traffic lights would have required an oscillator
circuit, counting and decoding circuits plus an assortment of logic gate ICs.
In order to use this exciting new technology we must learn how to program
these Microcontrollers.
The Microcontroller I have chosen to start with is the 16F84-04/P, which
means it is a flash device that can be electrically erased and reprogrammed
without using an Ultra Violet Eraser. It can be used up to an oscillation
frequency of 4MHz and comes in a standard 18pin Plastic package.
It has 35 instructions in its vocabulary, but like all languages not all of the
instructions are used all of the time you can go a long way on just a few.
In order to teach you how to use these instructions I have started off with a
simple program to flash an LED on and off continually. This program
introduces you to 4 instructions in 5 lines of code.
You are then encouraged to write your own program to flash two LEDs on
and off alternately. The idea being, when you have understood my code you
can then modify it for your own program, thus understanding better. Once
you have written your first program you are then off and running. The book
then continues with further applications such as traffic lights and disco lights
to introduce more of the instructions increasing your microcontroller
vocabulary.
Instructions used in this chapter:
BCF
BSF
CALL
GOTO

Microcontroller inputs and outputs (I/O)
The microcontroller is a very versatile chip and can be programmed to operate
in a number of different configurations. The 16F84 is a 13 I/O device, which
means it has 13 Inputs and Outputs. The I/O can be configured in any combi-
nation i.e. 1 input 12 outputs, 6 inputs 7 outputs, or 13 outputs depending
on your application. These I/O are connected to the outside world through
registers called Ports. The 16F84 has two ports, PORTA and PORTB. PORTA
is a 5-bit port it has 5 I/O lines and PORTB has 8 I/O.
Timing with the microcontroller
All microcontrollers have timer circuits onboard; some have 4 different timers.
The 16F84 has one timer register called TIMER0. These timers run at a speed
of ¼ of the clock speed. So if we use a 32,768Hz crystal the internal timer
will run at ¼ of 32768Hz i.e. 8192Hz. If we want to turn an LED on for say
1 second we would need to count 8192 of these timing pulses. This is a lot
of pulses! Fortunately within the microcontroller there is a register called an
OPTION Register, that allows us to slow down these pulses by a factor of 2, 4,
8, 16, 32, 64, 128 or 256. The OPTION Register is discussed in the Instruction
Set, Files and Register section in Chapter 19. Setting the prescaler, as it is called
to divide by 256 in the OPTION register means that our timing pulses are now
8192/256 ¼ 32Hz, i.e. 32 pulses a second. So to turn our LED on for 1 second
we need only to count 32 pulses in TIMER0, or 16 for 0.5 seconds, or 160 for
5 seconds etc.
Programming the microcontroller
In order to program the microcontroller we need to:
Write the instructions in a program.
Change the text into machine code that the microcontroller understands
using a piece of software called an assembler.
Blow the data into the chip using a programmer.
Let’s consider the first task, writing the program. This can be done on any text
editor, such as notepad. I prefer to use an editor supplied by the micro-
controller manufacturers, ‘Microchip’. This software is called MPLAB and is
available free on www.microchip.com.
As you have seen above we need to configure the I/O and set the Prescaler
for the timing. If we do not set them the default conditions are that all PORT
bits are inputs. A micro with no outputs is not much use! The default for the
Prescaler is that the clock rate is divided by 2.
12 Programming the 16F84 microcontroller

The program also needs to know what device it is intended for and also what
the start address in the memory is.
If this is starting to sound confusing – do not worry, I have written a header
program, which sets the all the above conditions for you to use. These con-
ditions can be changed later when you understand more about what you are
doing.
The header for the 16F84 sets the 5 bits of PORTA as inputs and the 8 bits
of PORTB as outputs. It also sets the prescaler to divide by 256. We will use the
32,768Hz crystal so our timing is 32 pulses per sec. The program instructions
will run at ¼ of the 32,768Hz clock, i.e. 8192 instructions per second. The
header also includes two timing subroutines for you to use they are DELAY1 –
a 1 second delay and DELAYP5 – a half-second delay. A subroutine is a
section of code that can be called, when needed, to save writing it again.
For the moment do not worry about how the header or the delay subroutines
work. We will work through them, in Chapter 6, once we have programmed
a couple of applications.
Just one more point, the different ways of entering data.
Entering data
Consider the decimal number 37, this has a Hex value of 25 or a Binary value
of 0010 0101. The assembler will accept this as .37 in decimal (note the . is not
a decimal point) or as 25H in hex or B’00100101’ in binary.
181 decimal would be entered as .181 in decimal, 0B5H in hex or B’10110101’
in binary. NB. If a hex number starts with a letter it must be prefixed with a
0, i.e. 0B5H not B5H.
NB. The default radix for the assembler MPASM is hex.
Appendix C. illustrates how to change between Decimal, Binary and
Hexadecimal numbers.
The PIC Microcontrollers are 8 bit micros. This means that the memory
locations, i.e. user files and registers contain 8 bits. So the smallest 8 bit number
is of course 0000 0000 which is equal to a decimal number 0 (of course). The
largest 8 bit number is 1111 1111 which is equal to a decimal number of 255.
To use numbers bigger than 255 we have to combine memory locations. Two
memory locations combine to give 16 bits with numbers up to 65,536. Three
memory locations combine to give 24 bits allowing numbers up to 16,777,215
Programming the 16F84 microcontroller 13

and so on. These large numbers are introduced in Chapter 8, Numbers Larger
than 255.
The Header for the 16F84, HEADER84.ASM
The listing below shows the header for the 16F84 microcontroller. I suggest
you start all of your programs, for this chip, with this header, or a modified
version of it. A full explanation of this header file is given in Chapter 6.
; HEADER84.ASM for 16F84. This sets PORTA as an INPUT (NB 1means
input).
; and PORTB as an OUTPUT (NB 0 means output).
;The OPTION Register is set to /256 to give timing pulses of 1/32 of a second.
;1second and 0.5 second delays are included in the subroutine section.
;*********************************************************
; EQUATES SECTION
TMR0 EQU 1 ;means TMR0 is file 1.
STATUS EQU 3 ;means STATUS is file 3.
PORTA EQU 5 ;means PORTA is file 5.
PORTB EQU 6 ;means PORTB is file 6.
TRISA EQU 85H ;TRISA (the PORTA I/O selection) is file 85H
TRISB EQU 86H ;TRISB (the PORTB I/O selection) is file 86H
OPTION_R EQU 81H ;the OPTION register is file 81H
ZEROBIT EQU 2 ;means ZEROBIT is bit 2.
COUNT EQU 0CH ;COUNT is file 0C, a register to count events.
;*********************************************************
LIST P ¼ 16F84 ; we are using the 16F84.
ORG 0 ;the start address in memory is 0
GOTO START ; goto start!
;******************************************************************
; Configuration Bits
__CONFIG H’3FF0’ ;selects LP oscillator, WDT off, PUT on,
;Code Protection disabled.
;*********************************************************
;SUBROUTINE SECTION.
;1 second delay.

DELAY1 CLRF TMR0 ;START TMR0.
LOOPA MOVF TMR0,W ;READ TMR0 INTO W.
SUBLW .32 ;TIME - 32
BTFSS STATUS,
ZEROBIT ; Check TIME-W ¼ 0
GOTO LOOPA ;Time is not ¼ 32.
RETLW 0 ;Time is 32, return.
; 0.5 second delay.
DELAYP5 CLRF TMR0 ;START TMR0.
LOOPB MOVF TMR0,W ;READ TMR0 INTO W.
BTFSS STATUS,
GOTO LOOPB ;Time is not ¼ 16.
;*********************************************************
;CONFIGURATION SECTION
START BSF STATUS,5 ;Turns to Bank1.
MOVLW B’00011111’ ;5bits of PORTA are I/P
MOVWF TRISA
MOVLW B’00000000’
MOVWF TRISB ;PORTB is OUTPUT
MOVLW B’00000111’ ;Prescaler is /256
MOVWF OPTION_R ;TIMER is 1/32 secs.
BCF STATUS,5 ;Return to Bank0.
CLRF PORTA ;Clears PortA.
CLRF PORTB ;Clears PortB.
;*********************************************************
;Program starts now.
END ;This must always come at the end of your code
NB. In the program any text on a line following the semicolon (;) is ignored by
the assembler software. Program comments can then be placed there.
The section is saved as HEADER84.ASM you can use it to start all of your
16F84 programs. HEADER84 is the name of our program and ASM is its
extension.

Program example
The best way to begin to understand how to use a microcontroller is to start
with a simple example and then build on this.
Let us consider a program to flash an LED ON and OFF at 0.5 second
intervals. The LED is connected to PortB bit 0 as shown in Figure 2.1.
Notice from Figure 2.1 how few components the microcontroller needs – 2
68pF capacitors, a 32.768kHz crystal for the oscillator and a 0.1mF capacitor
for decoupling the power supply. Other oscillator and crystal configurations
are possible – see Microchip’s data sheets for other combinations. I have
chosen the 32kHz crystal because it enables times of seconds to be produced
easily.
The program for this circuit can be written on any text editor, such as Notepad
or on Microchip’s editor MPLAB.
Open HEADER84.ASM or start a new file and type the program in, saving as
HEADER84.ASM If using Notepad saveas type ‘‘All Files’’ to avoid Notepad
adding the extension .TXT
Once you have HEADER84.ASM saved on disk and loaded onto the screen
alter it by including your program as shown below:-
; HEADER84.ASM for 16F84. This sets PORTA as an INPUT (NB 1means
input).
68p
68p
0v
32kHz 16
15
0v
470R
LED1
6
B0
V+
MCLR
T0CKI
0v
14
4
5v
0v
0.1µ
16F84
3
5
Figure 2.1 Circuit diagram of the microcontroller flasher

; and PORTB as an OUTPUT (NB 0 means output).
;*******************************************************
; EQUATES SECTION
COUNT EQU 0CH ; COUNT is file 0C, a register to count events.
;*********************************************************
LIST P ¼ 16F84 ;we are using the 16F84.
GOTO START ;goto start!
;******************************************************************
;*****************************************************
; 1 second delay.
BTFSS STATUS,
; 0.5 second delay.

BTFSS STATUS,
;*********************************************************
;CONFIGURATION SECTION.
MOVWF TRISA
MOVLW B’00000000’
;*********************************************************
BEGIN BSF PORTB,0 ;Turn ON B0.
CALL DELAYP5 ;Wait 0.5 seconds
BCF PORTB,0 ;Turn OFF B0.
GOTO BEGIN ;Repeat
END ;YOU MUST END!!
How Does It Work?
The 5 lines of code starting at BEGIN are responsible for flashing the LED ON
and OFF. This is all the code we will require for now. The rest of the code, the
header is explained in Chapter 6 once you have seen the program working.
BEGIN is a label. A label is used as a location for the program to go to.
Line1 the instruction BSF and its data PORTB,0 is shorthand for Bit Set
in File, which means Set the Bit in the File PORTB, where bit0 is the
designated bit. This will cause PORTB,0 to be Set to a logic1, in hardware
terms this means pin6 in Figure 2.1 is at 5v turning the LED on.

NB. There must not be any spaces in a label, an instruction or its data. I keep
the program tidy by using the TAB key on the keyboard.
Line2 CALL DELAYP5 causes the program to wait 0.5 seconds while the
subroutine DELAYP5 in the header is executed.
Line3 BCF PORTB,0 is the opposite of Line1, this code is shorthand for
Bit Clear in File, which means Clear the Bit in the File PORTB, where bit0
is the designated bit. This will cause PORTB,0 to be Cleared to a logic0,
in hardware terms this means pin6 in Figure 2.1 is at 0v turning the LED off.
Line4 CALL DELAYP5 is the same as Line2.
Line5 GOTO BEGIN sends the program back to the label BEGIN to repeat
the process of flashing the LED on and off.
Any of the 8 outputs can be turned ON and OFF using the 2 instructions BSF
and BCF for example:
BSF PORTB,3 makes PORTB,3 (pin9) 5v.
BCF PORTB,7 makes PORTB,7 (pin13) 0v.
Saving and assembling the code
The program is then saved as FLASHER.ASM. The next task is to assemble
this text into the HEX code that the microcontroller understands.
Open MPLAB the screen shown below in Figure 2.2 will open up.
Open the file FLASHER.ASM using the FILE menu as shown in Figure 2.3.
From the CONFIGURE Menu, Select Device then choose the micro 16F84 in
this example, as indicated in Figure 2.4.
Next choose CONFIGURE – Configuration Bits as shown in Figure 2.5 and
set as indicated.
Our configuration bits setting, select the LP Oscillator, turn the Watchdog
Timer Off, turn the Power Up Timer on and turn Code Protect off.
Notice the value of this configuration is 3FF0 in hex. This configuration setting
can be written into the header program so there is no need to here. The code is
__CONFIG H’3FF0’
The choice of configuration bit settings for the 16F84 are:
the Oscillator, RC, LP, XT, HS. i.e. LP
Watchdog Timer ON/OFF i.e. OFF

Figure 2.2 MPLAB initial screen
Figure 2.3 Opening FLASHER.ASM

Figure 2.4 CONFIGURE – select device
Figure 2.5 Configuration bits setting

Power Up Timer ON/OFF i.e. ON
Code Protect ON/OFF i.e. OFF
Then we have to convert our text, FLASHER.ASM into a machine code file
FLASHER.HEX to do this choose PROJECT – Quickbuild Flasher.ASM as
If the program has compiled without any errors then MPLAB will return with
a message Build Succeeded as indicated in Figure 2.7. There may be some
warnings and messages but do not worry about them, the compiler has seen
something it wasn’t expecting.
Incidently, I always have line numbers on my code to find my way around,
especially in larger programs. Line numbers can be turned on and off with the
path: EDIT – PROPERTIES.
Suppose that you have a syntax error in your code. The message Build Failed
will appear as shown in Figure 2.8. You then have to correct the errors.
MPLAB has indicated the error in the message box. If you ‘double click’ on the
error message then MPLAB will indicate, with an arrow, where the error is
Figure 2.6 Compiling FLASHER.ASM to FLASHER.HEX

in your code. Correct the errors and compile (Quickbuild) again to produce
an error free build.
The error I have written into my code occurs in line 61, with the message,
‘symbol not previously defined (PORT)’. I should have written PORTB the
compiler does not understand ‘PORT’.
After successfully building the program, the HEX code is ready to be
programmed into the Microcontroller.
You can view your compilation using VIEW – PROGRAM MEMORY as
The FLASHER.HEX file is now ready to be programmed into the chip.
PICSTART PLUS programmer
If you do not have a programmer I would recommend Arizona Microchip’s
own PICSTART PLUS. When Arizona bring out a new microcontroller as
Figure 2.7 Build Succeeded

they do regularly, the driver software is updated and can be downloaded free
off the internet from MICROCHIP.COM.
Once installed on your PC it is opened from MPLAB i.e.
Switch on the PICSTART Plus Programmer.
Figure 2.8 Build failed
Figure 2.9 Program memory

Select, Programmer – Select Programmer – PICSTART Plus, shown in
Figure 2.10.
Select Enable Programmer from the Programmer box, Figure 2.10.
The final stage is to program your code onto the chip. To do this click the
programming icon shown in Figure 2.11 or via the menu on Programmer –
Program.
After a short while the message success will appear on the screen.
You will be greeted with the success statement for a few seconds only, if
you miss it check the program statistics for Pass 1 Fail 0 Total 1, which will be
continually updated.
The code has been successfully blown into your chip and is ready for use.
If this process fails – check the chip is inserted correctly in the socket, if it
is then try another chip.
So we are now able to use the microcontroller to switch an LED on and
off – Fantastic!
Figure 2.10 Selecting the PICSTART plus programmer
Figure 2.11 Programming icon

But use your imagination. There are 35 instructions in your micro voca-
bulary. The PIC Microcontroller range at the moment includes devices with
64k bytes of EPROM-program memory, 3938 bytes of RAM-data memory,
1024 bytes of EEPROM, 72 Input and Output pins, 11 interrupts, 15 channel
A/D converter, 20MHz. clock, real time clock, 4 counter/timers, 55 word
instruction set. See Appendix A for a detailed list. If the 64k of EPROM or
3938 bytes of RAM is not enough your system can be expanded using extra
EPROM and RAM. In the end the only real limits will be your imagination.
Programming flowchart
Problem: flashing two LEDs
There has been a lot to do and think about to get this first program into
the microcontroller and make it work in a circuit. But just so that you are
sure what you are doing – Write a program that will flash two LEDs on and
off alternately. Put LED0 on B0 and LED1 on B1. NB you can use the
file FLASHER.ASM it only needs two extra lines adding! Then save it as
FLASHER2.ASM
The circuit layout is shown in Figure 2.12.
Try not to look at the solution below before you have attempted it.
N
Y
Produce file FLASHER.ASM
Quickbuild Flasher.ASM
Open MPLAB
Program Microcontroller
Correct errors
Build
Errors?

Solution to the problem, flashing two LEDs
The header is the same as in FLASHER.ASM. just include in the section,
program starts now, the following lines:
BEGIN BSF PORTB,0 ;Turn ON B0.
BCF PORTB,1 ;Turn OFF B1
BSF PORTB,1 ;Turn ON B1.
GOTO BEGIN ;Repeat
END
68p
68p
0v
32kHz
16
15
V+
MCLR
0v
14
4
5v
0v
0.1µ
16F84
5
0v
0v
B1
B0
7 680R
680R
6
Figure 2.12 Circuit to flash 2 LEDs

Did you manage to do this? If not have a look at my solution and see what the
lines are doing. Now try flashing 4 LEDs on and off, with 2 on and two off
alternately. You might like to have them on for 1 second and off for half a
second. Can you see how to use the 1-second delay in place of the half-second
delay.
The different combinations of switching any 8 LEDs on PORTB should be
relatively easy once you have mastered these steps.
Perhaps the most difficult step in understanding any new technology is
getting started. The next chapter will introduce a few more projects similar to
Flasher.ASM to help you progress.

3
Introductory projects
New instructions used in this chapter:
MOVLW
MOVWF
DECFSZ
Let’s have a look at a few variations of flashing the LEDs on and off to
develop our programming skills.
LED_Flasher2
Suppose we want to switch the LED on for 2 seconds and off for 1 second.
Figure 2.1 shows the circuit diagram for this application. The code for this
would be:
BEGIN BSF PORTB,0 ;Turn on B0
CALL DELAY1 ;Wait 1 second
BCF PORTB,0 ;Turn off B0
GOTO BEGIN ;Repeat
END
NB. This code would be added to HEADER84.ASM into the section called,
‘‘Program starts now’’.
To do this open MPLAB, then FILE – OPEN – HEADER84.ASM
Add the code and saveas LED_FLASHER2.ASM
The text would then be assembled by the MPLAB software and then blown
into the Microcontroller as explained in Chapter 2.
How does it work?
The comments alongside the code explain what the lines are doing. Because we
do not have a 2 second delay we wait for 1 second twice. You can of course
write a 2 second delay routine but we will be looking at this later.

SOS
For our next example let us switch B0 on and off just as we have been doing
but this time we will use delays of ¼ second and ½ second. This is not
much different than we have done previously, but instead of turning an LED
on and off we will replace it by a buzzer. The program is not just going to
turn a buzzer on and off, but do it in a way that generates the signal, SOS.
Which is DOT,DOT,DOT DASH,DASH,DASH DOT,DOT, DOT. Where
the DOT is the buzzer on for ¼ second and the DASH is the buzzer on for
½ second with ¼ second between the beeps.
The circuit diagram for the SOS circuit is shown in Figure 3.1.
Code for SOS circuit
The complete code for the SOS circuit is shown below because an extra
subroutine, DELAYP25, has been added.
;SOS.ASM for 16F84. This sets PORTA as an INPUT (NB 1means input)
;and PORTB as an OUTPUT (NB 0 means output).
;1second, 0.5 second and 0.25 second delays are included in the subroutine
;section.
68p
68p
0v
32kHz 16
15
6
B0
V+
MCLR
T0CKI
0v
14
4
5v
0v
0.1µ
16F84
3
5
0v
Figure 3.1 SOS circuit diagram
30 Introductory projects

;*********************************************************
;EQUATES SECTION
;*********************************************************
;******************************************************************
;Configuration Bits
;*****************************************************
;SUBROUTINE SECTION
;1 second delay.
BTFSS STATUS,
ZEROBIT ;Check TIME-W ¼ 0
;0.5 second delay.
BTFSS STATUS,
Introductory projects 31

;0.25 second delay.
LOOPC MOVF TMR0,W ;READ TMR0 INTO W.
SUBLW .8 ;TIME - 8
BTFSS STATUS,
GOTO LOOPC ;Time is not ¼ 8.
;*********************************************************
MOVWF TRISA
MOVLW B’00000000’
;*********************************************************
BEGIN BSF PORTB,0 ;Turn ON B0, DOT
BSF PORTB,0 ;Turn ON B0, DOT
BSF PORTB,0 ;Turn ON B0, DASH

CALL DELAY1
CALL DELAY1 ;Wait 2 seconds before returning.
GOTO BEGIN ;Repeat
END ;YOU MUST END!!
How does it work?
I think the explanation of the code is clear from the comments. At the end of
the SOS the program has a delay of 2 seconds before repeating. This should be
a useful addition to any alarm project.
We will now move onto switching a number of outputs on and off. Consider
flashing all 8 outputs on PORTB on and off at ½ second intervals.
Flashing 8 LEDs
The circuit for this is shown in Figure 3.2.
This code is to be added to HEADER84.ASM as in LED_FLASHER2.ASM

BEGIN BSF PORTB,0 ;Turn ON B0
BSF PORTB,1 ;Turn ON B1
68p
68p
0v
32kHz
16
15
B5
B4
V+
MCLR
0v
14
4
5v
0v
0.1µ
16F84
5
11
10
9
8 × 680R
0v
0v
0v
0v
0v
0v
B2
B1
B7
B3
B0
8
7
6
0v
0v
B6
12
13
Figure 3.2 Flashing 8 LEDs

GOTO BEGIN
END
Save the program as FLASH8.ASM, assemble and program the 16F84 as
indicated in Chapter 2.
There is an easier way than this of switching all outputs on a port, which we
look at later in this chapter with a set of disco lights.
Chasing 8 LEDs
Let’s now consider the code to chase the 8 LEDs. The circuit of Figure 3.2 is
required for this. The code will switch B0 on and off, then B1, then B2 etc.

GOTO BEGIN
END
This code once again is added to the bottom of HEADER84.ASM and is
saved as
CHASE8A.ASM
Now that we have chased the LEDs one way let’s run them back the other
way and call the program CHASE8B.ASM. I think you know the routine
add the code to the bottom of HEADER84.ASM etc. So I will not mention
it again.
;CHASE8B.ASM

GOTO BEGIN
END
Just one last flasher program. Let us switch each output on in turn leaving
them on as we go and then switch them off in turn. Try this for yourselves
before looking at the solution!
The program is saved as UPANDDOWN.ASM

GOTO BEGIN
END
There are lots of other combinations for you to practice on. I’ll leave you to
experiment further.
Consider another example of the delay routine:
Traffic lights
If you have ever tried to design a ‘simple’ set of traffic lights then you will
appreciate how much circuitry is required. An oscillator circuit, counters and
logic decode circuitry.
The microcontroller circuit is a much better solution even for this ‘simple’
arrangement. The circuit is shown in Figure 3.3.
68p
68p
0v
32kHz
16
15
B5
B4
B3
V+
MCLR
0v
14
4
5v
0v
0.1µ
16F84
5
11
10
9
6 x 680R
0v
0v
0v
R1
A1
G1
0v
0v
0v
R2
A2
G2
B2
B1
B3
B3
B0
8
7
6
Figure 3.3 Traffic lights circuit

A truth table of the operation of the lights is probably a better aid to a solution
rather than a flowchart.
Traffic light truth table
Time B7 B6 B5 B4 B3 B2 B1 B0
R1 A1 G1 R2 A2 G2
2sec 0 0 1 0 0 1 0 0
2sec 0 0 1 1 0 1 0 0
5sec 0 0 0 0 1 1 0 0
2sec 0 0 0 1 0 1 0 0
2sec 0 0 1 0 0 1 0 0
2sec 0 0 1 0 0 1 1 0
5sec 0 0 1 0 0 0 0 1
2sec 0 0 1 0 0 0 1 0
REPEAT
Program listing for the traffic lights
;TRAFFIC.ASM
;*********************************************************
BEGIN MOVLW B’00100100’ ;R1, R2 on.
MOVWF PORTB
CALL DELAY2 ;Wait 2 Seconds.
MOVLW B’00110100’ ;R1, A1, R2 on.
MOVWF PORTB
MOVLW B’00001100’ ;G1, R2 on.
MOVWF PORTB
MOVLW B’00010100’ ;A1, R2 on.
MOVWF PORTB
MOVLW B’00100100’ ;R1, R2 on.
MOVWF PORTB

MOVLW B’00100110’ ;R1, R2, A2 on.
MOVWF PORTB
MOVLW B’00100001’ ;R1, G2 on.
MOVWF PORTB
MOVLW B’00100010’ ;R1, A2 on.
MOVWF PORTB
GOTO BEGIN
END
How does it work
In a previous examples we turned LEDs on and off with the two commands
BSF and BCF, but a much better way has been used with the TRAFFIC.ASM
program.
The basic difference is the introduction of two more commands:
MOVLW MOVe the Literal (a number) into the Working register.
MOVWF MOVe the Working register to the File.
The data, in this example, binary numbers, are moved to W and then to the file
which is the output PORTB to switch the LEDs on and off. Unfortunately
the data cannot be placed in PORTB with only one instruction it has to go
via the W register.
So:
MOVLW B’00100100’ clears B7,B6, sets B5, clears B4,B3, sets B2
and clears B1, B0 in the W register
MOVWF PORTB moves the data from the W register to PORTB
to turn the relevant LEDs on and off.
All 8 outputs are turned on/off with these 2 instructions.
CALL DELAY2 and CALL DELAY5 waits 2 seconds and 5 seconds before
continuing with the next operation. DELAY2 and DELAY5 need adding to
the subroutine section as:
; 5 second delay.

LOOPC MOVF TMR0,W ;READ TMR0 INTO W.
BTFSS STATUS,ZEROBIT ;Check TIME - W ¼ 0
GOTO LOOPC ;Time is not ¼ 160.
; 2 second delay.
LOOPD MOVF TMR0,W ;READ TMR0 INTO W.
BTFSS STATUS,ZEROBIT ;Check TIME - W ¼ 0
GOTO LOOPD ;Time is not ¼ 64.
The W register
The W or working register is the most important register in the micro. It is
in the W register were all the calculations and logical manipulations such as
addition, subtraction, and-ing, or-ing etc., are done.
The W register shunts data around like a telephone exchange re-routes tele-
phone calls. In order to move data from locationA to locationB, the data has
to be moved from locationA to W and then from W to location B
NB. If the three lines in the TRAFFIC.ASM program are repeated then any
pattern and any delay can be used to sequence the lights – you can make your
own disco lights!
Repetition (e.g. disco lights)
Instead of just repeating one sequence over and over, suppose we wish to repeat
several sequences before returning to the start as with a set of disco lights.
Consider the circuit shown in Figure 3.4. The 8 ‘Disco Lights’ B0-B7 are to be
run as two sequences.
Sequence 1 Turn all lights on.
Wait.
Turn all lights off
Wait
Sequence 2 Turn B7-B4 ON, B3-B0 OFF
Wait
Turn B7-B4 OFF, B3-B0 ON
Wait

Suppose we wish Sequence 1 to run 5 times before going onto Sequence 2 to
run 10 times and then repeat. A section of program is repeated a number of
times with 4 lines of code shown below:
MOVLW .5 ;Move 5 into W
MOVWF COUNT ;Move W into user file COUNT
.
SEQ1
.
DECFSZ COUNT ;decrement file COUNT skip if zero.
GOTO SEQ1 ;COUNT not yet zero, repeat sequence
The first two lines set up a file COUNT with 5. (Count is the first user file
and is found in memory location 0CH.) 5 is first of all moved into W then
from there to file COUNT.
SEQ1 is executed.
The DECFSZ COUNT instruction, DECrement File and Skip if Zero,
decrements, takes 1 off, the file COUNT and skips GOTO SEQ1 if the count
is zero, if not zero then do SEQ1 again.
68p
68p
0v
32kHz
16
15
B5
B4
V+
MCLR
0v
14
4
5v
0v
0.1µ
16F84
5
11
10
9
8 × 680R
0v
0v
0v
0v
0v
0v
B2
B1
B7
B3
B0
8
7
6
0v
0v
B6
12
13
Figure 3.4 Disco lights

This way SEQ1 is executed 5 times and COUNT goes from 5 to 4 to 3 to 2 to 1
to 0 when we skip and follow onto SEQ2. SEQ2 is then done 10 times, say, and
the code would be:
MOVLW .10 ;Move 10 into W
MOVWF COUNT ;Move W into user file COUNT
.
SEQ2
.
DECFSZ COUNT ;decrement file COUNT skip if zero.
GOTO SEQ2 ;COUNT not yet zero, repeat sequence
Program code for the disco lights
;DISCO.ASM
;*********************************************************
BEGIN MOVLW .5
MOVWF COUNT ;Set COUNT ¼ 5
SEQ1 MOVLW B’11111111’
MOVWF PORTB ;Turn B7-B0 ON
MOVLW B’00000000’
MOVWF PORTB ;Turn B7-B0 OFF
DECFSZ COUNT ;COUNT-1, skip if 0.
GOTO SEQ1
MOVLW .10
MOVWF COUNT ;Set COUNT ¼ 10
SEQ2 MOVLW B’11110000’
MOVWF PORTB ;B7-B4 on, B3-B0 off
MOVLW B’00001111’
MOVWF PORTB ;B7-B4 off, B3-B0 on
DECFSZ COUNT ;COUNT-1, skip if 0.
GOTO SEQ2
GOTO BEGIN
END

Using the idea of repeating sequences like this any number of combinations
can be repeated. The times of course do not need to be of 0.5 seconds duration.
The flash rate can be speeded up or slowed down depending on the
combination.
Try programming a set of your own Disco Lights. This should keep you quiet
for hours (days!).
More than 8 outputs
Suppose we wish to have a set of disco lights in a 3 3 matrix as shown in
Figure 3.5. This configuration of course requires 9 outputs. We have 8 outputs
on PORTB so we need to make one of the PORTA bits an output also, say
PORTA bit0.
68p
68p
0v
32kHz
16
15
B5
B4
B3
V+
MCLR
0v
14
4
5v
0v
0.1µ
16F
84
5
10
9
9 x 680R
B2
B1
B3
B3
B0
8
7
6
0v
0v 0v
0v 0v 0v
0v 0v 0v
B6
B7
A0
11
13
17
12
Figure 3.5 9 Disco light set

To change PORTA bit0 from an input to an output change the lines in the
Configuration section from:
MOVLW B’00011111’
MOVWF TRISA
to
MOVLW B’00011110’
MOVWF TRISA
NB a 1 signifies an input a 0 signifies an output.
So to set a ‘þ’ pattern in the lights we turn on B7, B4, B1, B3 and B5, keeping
the others off. The code for this would be:
MOVLW B’00000000’
MOVWF PORTA ;A0 is clear
MOVLW B’10111010’
MOVWF PORTB ;B7, B5, B4, B3 and B1 are on
So to set an ‘X’ pattern in the lights we turn on B6, B4, B2, A0 and B0, keeping
the others off. The code for this would be:
MOVLW B’00000001’
MOVWF PORTA ;A0 is on
MOVLW B’01010101’
MOVWF PORTB ;B6, B4, B2 and B0 are on
There are endless combinations you can make with 9 lights. In fact there are
512. That is 29
. This should give you something to go at!

4
Headers, porting
code – which micro?
Arizona Microchip the manufacturers of the PIC Microcontroller make
over 100 different types of microcontroller. How do we choose the correct one
for the job?
Factors affecting the choice of the microcontroller
When deciding on which Microcontroller to use for your application there are
a number of factors you will need to consider.
How many inputs and outputs do you need. If you are using the program
FLASHER.ASM which only flashes 1 LED on and off then any PIC will
do this. If you are turning 8 outputs on and off then you will need
a microcontroller that has at least 8 I/O (of course). So an 8pin micro
i.e. 12F629 will not do because it only has 6 I/O.
Do you need accurate timing? If so then you will need to add a crystal
to your micro to provide the clock. If timing is not that critical then
you can use a micro that has an on board oscillator such as the 16F818. You
can then omit the crystal and 2 capacitors. The timing accuracy is about
1%. This would do for FLASHER.ASM but not for a 24 hour clock.
1% is about 14 minutes a day.
Are you making analogue measurements? If so you will need a micro with an
AtoD converter on it. The 16F818 has a 5 channel, 10 bit AtoD converter.
If you need more that 5 channels then you will need to use a micro with
more AtoD channels such as the 16F877 which has 8.
What operating frequency do you require? The greater the frequency the
faster your code will execute. Most newer devices can operate up to 20MHz,
some even faster. Some older devices can only achieve 4MHz. The programs
in this book only require an operating speed of 4MHz.
How many instructions are there in your program? The 16F818 has
space for 1k i.e. 1024 instructions. The 16F877 has 8k program memory
locations. All programs in this book require less than 1k of program
memory space.
How many memory locations are required to store data? The 16F818 has
128 bytes of data memory, the 16F877 has 368.

Do you need to store data so that it will be saved if the power is removed or
lost? If so you need a micro with EEPROM data memory. The 16F818 has
128 bytes of EEPROM memory, the 16F877 has 256.
There may be other requirements that you need from your micro, which are
not considered in this book, such as:
Number of timers
Comparators
Pulse width modulation
In circuit debugging
USB drivers.
Choosing the microcontroller
As I mentioned previously the FLASHER.ASM program which flashes 1 LED
on and off can be performed by any Micro. Well, that has narrowed the field
down! So which microcontroller do we use for that application? If you were
mass producing these flasher units the answer would probably be – use the
cheapest and smallest – the 12C508 is possibly the device then. But for small
scale production or one offs you will probably have (or develop) a favorite.
Probably the most common chip used by the beginner is the 16F84; this has
been around since about 1998. This micro has built up a very large fan base
which is why it is still widely used. People are using this chip because they are
used to using it! There is now another micro on the market which will do
everything that the 16F84 can do and more. This device is the 16F818.
The data sheets for the 16F84 and 16F818 are shown in Figures 4.1 and 4.2
respectively.
The main differences are that the 16F818 has 16 I/O, an on board oscillator
with 8 selectable frequencies, 128 bytes of data RAM, 128 bytes of EEPROM,
3 Timers one of them a 16 bit, 5 channel 10 bit AtoD converter. The 16F84 has
13 I/O, no on board oscillator, 68 bytes of data RAM, 64 bytes of EEPROM,
1 timer, no AtoD. The most surprising difference of all is that the 16F84 is
about 3 times the price of the 16F818!!
The programs in this book consist of 2 parts:
A header section which tells the ‘build’ software which device we are using,
configures the device, i.e. defines which pins are inputs and outputs, sets the
timer rate and includes some timing delays if you require them in a
subroutine section.
48 Headers, porting code – which micro?

The second part of the program, entitled, ‘Program starts now’, is where you
write the code to perform your application.
The header program is unique to the particular microcontroller being used, but
the ‘application code’ entered after ‘‘Program starts now’’, is specific to the
application not the microcontroller. So any microcontroller that has i.e. the
required number of I/O or A/D can be used. As I mentioned before any
microcontroller can be used to execute the FLASHER.ASM code.
Headers
Just one point before we look at the headers. The 8 pin micros only have 6 I/O,
they do not have PORTA and PORTB pins, they have what is called a General
Devices included in this Data Sheet:
· PIC16F83
· PIC16F84
· PIC16CR83
· PIC16CR84
· Extended voltage range devices available
(PIC16LF8X, PIC16LCR8X)
High Performance RISC CPU Features:
· Only 35 single word instructions to learn
· All instructions single cycle except for program
branches which are two-cycle
· Operating speed: DC - 10 MHz clock input
DC - 400 ns instruction cycle
· 14-bit wide instructions
· 8-bit data path
· 15 special function hardware registers
· Eight-level deep hardware stack
· Direct, indirect and relative addressing modes
· Four interrupt sources:
- External RB0/INT pin
- TMR0 timer overflow
- PORTE7:4 interrupt on change
- Data EEPROM write complete
· 1000 erase/write cycles Flash program memory
· 10,000,000 erase/write cycles EEPROM data memory
· EEPROM Data Retention 40 years
Peripheral Features:
· 13 I/O pins with individual direction control
· High current sink/source for direct LED drive
- 25 mA sink max. per pin
- 20 mA source max. per pin
· TMR0: 8-bit timer counter with 8-bit
programmable prescaler
Special Microcontroller Features:
· In-Circuit Serial Programming (ICSPTM) - via two
pins (ROM devices support only Data EEPROM
programming)
· Power-on Reset (POR)
· Power-up Timer (PWRT)
· Oscillator Start-up Timer (OST)
· Watchdog Timer (WDT) with its own on-chip RC
oscillator for reliable operation
· Code-protection
· Power saving SLEEP mode
· Selectable oscillator options
CMOS Flash/EEPROM Technology:
· Low-power, high-speed technology
· Fully static design
· Wide operating voltage range:
- Commercial: 2.0V to 6.0V
- Industrial: 2.0V to 6.0V
· Low power consumption:
- 2 mA typical @ 5V, 4 MHz
- 15 µA typical @ 2V, 32 kHz
- 1 µA typical standby current @ 2V
Device
PIC16F83 512 Flash 36
68
36
68
64
64
64
64
10
10
10
10
1 K Flash
512 ROM
1 K ROM
PIC16F84
PIC16CR83
PIC18CR84
Program
Memory
(words)
Data
RAM
(bytes)
Date
EEPROM
(bytes)
MAX.
Freq
(MHz)
Pin Diagrams
PDIP, SOIC
RA2 RA1
RB7
RB6
RB5
RB4
RA0
OSC1/CLKIN
OSC2/CLKOUT
VDD
RA3
RA4/T0CKI
MCLR
RB0/INT
RB1
RB2
RB3
VSS
1
2
3
4
5
6
7
8
9
PIC16F8X
PIC16CR8X
18
17
16
15
14
13
12
11
10
Figure 4.1 The PIC 16F84 data sheet
Headers, porting code – which micro? 49

Purpose I/O or GPIO. So the instruction BSF PORTB,0 would have to be
changed to BSF GPIO,0.
The following headers will be used in this book:
HEAD12C508.ASM ; for the 12C508 and 12C509
HEAD12F629.ASM ; for the 12F629
HEAD12F675.ASM ; for the 12F675
HEAD16F627.ASM ; for the 16F627 and 16F628
HEADER84.ASM ; for the 16F84
HEAD16F818.ASM ; for the 16F818 and 16F819
HEAD16F872.ASM ; for the 16F872, 16F874 and 16F877
Low-Power Features:
· Power Managed modes:
- Primary RUN: XT, RC oscillator,
87 µA, 1 MHz, 2V
- INTRC: 7 µA, 31.25 kHz, 2V
- SLEEP: 0.2 µA, 2V
· Timer1 oscillator 1.8 µA, 32 kHz, 2V
· Watchdog Timer: 0.7 µA, 2V
· Wide operating voltage range:
- Industrial: 2.0V to 5.5V
Oscillators:
· Three Crystal modes:
– LP, XT, HS: up to 20 MHz
· Two External RC modes
· One External Clock mode:
- ECIO: up to 20 MHz
· Internal oscillator block:
- 8 user selectable frequencies: 31 kHz, 125 kHz,
250 kHz, 500 kHz, 1 MHz, 2 MHz, 4 MHz, 8 MHz
· 16 I/O pins with individual direction control
· High sink/source current: 25 mA
· Timer0: 8-bit timer/counter with 8-bit prescaler
· Timer1: 16-bit timer/counter with prescaler,
can be incremendet during Sleep via external
crystal/clock
· Timer2: 8-bit timer/counter with 8-bit period
register, prescaler and postscaler
· Capature, Compare, PWM (CCP) module:
- Capature is 16-bit, max. resolution is 12.5 ns
- Coampare is 16-bit, max. resolution is 200 ns
- PWM max. resolution is 10-bit
· 10-bit, 5-channel Analog-to-digital converter
· Synchronous Serial Port (SSP) with
SPI (Master/Slave) and I2
C (Slave)
Special Microcontroller Features:
· 100,000 erase/write cycles Enhanced FLASH
program memory typical
· 1,000,000 typical erase/write cycles EEPROM
data memory typical
· EEPROM Data Retection: 40 years
· In-Circuit Serial Proramming (ICSP)-
via two pins
· Processor read/write access to program memory
· Low Voltage Programming
· In-Circuit Debugging via two pins
Device
PIC16F818
PIC16F819 3584 2048
1792 1024
256
126
256
128
16
16
5
5
1
1
FLASH
(bytes)
Program Memory Data Memory
# Single Word
Instructions
SRAM
(bytes)
EEPROM
(bytes)
I/O Pins
Y
Y
Y
Y
2/1
2/1
10-bit
A/D (ch)
CCP
(PWM)
SSP
SPI Slave
12
C
Timers
8/16-bit
Pin Diagram
18-pin DIP, SOIC
PIC16F818/819
RA2/AN2/VREF- RA1/AN1
RA3/AN3/VREF+ RA0/AN0
RA4/AN4/T0CKI RA7/OSC1/CLKI
RA6/OSC2/CLKO
VDD
RA5/MCLR/VPP
VSS
RB0/INT RB7/T1OSI/PGD
RB1/SDI/SDA RB6/T1OSO/T1CKI/PGC
RB2/SDO/CCP1 RB5/SS
RB3/CCP1/PGM RB4/SCK/SCL
1 18
17
16
15
14
13
12
11
10
2
3
4
5
6
7
8
9
Figure 4.2 The PIC 16F818 and 16F819 data sheet

;HEAD12C508.ASM FOR 12C508/9.
;Uses the internal 4MHz clock.
TMR0 EQU 1 ;TMR0 is FILE 1.
OSCCAL EQU 5
GPIO EQU 6 ;GPIO is FILE 6.
STATUS EQU 3 ;STATUS is FILE 3.
ZEROBIT EQU 2 ;ZEROBIT is Bit 2.
COUNT EQU 07H ;USER RAM LOCATION.
TIME EQU 08H ;TIME IS 39
;**********************************************************
LIST P ¼ 12C508 ;We are using the 12C508.
ORG 0 ;0 is the start address.
;***************************************************
;Configuration Bits
__CONFIG H’0FEA’ ;selects Internal RC oscillator, WDT off,
;**********************************************************
;1/100 SECOND DELAY
DELAY CLRF TMR0 ;START TMR0
LOOPA MOVF TMR0,W ;READ TMR0 IN W
SUBWF TIME,W ;TIME - W
BTFSS STATUS,ZEROBIT ;CHECK TIME-W ¼ 0
GOTO LOOPA
RETLW 0 ;RETURN AFTER TMR0 ¼ 39
;P5 SECOND DELAY
DELAYP5 MOVLW .50
MOVWF COUNT
TIMEC CALL DELAY
DECFSZ COUNT
GOTO TIMEC
RETLW 0
;1 SECOND DELAY
DELAYP5 MOVLW .100
MOVWF COUNT
TIMED CALL DELAY
DECFSZ COUNT
GOTO TIMED
RETLW 0

;**********************************************************
START MOVWF OSCCAL
MOVLW B’00001000’ ;5 bits of GPIO are O/Ps.
TRIS GPIO
MOVLW B’00000111’
OPTION ;PRESCALER is /256
CLRF GPIO ;Clears GPIO
MOVLW .39
MOVWF TIME
**********************************************************
END
HEAD12F629.ASM FOR 12F629 using 4MHz internal RC
TRISIO EQU 85H
GO EQU 1
OPTION_R EQU 81H
CMCON EQU 19H
OSCCAL EQU 90H
;**********************************************************
LIST P ¼ 12F629 ;We are using the 12F629.
;***************************************************
;Configuration Bits
__CONFIG H’3F84’ ;selects Internal RC oscillator, WDT off,
;**********************************************************
;1/100 SECOND DELAY

SUBLW .39 ;TIME - W
GOTO LOOPA
;P1 SECOND DELAY
DELAYP1 MOVLW .10
MOVWF COUNT
TIMEC CALL DELAY
DECFSZ COUNT
GOTO TIMEC
RETLW 0
;**********************************************************
START BSF STATUS,5 ;BANK1
MOVLW B’00001001’ ;BITS 0,3 are I/P
MOVWF TRISIO
MOVLW B’00000111’
MOVWF OPTION_R ;PRESCALER is /256
CALL 3FFH
MOVWF OSCCAL ;Calibrates 4MHz oscillator
BCF STATUS,5 ;BANK0
MOVLW 7H
MOVWF CMCON ;Turns off comparator
;**********************************************************
END
;HEAD12F675.ASM FOR 12F675 using 4MHz internal RC.
TRISIO EQU 85H

GO EQU 1
ADSEL EQU 9EH
ADCON0 EQU 1FH
ADRESH EQU 1EH
OPTION_R EQU 81H
CMCON EQU 19H
OSCCAL EQU 90H
;**********************************************************
;***************************************************
;Configuration Bits
;**********************************************************
;1/100 SECOND DELAY
SUBLW .39 ;TIME - W
GOTO LOOPA
;P1 SECOND DELAY
DELAYP1 MOVLW .10
MOVWF COUNT
TIMEC CALL DELAY
DECFSZ COUNT
GOTO TIMEC
RETLW 0
;**********************************************************
MOVLW B’00010001’ ;A0 IS ANALOGUE,FOSC/8
MOVWF ADSEL

MOVLW B’00001001’ ;BITS 0,3 are I/P
MOVWF TRISIO
MOVLW B’00000111’
CALL 3FFH
BCF STATUS,5 ;BANK0
MOVLW 7H
BSF ADCON0,0 ;Turns on A/D converter.
;**********************************************************
END
;HEAD16F627.ASM for the 16F627/8, using the 37kHz internal RC
;PortA bits 0 to 7 are inputs
;PortB bits 0 to 7 are outputs
;Prescaler/32
;********************************************
;EQUATES SECTION
TMR0 EQU 1
OPTION_R EQU 1
PORTA EQU 5
PORTB EQU 6
TRISA EQU 5
TRISB EQU 6
STATUS EQU 3
ZEROBIT EQU 2
CARRY EQU 0
EEADR EQU 1BH
EEDATA EQU 1AH
EECON1 EQU 1CH
EECON2 EQU 1DH
RD EQU 0
WR EQU 1

WREN EQU 2
PCON EQU 0EH
COUNT EQU 20H
;*****************************************************
LIST P ¼ 16F627 ;using the 627
ORG 0
GOTO START
;***************************************************
;Configuration Bits
;*******************************************************
;0.1 SECOND DELAY
DELAYP1 CLRF TMR0 ;Start TMR0
LOOPA MOVF TMR0,W ;Read TMR0 into W
SUBLW .29 ;TIME - W
BTFSS STATUS,ZEROBIT ;Check TIME-W ¼ 0
GOTO LOOPA
RETLW 0 ;Return after TMR0 ¼ 29
;0.5 SECOND DELAY
DELAYP5 MOVLW 5
MOVWF COUNT
LOOPB CALL DELAYP1 ;0.1s delay
DECFSZ COUNT
GOTO LOOPB
RETLW 0 ;Return after 5 DELAYP1
;1 SECOND DELAY
DELAY1 MOVLW .10
MOVWF COUNT
LOOPC CALL DELAYP1 ;0.1s delay
DECFSZ COUNT
GOTO LOOPC
;**********************************************************

START BSF STATUS,5 ;Bank1
MOVLW B’11111111’
MOVWF TRISA ;PortA is input
MOVLW B’00000000’
MOVWF TRISB ;PortB is output
MOVLW B’00000100’
MOVWF OPTION_R ;Option Register, TMR0/32
CLRF PCON ;Select 37kHz oscillator.
BCF STATUS,5 ;Bank0
CLRF PORTA
CLRF PORTB
MOVLW 7
MOVWF 1FH ;CMCON turns off comparators.
;*********************************************************
END
;HEADER84.ASM for the 16F84 using a 32kHz crystal
;EQUATES SECTION
PORTA EQU 5 ;PORTA is FILE 5.
PORTB EQU 6 ;PORTB is FILE 6.
TRISA EQU 85H ;TRISA (the PORTA I/O selection)
TRISB EQU 86H ;TRISB (the PORTB I/O selection)
COUNT EQU 0CH ;USER RAM LOCATION.
;**********************************************************
;**********************************************************

;Configuration Bits
;*****************************************************
;1 SECOND DELAY
DELAY1 CLRF TMR0 ;START TMR0
SUBLW .32 ;TIME - W
GOTO LOOPA
;0.5 SECOND DELAY
DELAYP5 CLRF TMR0 ;START TMR0
LOOPB MOVF TMR0,W ;READ TMR0 IN W
SUBLW .16 ;TIME - W
GOTO LOOPB
;**********************************************************
START BSF STATUS,5 ;Turn to BANK1
MOVLW B’00011111’ ;5 bits of PORTA are I/Ps.
MOVWF TRISA
MOVLW B’00000000’
MOVWF TRISB ;PORTB IS OUTPUT
MOVLW B’00000111’
BCF STATUS,5 ;Return to BANK0
CLRF PORTA ;Clears PORTA
CLRF PORTB ;Clears PORTB
CLRF COUNT
;**********************************************************
END

; HEAD818.ASM for 16F818. This sets PORTA as digital INPUT.
;PORTB is an OUTPUT.
;Internal oscillator of 31.25kHz chosen
;The OPTION register is set to /256 giving timing pulses 32.768ms.
;*********************************************************
;EQUATES SECTION
ADCON0 EQU 1FH ;A/D Configuration reg.0
ADRES EQU 1EH ;A/D Result register.
CARRY EQU 0 ;CARRY IS BIT 0.
TRISA EQU 85H ;PORTA Configuration Register
TRISB EQU 86H ;PORTB Configuration Register
OPTION_R EQU 81H ;Option Register
OSCCON EQU 8FH ;Oscillator control register.
COUNT EQU 20H ;COUNT a register to count events.
;*********************************************************
;*********************************************************
;Configuration Bits
__CONFIG H’3F10’ ;sets INTRC-A6 is port I/O, WDT off, PUT
;on, MCLR tied to VDD A5 is I/O
;BOD off, LVP disabled, EE protect disabled,
;Flash Program Write disabled,
;Background Debugger Mode disabled,
;CCP function on B2,
;*****************************************************
;0.1 second delay, actually 0.099968s

SUBLW .3 ;TIME-3
NOP ;add extra delay
NOP
;0.5 second delay.
DELAYP5 MOVLW .5
MOVWF COUNT
LOOPC CALL DELAYP1
DECFSZ COUNT
GOTO LOOPC
RETLW 0
;1 second delay.
DELAY1 MOVLW .10
MOVWF COUNT
LOOPA CALL DELAYP1
DECFSZ COUNT
GOTO LOOPA
RETLW 0
;*********************************************************
;Configuration Section
MOVLW B’11111111’ ;8 bits of PORTA are I/P
MOVWF TRISA
MOVLW B’00000110’ ;PORTA IS DIGITAL
MOVWF ADCON1
MOVLW B’00000000’
MOVLW B’00000000’
MOVWF OSCCON ;oscillator 31.25kHz

;*********************************************************
END
;HEAD872.ASM Header for 16F872 using 32kHz oscillator
;EQUATES SECTION
TMR0 EQU 1
OPTION_R EQU 1
PORTA EQU 5
PORTB EQU 6
PORTC EQU 7
TRISA EQU 5
TRISB EQU 6
TRISC EQU 7
STATUS EQU 3
ZEROBIT EQU 2
CARRY EQU 0
EEADR EQU 0DH
EEDATA EQU 0CH
EECON1 EQU 0CH
EECON2 EQU 0DH
RD EQU 0
WR EQU 1
WREN EQU 2
ADCON0 EQU 1FH
ADCON1 EQU 1FH
ADRES EQU 1EH
CHS0 EQU 3
GODONE EQU 2
COUNT EQU 20H
;*****************************************************
LIST P ¼ 16F872
ORG 0
GOTO START
;*******************************************************
;Configuration Bits
__CONFIG H’3F30’ ;selects LP oscillator, WDT off, PUT on,

;*****************************************************
;1 SECOND DELAY
DELAY1 CLRF TMR0 ;Start TMR0
SUBLW .32 ;TIME - W
GOTO LOOPA
;0.5 SECOND DELAY
LOOPB MOVF TMR0,W ;Read TMR0 into W
SUBLW .16 ;TIME - W
GOTO LOOPB
;**********************************************************
MOVLW B’11111111’
MOVLW B’00000000’
MOVLW B’11111111’
MOVWF TRISC ;PortC is input
MOVLW B’00000111’
MOVLW B’00000000’
MOVWF ADCON1 ;PortA bits 0,1,2,3,5 are analogue
BSF STATUS,6 ;BANK3
BCF EECON1,7 ;Data memory on.
BCF STATUS,5
BCF STATUS,6 ;BANK0 return
BSF ADCON0,0 ;turn on A/D
CLRF PORTA

CLRF PORTB
CLRF PORTC
;*********************************************************
END
These headers can be used for applications that use the corresponding
microcontrollers. E.g. Any one of them can be used with FLASHER.ASM.
Other applications may require functions that are not in all of the devices i.e.
AtoD.
The explanation of the operation of the headers will be dealt with later when
the individual micros are examined.

5
Using inputs
A control program usually requires more than turning outputs on and off.
They switch on and off because an event has happened. This event is then
connected to the input of the microcontroller to ‘tell’ it what to do next.
The input could de derived from a switch or it could come from a sensor
measuring temperature, light levels, soil moisture, air quality, fluid pressure,
engine speed etc.
Analogue inputs are dealt with later, in this chapter we will concern ourselves
with digital on/off inputs.
BTFSC
BTFSS
CLRF
MOVF
SUBLW
SUBWF
RETLW
As an example let us design a circuit so that switch, SW1 will turn an LED
on and off.
The circuit diagram is shown in Figure 5.1.
This circuit is using the 16F84 microcontroller with a 32kHz crystal.
It can of course also be performed with any of the microcontrollers discussed
previously. Including the 16F818 using its internal oscillator, in which case
the crystal and 2 68pF capacitors are not required.
The program to control the hardware would use the following steps:
1. Wait for SW1 to close.
2. Turn on LED1.
3. Wait for SW1 to open.

4. Turn off LED1.
5. Repeat.
In the circuit diagram SW1 is connected to A0 and LED1 to B0.
When the switch is closed A0 goes low or clear. So we wait until A0 is clear.
The code for this is:
BEGIN BTFSC PORTA,0 (test bit 0 in file PORTA skip if clear)
GOTO BEGIN
BSF PORTB,0
The command BTFSC is Bit Test in File Skip if Clear, and the instruction
BTFSC PORTA,0 means Test the Bit in the File PORTA, i.e. Bit0, Skip
the next instruction if Clear. If A0 is Clear Skip the next instruction
(GOTO BEGIN) if it isn’t Clear then do not Skip and GOTO BEGIN to
check the switch again.
The program will check the switch thousands maybe millions of times a second,
depending on your clock.
When the switch is pressed the program moves on and executes the
instruction BSF PORTB,0 to turn on the LED.
5v
0v
1K
SW1
17
A0
16F84
V+
MCLR
0v
14
4
5v
0v
0.1µ
B0 6
0v
LED1
470R
68p
68p
0v
32kHz
16
15
Figure 5.1 Circuit diagram of the microcontroller switch
Using inputs 65

We then wait for the switch to open.
When the switch is open A0 goes Hi or Set, we then wait until A0 is Set i.e.
SWOFF BTFSS PORTA,0
GOTO SWOFF
BCF PORTB,0
GOTO BEGIN
The command BTFSS is Bit Test in File Skip if Set, and the instruction
BTFSS PORTA,0 means Test the Bit in the File PORTA, i.e. Bit0, Skip
the next instruction if Set. If A0 is Set Skip the next instruction (GOTO
SWOFF) if it isn’t Set then do not Skip and GOTO SWOFF to check the
switch again.
When the switch is set the program moves on and executes the instruction
BCF PORTB,0 to switch off the LED.
The program then goes back to the label BEGIN, to repeat.
The program is now added to the header. (NB. Use the TAB to make your
listing easy to read.) It is then saved as SWITCH.ASM.
;SWITCH.ASM
;*********************************************************
BEGIN BTFSC PORTA,0 ;Wait for SW1 to be pressed
GOTO BEGIN
BSF PORTB,0 ;Turn on LED1.
SWOFF BTFSS PORTA,0 ;Wait for SW1 to be released.
GOTO SWOFF
BCF PORTB,0 ;Switch off LED1.
GOTO BEGIN ;Repeat sequence.
END
Switch flowchart
It will be obvious from the program listing of the solution to the switch
problem that listings are difficult to follow. A picture is worth a thousand
words has never been more apt than it is with a program listing. The picture of
the program is shown below in the flowchart for the solution to our initial
switch problem, Figure 5.2. Before a programming listing is attempted it is
very worthwhile drawing a flowchart to depict the program steps. Diamonds
are used to show a decision (i.e. a branch) and rectangles are used to show
66 Using inputs

a command. Each shape may take several lines of program to implement. But
the idea of the flowchart should be evident. Note that the flowchart describes
the problem – you can draw it without any knowledge of the instruction set.
Program development
From our basic switch circuit an obvious addition would be to include a
delay so that the LED would go off automatically after a set time.
Suppose we wish to switch the light on for 5 seconds, using A0 as the switch
input. Figure 5.3 shows this Delay Flowchart.
The complete listing for this program for the 16F84 is shown below. I have
shown the complete code including the header because I have added a 5 second
delay in the subroutine section.
;DELAY.ASM
;EQUATES SECTION
Start
Is SW1
Closed?
Turn on LED1
Is SW1
Open?
Turn off LED1
N
N
Y
Y
Figure 5.2 Flowchart for the switch
Using inputs 67

STATUS EQU 3 ;STATUS is FILE3.
;**********************************************************
;**********************************************************
;Configuration Bits
;*****************************************************
;5 second delay.
DELAY5 CLRF TMR0 ;Start TMR0.
LOOPA MOVF TMR0,W ;Read TMR0 into W.
;**********************************************************
MOVWF TRISA
MOVLW B’00000000’
MOVLW B’00000111’
CLRF COUNT
;*********************************************************
68 Using inputs

ON BTFSC PORTA,0 ;Check button pressed.
GOTO ON
BSF PORTB,0 ;Turn on LED.
CALL DELAY5 ;CALL 5 second delay
BCF PORTB,0 ;Turn off LED.
GOTO ON ;Repeat
END
How does it work?
We check to see if the switch has been pressed (clear). If not GOTO ON
and check again. If it has skip that line and Turn on the LED on B0.
The code is:
GOTO ON
Wait 5 seconds. The 5 second delay has been included for you in the
subroutine section. Code:
CALL DELAY5
Set up PORTB as output.
Set PRESCALER to /256.
Button
Pressed
Wait 5 seconds
Turn OFF LED.
Turn ON LED.
Y
N
Figure 5.3 Delay flowchart
Using inputs 69

Turn the LED off and go back to the beginning. Code:
BCF PORTB,0 ;Turn off LED.
GOTO ON
Try this next problem for yourselves, before looking at the solution.
Problem 1: Using Port A bit 0 as a start button and outputs on PortB
bits 0-3. Switch on Port B bits 0 and 2 for ¼ second, switch
off bits 0 and 2.
Switch on Port B bits 1 and 3 for ¼ second, switch off bits
1 and 3.
Repeat continuously.
The ¼ second delay is provided for you.
The flowchart for the solution to problem1 is shown in Figure 5.4
Program solution to problem1 for the 16F84
;PROBLEM1.ASM
;EQUATES SECTION
;*********************************************************
;*********************************************************
70 Using inputs

;Configuration Bits
__CONFIG H’3FF0’ ;selects LP oscillator, WDT off, PUT on
;*********************************************************
;0.25 second delay.
DELAY CLRF TMR0 ;START TMR0.
SUBLW .8 ;TIME - 8
;*********************************************************
MOVWF TRISA
MOVLW B’00000000’
MOVLW B’00000111’
;*********************************************************
GOTO ON
REPEAT MOVLW B’00000101’
MOVWF PORTB ;Turn on bits 0 and 2
CALL DELAY ;¼ second delay
MOVLW B’00001010’
MOVWF PORTB ;Turn on bits 1 and 3
CALL DELAY ;¼ second delay
GOTO REPEAT ;Repeat
END
Using inputs 71

How does it work?
Wait for the switch on PORTA,0 to clear, with BTFSC PORTA,0 then
skip to
MOVLW B’00000101’ this sets up the data in the W register.
MOVWF PORTB transfers the W register to PORTB and puts 5v on
B0 and B2 only.
CALL DELAY waits for ¼ second.
MOVLW B’00001010’ this sets up the data in the W register.
MOVWF PORTB transfers the W register to PORTB and puts 5v on
B1 and B3 only.
CALL DELAY waits for ¼ second.
Set PORTB as OUTPUT.
Set PRESCALER to /256.
Is
Switch
pressed?
Turn on B0, B2.
Turn OFF B0, B2.
Turn ON B1, B3.
Wait 1/4 second.
Turn OFF B1, B3.
Wait 1/4 second.
N
Y
Figure 5.4 Flowchart for problem
72 Using inputs

GOTO REPEAT sends the program back to (my) label, REPEAT.
This will keep the lights flashing all the time without checking the switch
again.
Question. How do we make the program look at the switch, so that we can
control whether or not the program repeats?
Answer: Instead of GOTO REPEAT use GOTO BEGIN. The program will
then goto the label BEGIN instead of REPEAT and will wait for the switch
to be Clear before repeating.
Extra Work. Try and make the flashing routine more interesting by adding
more combinations.
Scanning (using multiple inputs)
Scanning (also called polling) is when the microcontroller looks at the condi-
tion of a number of inputs in turn and executes a section of program depending
on the state of those inputs.
Applications include:
Burglar Alarms – when sensors are monitored and a siren sounds either
immediately or after a delay depending on which input is active.
Keypad scanning – a key press could cause an LED to light, a
buzzer to sound or a missile to be launched. Just do not press the
wrong key!
Let’s consider a simple example:
Switch scanning
Design a circuit so that if a switch is pressed a corresponding LED will light. i.e.
If SW0 is Hi, (logic1 or Set) then LED0 is on.
If SW0 is Low, (logic 0 or Clear) then LED0 is off.
If SW1 is Hi, (logic1 or Set) then LED1 is on.
If SW1 is Low, (logic 0 or Clear) then LED1 is off.
etc.
The circuit diagram for this is shown if Figure 5.5 and the corresponding
flowchart in Figure 5.6.
Using inputs 73

The program for this switch scan is:
;SWSCAN.ASM using 16F84 and 32kHz crystal.
;EQUATES SECTION
5v
0v
1K
SW0
17
A0
68p
68p
0v
32kHz
16
15
B0
B1
B2
B3
V+
MCLR
0v
14
4
5v
0v
0.1µ
16F84
5
6
7
8
9
4 × 680R
5v
0v
1K
SW1
5v
0v
1K
SW2
5v
0v
1K
SW3
18
1
2
A1
A2
A3
0v
0v
0v
0v
LED0
LED1
LED2
LED3
Figure 5.5 Switch scanning circuit
74 Using inputs

STATUS EQU 3 ;STATUS is FILE3.
;**********************************************************
;**********************************************************
;Configuration Bits
;*****************************************************
MOVWF TRISA
MOVLW B’00000000’
MOVLW B’00000111’
CLRF COUNT
;*********************************************************
SW0 BTFSC PORTA,0 ;Switch0 pressed?
GOTO TURNON0 ;Yes
BCF PORTB,0 :No, Switch off LED0.
GOTO TURNON1 ;Yes
BCF PORTB,1 :NO Switch off LED1.
Using inputs 75

GOTO TURNON2 ;Yes
GOTO TURNON3 ;Yes
GOTO SW0 ;Rescan.
TURNON0 BSF PORTB,0 ;Turn on LED0
GOTO SW1
GOTO SW2
GOTO SW3
GOTO SW0
END
How does it work?
SW0 is checked first with the instruction BTFSC PORTA,0. If the switch is
closed when the program is executing this line then we GOTO TURNON0.
That is the program jumps to the label TURNON0 which turns on
LED0 and then jumps the program back to check SW1 at, of course, the
label, SW1.
SW1 is then checked in the same manner and then SW2 and SW3.
Suppose we press the switch when the program is not looking at it. The
program lines are being executed at ¼ of the clock frequency i.e. 32,768Hz
that is 8192 lines a second. The program will always catch you!
Try modifying the program so that the switches can flash 4 different routines
e.g. SW0 flashes all lights on and off 5 times for 1 second.
76 Using inputs

Control application – a hot air blower
The preceding section outlined how to monitor inputs by looking at them
in turn. This application will ‘read’ all the bits on the port at once,
because we will be concerned with particular combinations of inputs
rather than individual ones.
Is SW0
SET?
Is SW1
SET?
Is SW2
SET?
IS SW3
SET?
Turn on LED0
Turn off LED0
Turn on LED1
Turn off LED1
Turn on LED2
Turn off LED2
Turn on LED3
Turn off LED3
N
Y
N
Y
Y
N
N
Y
Figure 5.6 Flowchart for switch scan
Using inputs 77

The bits on the Input Port will be 0s or 1s and we can treat this binary pattern
like any other number in a file.
Consider a controller for a hot air radiator. When the water is warm the fan
will blow the warm air into the room. The heater and fan are controlled by
3 temperature sensors: (a) a room temperature sensor, (b) a boiler water
temperature sensor and (c) a safety overheating sensor. The truth table for
the system is shown in Table 5.1, where a 1 means hot and a 0 means cold
for the sensors.
The block diagram for the system is shown in Figure 5.7.
Note A3, A4, A5, A6 and A7 are inputs and need to be connected to 0v. Do
not leave them floating – you would not know if they were 0 or 1! Even though
A5
A6
A7
A0
A1
A2
B0
B1
A3
A4
0v
Over heat sensor
Water temp sensor
Room temp sensor
Fan
Heater
Figure 5.7 Block diagram for the hot air system
INPUTS OUTPUTS
A
7
A
6
A
5
A
4
A3 Room
A2
Water
A1
OverH
A0
Heater
B1
Fan
B0
0 0 0 0 0 0 0 0 1 0
0 0 0 0 0 0 0 1 0 1
0 0 0 0 0 0 1 0 1 1
0 0 0 0 0 0 1 1 0 1
0 0 0 0 0 1 0 0 0 0
0 0 0 0 0 1 0 1 0 1
0 0 0 0 0 1 1 0 0 0
0 0 0 0 0 1 1 1 0 1
Table 5.1 Truth table for the hot air system
78 Using inputs

they are not being used they are still being read. NB. The inputs A5, A6 and
A7 do not exist on the 16F84.
There are 8 input conditions from our 3 sensors. So all 8 must be checked to
determine which condition is true.
Consider the first condition A2 ¼ A1 ¼ A0 ¼ 0, i.e. PORTA reads 0000 0000.
How do we know that PORTA is 0000 0000? We do not have an instruction
which says ‘‘is PORTA equal to 0000 0000’’ or any other value for that matter.
So we need to look at our 35 instructions and come up with a way of finding
out what is the binary value of PORTA.
We check for this condition by subtracting 00000000 from it, if the answer is
zero then PORTA reads 00000000. I.e. 0000 0000 0000 0000 ¼ 0 (obviously).
But how do we subtract the two numbers and how do we know if the answer
is zero?
This is a very important piece of programming so read the next few lines
carefully.
We first of all read PORTA into the W register with the instruction MOVF
PORTA,W that moves the data, (setting of the switches, 1s or 0s), into W.
We then subtract the number we looking for in this case 00000000 from W.
We then need to know if the answer to this subtraction is zero. If it is
then the value on PORTA was 00000000. If the answer is not zero then the
value of the data on PORTA was not zero.
So is the answer zero? Yes or No? The answer is held in a register called
the Status Register, in bit 2 of this register, called the zero bit. If the zero bit,
called a flag is 1, it is indicating that the statement is true the calculation
was zero. If the zero bit is 0 that indicates the statement is false the answer
was not zero.
We test the zero bit in the status register just like we tested the bit on
the switch connected to PORTA at the start of this chapter. We use the
command BTFSC and the instruction BTFSC STATUS,ZEROBIT.
If the zero bit is clear we skip the next instruction if it is set we have a
match and do not skip.
MOVLW B’00000000’ ;put 000000 in W
SUBWF PORTA ;subtract W from PORTA
BTFSC STATUS,ZEROBIT ;PORTA ¼ 00000000?
CALL CONDA ;yes
Using inputs 79

CONDA is short for condition A where we require the heater on and the fan off.
To check for A2 ¼ A1 ¼ 0 and A0 ¼ 1 we subtract 00000001. To check for
the next condition A2 ¼ 0, A1 ¼ 1, A0 ¼ 0 we subtract 00000010, and so on
for the other 5 conditions.
MOVLW B’00000001’ ;put 00000001 in W
SUBWF PORTA ;subtract W from PORTA
BTFSS STATUS,ZEROBIT ;PORTA ¼ 00000001?
CALL CONDB ;yes
etc.
The opcode for this program CONTROL.ASM is:
;CONTROL.ASM
CONDA BCF PORTB,0 ;turns fan off
BSF PORTB,1 ;turns heater on
RETLW 0
CONDB BSF PORTB,0 ;turns fan on
BCF PORTB,1 ;turns heater off
RETLW 0
CONDC BSF PORTB,0 ;turns fan on
BSF PORTB,1 ;turns heater on
RETLW 0
CONDD BCF PORTB,0 ;turns fan off
BCF PORTB,1 ;turns heater off
RETLW 0
;*********************************************************
BEGIN MOVLW B’00000000’ ;put 00000000 in W
SUBWF PORTA ;PORTA - W
CALL CONDA ;yes
MOVLW B’00000001’ ;put 00000001 in W
CALL CONDB ;yes
80 Using inputs

MOVLW B’00000010’ ;put 00000010 in W
CALL CONDC ;yes
MOVLW B’00000011’ ;put 00000011 in W
CALL CONDB ;yes
MOVLW B’00000100’ ;put 00000100 in W
CALL CONDD ;yes
MOVLW B’00000101’ ;put 00000101 in W
CALL CONDB ;yes
MOVLW B’00000110’ ;put 00000110 in W
CALL CONDD ;yes
MOVLW B’00000111’ ;put 00000111 in W
CALL CONDB ;yes
GOTO BEGIN
END
Notice that the SUBROUTINE SECTION needs to be changed to include the
conditions, CONDA, CONDB, CONDC and CONDD. The DELAY
subroutines are not required in this example.
The program can be checked by using switches for the input sensors and
LEDs for the outputs.
There is more than one way of skinning a cat, another way of writing this
program is shown in Chapter 8, in the section on look up tables.
Using inputs 81

6
Understanding the headers
The 16F84
HEADER84.ASM The header for the 16F84.
Now that we have looked at a number of applications we are ready to under-
stand HEADER84.ASM introduced in Chapter 2.
The header starts with a title that includes the name of the file, this is useful
when you are printing it out and details about what the program is doing.
;HEADER84.ASM for 16F84. This sets PORTA as an INPUT (NB 1
; means input) and PORTB as an OUTPUT
; (NB 0 means output). The OPTION
; register is set to /256 to give timing pulses
; of 1/32 of a second.
; 1second and 0.5 second delays are
; included in the subroutine section.
;*********************************************************
The EQUATES section tells the software what numbers your words
represent. When you write your program you use mnemonics such as
PORTA, PORTB, TMR0, STATUS, ZEROBIT, COUNT, MYAGE. The
Assembler Program does not understand your words; it is looking for the
file number or the bit number. You have to tell it what these mean in
the Equates Section i.e. COUNT is File 0C, PortA is file 5, the STATUS
register is file 3, ZEROBIT is bit 2, etc. The memory map of the 16F84
in Table 6.1 shows the addresses of the registers and user files. The file
with address 0C is the first of the user files and I have called it COUNT,
it stores the number of times certain events have happened in my program.
I could have file 0D as COUNT2, file 0E as COUNT3, file 0F as SECONDS
or WAIT etc.

;EQUATES SECTION
What chip are we using?
LIST P¼16F84 ;we are using the 16F84.
LIST P¼16F84 tells the assembler what chip to assemble the code for. ORG 0
means put the next line of code into program memory address 0, then follow
with next line in address1 etc.
GOTO START makes the program bypass the subroutine section and GOTO
the label START which is where the device is configured before executing
the body of the program. The instruction GOTO START is placed in EPROM
address 0 by ORG 0.
The line DELAY1 CLRF TMR0 is then placed in program memory
address 1, etc.
CONFIGURATION BITS
To avoid having to set the configuration bits when we come to program the
device they can be set in the code. You can change these bits if you require
in MPLAB, note the new number and substitute it in the code.
SUBROUTINE SECTION.
The subroutine section consists of 2 subroutines DELAY1 and DELAYP5.
A subroutine is a section of program, which is, used a number of times
instead of rewriting it and using up program memory. Just call it i.e. CALL
Understanding the headers 83

DELAY1, at the end you RETURN to the program in the position you left
it. The stack is the register that remembers where you came from and returns
you back.
The DELAY1 code is:
DELAY1 CLRF TMR0 ;Start TMR0.
LOOPA MOVF TMR0,W ;Read TMR0 into W.
DELAY1 starts by clearing the register TMR0 (timer 0), with CLRF TMR0,
i.e. CleaR the File TMR0. This sets the timer to zero and will be counting
TMR0 pulses every 1/32 of a second.
LOOPA MOVF TMR0,W is move file TMR0 into the working register, W.
We want to know when TMR0 is 32, because then we will have had 32
TIMER0 pulses, which is 1 second. This is done with a subtraction as in the
example earlier in this Chapter 5, in the section on the hot air blower.
The label LOOPA is there because we keep returning to it until TMR0 reaches
the required value.
There is no instruction, which asks the micro is TMR0 equal to 32. So we
have to use the instructions available. We subtract a number from W and ask
is the answer 0. If for example we subtract 135 from W and the answer is 0
then W contained 135 if the answer was not 0 then W did not contain 135.
The status register contains a bit called a zerobit, it is bit2. Notice in the
EQUATES section I have put ZEROBIT EQU 2. So I can use ZEROBIT
in my code instead of 2 I would soon forget what the 2 was supposed to
mean. The zerobit is set to a 1 when the result of a previous calculation is 0.
So a 1 means result was 0!!!! Think of this as a flag (because that’s what it
is called), the flag is waving (a 1) to indicated the result is zero. We can
test this zerobit, i.e. look at it and see if it is a 1 or 0. We can skip the next
instruction if it is set, (a zero has occurred), by BTFSS STATUS,ZEROBIT
or skip if clear, (a zero has not occurred), by BTFSC STATUS,
ZEROBIT. Doesn’t this read better than BTFSC 3,2 STATUS is Register3,
ZEROBIT is bit 2.
84 Understanding the headers

Lets look at this subroutine again.
BTFSS STATUS,ZEROBIT ; Check TIME-W ¼ 0
We clear TMR0 (CLRF TMR0).
Then move TMR0 into W (MOVF TMR0,W)
SUBTRACT 32 from W which now holds TMR0 value. (SUBLW .32)
If W (hence TMR0) is 32 the zerobit is set, we skip the next instruction
and return from the subroutine with 0 in W (RETLW 0)
If W is not 32 then we do not skip and we GOTO LOOPA and put TMR0
in W and repeat until TMR0 is 32.
DELAYP5 is a similar code but TMR0 now is only allowed to count upto 16
i.e. a half-second (with 32 pulses a second). Note if you copy and paste, change
the name of the subroutine from DELAY1 to DELAYP5, change the 32 to
16 and do not forget to change LOOPA to LOOPB. You cannot goto room 27
if there are two room 27s!
CONFIGURATION SECTION:
MOVLW B0
000111110
;5 bits of PORTA are I/Ps.
MOVWF TRISA
MOVLW B0
000000000
MOVLW B0
000001110
CLRF COUNT
The instruction BSF STATUS,5 sets bit 5 in the Status Register. As you can
see from the explanation of the Status Register bits in Chapter 19, bit 5 is
a page select bit which selects page1 giving us access to the Registers in the
page 1 (Bank1) column of the memory map in Table 6.1. The reason for pages
or banks is that we have an 8 bit micro. 8 bits can only address 256 files so

to identify a file we have it on a page, like a line in a book i.e. line 17 on page 40
instead of line 2475.
MOVWF TRISA
These 2 lines move 11111 into the data direction register to set the 5 bits of
PORTA as inputs. The 11111 is first moved to W (MOVLW B’00011111’)
and then into the data direction register with MOVWF TRISA. A 1 signifies
an input a 0 an output.
MOVLW B’00000000’ ;8bits of PORTB are O/P
MOVWF TRISB
These 2 lines move 00000000 into the data direction register to set the 8 bits
of PORTB as outputs. The 000000 is first moved to W and then into the
data direction register with MOVWF TRISB.
PortA and PortB can be configured differently if required. E.g. to make
the lower 4 bits of PortB outputs and the upper 4 bits inputs - alter the 2 lines
of the program with:
MOVLW B’11110000’
MOVWF TRISB
The header also sets the internal clock to divide by 256 i.e. a 32.768kHz
clock gives a program execution of 32.768 kHz/4 ¼ 8.192 kHz. If the prescaler is
set to divide by 256 this gives timing pulses of 32 a second.
The prescaler is configured with the 2 lines:
MOVWF OPTION_R ;TIMER is 1/32 sec.
The OPTION register can be altered in the header to give faster timing pulses
if required, as described in the OPTION Register section in Chapter 19.
The line BCF STATUS,5 ;Return to Bank0.
then returns to page 0 on the memory map. The good news here is in the
programs in this book we only need to go into page 1 in the Configuration
Section. The body of the program, your section, resides in page 0.

We then finish the configuration section by clearing any outputs in PORTA
and PORTB with,
This will not affect any bits that are configured as inputs.
Just for good measure the COUNT file is also cleared with CLRF COUNT.
16F84 memory map
The Memory Map of the 16F84 is shown in Table 6.1.
This diagram shows the position of the Special Function Registers, i.e.
PORTA, PORTB, TMR0 etc. in addresses 00 to 0B and the location of
the User Files i.e. COUNT (the only one we have used up to now) occupying
locations 0C through to 4F.
These files are very important when writing our code. The Special Function
Registers enable us to tell the microcontroller to do things, i.e. set PORTB
up as an output port with TRISB, alter the rate of TMR0 with the OPTION
FILE
ADDRESS
FILE NAME FILE NAME
00 INDIRECT
ADDRESS
INDIRECT
ADDRESS
01 TMR0 OPTION
02 PCL PCL
03 STATUS
FST
STATUS
04 FSR
05 PORTA
PORTB
-
TRISA
06 TRISB
07 -
08 EEDATA EECON1
09 EDADR EECON2
0A PCLATH
PCLATH
0B INTCON INTCON
0C
4F
68
USER
FILES
BANK0 BANK1
Table 6.1 16F84 memory map

register, find out if the result of a calculation is zero, þve or ve using the
STATUS register. TMR0 of course tells us how much time has elapsed.
The other microcontroller which features frequently in this book, my
favourite, is the 16F818. We will look at its header and memory map now
and compare it to the 16F84 to see how they differ. After that you will be
able to distinguish between other micros.
The 16F818
HEAD818.ASM The header for the 16F818.
The code shown below is the header for the 16F818 that we first saw in
Chapter 4.
;HEAD818.ASM for 16F818. This sets PORTA as digital INPUT.
;PORTB is an OUTPUT.
;Internal oscillator of 31.25kHz chosen
;The OPTION register is set to /256 giving timing pulses of 32.768 ms.
;*********************************************************
; EQUATES SECTION
TRISB EQU 86H ;PORTB Configuration Register OPTION_R
;*********************************************************
;*********************************************************

;Configuration Bits
;Background Debugger Mode disabled, CCP
;function on B2,
;*****************************************************
SUBLW .3 ;TIME - 3
BTFSS STATUS,ZEROBIT ; Check TIME-W ¼ 0
NOP
;0.5 second delay.
DELAYP5 MOVLW .5
MOVWF COUNT
LOOPC CALL DELAYP1
DECFSZ COUNT
GOTO LOOPC
RETLW 0
;1 second delay.
DELAY1 MOVLW .10
MOVWF COUNT
LOOPA CALL DELAYP1
DECFSZ COUNT
GOTO LOOPA
RETLW 0
;*********************************************************
MOVLW B11111111’ ;8 bits of PORTA are I/P
MOVWF TRISA

MOVWF ADCON1
MOVLW B’00000000’
MOVLW B’00000000’
;*********************************************************
END
We will now consider only the new additions to the previous HEADER84.
ASM for the 16F84.
PORTA is now an 8 I/O port, NB. PORTA,5 is input only.
ADCON0, ADCON1 and ADRES are Special Function Registers that will
enable us to instruct the microcontroller on how we want the A/D converter
to function. We will discuss these when we consider A/D conversion in
Chapter 11.
OSCCON allows us to set the value of the internal oscillator. We can
choose from 8MHz, 4MHz, 2MHz, 1MHz, 500kHz, 250kHz, 125kHz or
31.25 kHz. The use of OSCCON is described in the Register section in
Chapter 19.
CONFIGURATION BITS. There are more functions on the 16F818
than the 16F84 so there are more choices in the way it is configured.
Here we have selected the internal oscillator so we do not need the
crystal, that has freed up 2 I/O lines. The master clear, MCLR has been
switched internally to Vdd (5v) freeing up another I/O line, giving 16 I/O.
We have switched the brown out off this would reset the micro if the
supply voltage fell below a critical point avoiding erratic behaviour.
Low voltage programming has been switched off. EEPROM protection
and Program Write Protection has been disabled. Background Debugger
Mode has been disabled. The 16F818 is capable of working with
the Microchip In Circuit Debugger (ICD2). Capture and Compare

Pulse Width Module (CCP) not discussed in this book has been switched
onto B2.
SUBROUTINE SECTION.
The 16F818 header described uses the internal 31.25kHz oscillator, which
does not lend itself so easily to times of seconds. I have had to write a
different code for the delays. A 31.25kHz clock gives timing pulses of
32ms which do not add up exactly to give a second. The delay loop similar
in its action to the 16F84 delay has had 2 NOP (no operation) instructions
added to make up the shortfall. The 0.1 second delay is therefore 0.099968s
which is as close as I could get it. If you really need accurate times you will
need to use a crystal for your timing. The internal oscillators are only about
1% accurate.
CONFIGURATION SECTION.
Because the 16F818 has an A/D converter on board you need to tell it
which PORTA inputs are analogue and which are digital. Analogue inputs
are dealt with in Chapter 11 for now PORTA has been set to all digital
inputs with:
MOVWF ADCON1
The internal oscillator is set to 31.25kHz with:
MOVLW B’00000000’
This is a default condition and is therefore not required. I have included it
incase you are wondering how the frequency is set. You need to alter the
data in OSCCON to change the frequency, see Chapter 19.
Because the 16F818 has more functions than the 16F84 it follows that there
are more Special Function Registers to handle these extra functions. It also
has more user files.
These files are now arranged over 4 banks, BANK0, BANK1, BANK2 and
BANK3. The Banks are selected by the Bank Select bits (page select bits)
in the Status Register, RP0 and RP1, bits 5 and 6, shown in Figure 6.1.
IRP RP1 RP0 TO PD Z DC C
bit7 bit0
Figure 6.1 Status register bits

So 00 selects Bank0
01 selects Bank1
10 selects Bank2
11 selects Bank3
For most applications in this book once we have configured the device we
will not need to change banks. The only time we do change is when we look
at applications involving the Data EEPROM.
The 16F818 memory Map is shown below in Figure 6.2.
We will now continue with some more applications and introduce some
more instructions and ideas. Each of these programs will be able to be executed
using a number of micros using the appropriate headers.
File
Address
File
Address
File
Address
File
Address
Indirect addr.(*) 00h Indirect addr.(*) Indirect addr.(*)
80h Indirect addr.(*) 180h
TMR0 01h OPTION 81h TMRC OPTION 181h
PCL 02h PCL 82h PCL 182h
STATUS 03h STATUS 83h STATUS 183h
FSR 04h FSR 84h FSR
PCL
STATUS
FSR
104h
103h
100h
101h
102h
184h
PORTA 05h TRISA 85h 105h 185h
PORTB 06h TRISB 86h PORTB 106h TRISB 186h
07h 87h 107h 187h
08h 88h 108h 188h
09h 89h 109h 189h
PCLATH 0Ah PCLATH 8Ah PCLATH 10Ah
INTCON 0Bh INTCON 8Bh INTCON
PCLATH
INTCON
10Bh 18Bh
PIR1 0Ch PIE1 8Ch EEDATA 10Ch EECON1 18Ch
PIR2 0Dh PIE2 8Dh EEADR 10Dh EECON2 18Dh
TMR1L 0Eh PCON 8Eh EEDATH 10Eh Reserved(1) 18Eh
TMR1H 0Fh OSCCON 8Fh EEADRH 10Fh Reserved(1) 18Fh
T1CON 10h OSCTUNE 90h 110h 190h
TMR2 11h 91h
T2CON 12h PR2 92h
SSPBUF 13h SSPADD 93h
SSPCON 14h SSPSTAT 94h
CCPR1L 15h 95h
CCPR1H 16h 96h
CCP1CON 17h 97h
18h 98h
19h 99h
1Ah 9Ah
1Bh 9Bh
1Ch 9Ch
1Dh 9Dh
ADRESH 1Eh ADRESL 9Eh
ADCON0 1Fh ADCON1 9Fh 11Fh 19Fh
General Purpose
Register
A0h
20h
32 Bytes
120H 1A0h
General
Purpose
Register
96 Bytes
BFh
C0h
accesses
20h-7Fh
accesses
20h-7Fh
accesses
40h-7Fh
Bank 0
7Fh
Bank 1
FFh
Bank 2
17Fh
Bank 3
1FFh
18Ah
Figure 6.2 16F818 memory map

7
Keypad scanning
There are no new instructions used in this chapter
Keypads are an excellent way of entering data into the microcontroller. The
keys are usually numbered but they could be labeled as function keys for
example in a remote control handset in a TV to adjust the sound or colour etc.
As well as remote controls, keypads find applications in burglar alarms, door
entry systems, calculators, microwave ovens etc. So there are no shortage
of applications for this section.
Keypads are usually arranged in a matrix format to reduce the number of
I/O connections.
A 12 key keypad is arranged in a 3 4 format requiring 7 connections.
A 16 key keypad is arranged in a 4 4 format requiring 8 connections.
Consider the 12 key keypad. This is arranged in 3 columns and 4 rows as shown
in Table 7.1. There are 7 connections to the keypad – C1, C2, C3, R1, R2, R3
and R4.
This connection to the micro is shown in Figure 7.1.
The keypad works in the following way:
If for example key 6 is pressed then B2 will be joined to B4. For key 1 B0 would
be joined to B3 etc. as shown in Figure 7.1.
The micro would set B0 low and scan B3, B4, B5 and B6 for a low to see
if keys 1, 4, 7 or * had been pressed.
Column1, C1 Column2, C2 Column3, C3
Row1, R1 1 2 3
Row2, R2 4 5 6
Row3, R3 7 8 9
Row4, R4 * 0 #
Table 7.1 12 Key keypad

The micro would then set B1 low and scan B3, B4, B5 and B6 for a low to
see if keys 2, 5, 8 or 0 had been pressed.
Finally B2 would be set low and B3, B4, B5 and B6 scanned for a low to see
if keys 3, 6, 9 or # had been pressed.
Programming example for the keypad
As a programming example when key 1 is pressed display a binary 1 on
PORTA, when key 2 is pressed display a binary 2 on PORTA etc.
Key 0 displays 10. Key * displays 11. Key # displays 12.
This program could be used as a training aid for decimal to binary conversion.
The flowchart is shown in Figure 7.2.
68p
68p
0v
32kHz
16
15
V+
0v
14
4
5v
0v
0.1µ
5
0v
0v
0v
0v
LED0
LED1
LED2
LED3
5v
4 × 680R
7 × 100k
A0
A1
A2
A3
MCLR
6
7
8
9
B0
B1
B2
B3
B4
B5
B6
1 2 3
4 5 6
7 8
0
9
* #
5v
16F84
Figure 7.1 Keypad connection to the microcontroller
94 Keypad scanning

Set PORTA as Output.
Set PORTB as MIXED I/O
CLEAR PORTA
PORTB = FF
Clear B0.
Is
B3 = 0?
Is
B4 = 0?
Is
B5 = 0?
Is
B6 = 0?
PORTA = 4
**
PORTA = 11
N
Y
PORTA = 7
N
Y
N
Y
PORTA = 1
N
Y
Figure 7.2 Keypad scanning flowchart
Keypad scanning 95

PORTB = FF
Clear B1
Is
B4 = 0?
Is
B3 = 0?
Is
B5 = 0?
Is
B6 = 0?
PORTA = 5
PORTA = 8
PORTA = 2
PORTA = 10
Y
N
N
N
N
Y
Y
Y
Figure 7.2 Continued
96 Keypad scanning

The program listing for the Keypad example for the 16F84 is shown below but
can be used with any ‘suitable’ microcontroller using the appropriate header.
N.B. PORTA has been configured as an output port and PORTB has been
configured with 3 outputs and 5 inputs, so the header will require modifying as
shown.
N
N
N
Y
N
Return to **
Y
Y
Y
PORTB = FF
Clear B2
Is
B6 = 0?
Is
B3 = 0?
Is
B4 = 0?
Is
B5 = 0?
PORT A = 6
PORT A = 9
PORT A = 3
PORT A = 12
Keypad scanning 97

PORTB has internal pull up resistors so that the resistors connected to PORTB
in Figure 7.1 are not required.
;KEYPAD.ASM
;EQUATES SECTION
TRISA EQU 85H
TRISB EQU 86H
OPTION_R EQU 81H
;*********************************************************
;*********************************************************
;CONFIGURATION BITS
__Config H’3FF0’ ;selects LP Oscillator, WDT off,
;Put on,
;code protection disabled.
;*********************************************************
MOVLW B’00000000’ ;PORTA is OUTPUT
MOVWF TRISA
MOVLW B’11111000’
MOVWF TRISB ;PORTB is mixed I/O.
BCF OPTION_R,7 ;Turn on pull ups.
;*********************************************************
COLUMN1 BCF PORTB,0 ;Clear B0
BSF PORTB,1 ;Set B1
BSF PORTB,2 ;Set B2
98 Keypad scanning

CHECK1 BTFSC PORTB,3 ;Is B3 Clear?
GOTO CHECK4 ;No
MOVLW .1 ;Yes, output 1.
MOVWF PORTA
GOTO CHECK7 ;No
MOVWF PORTA
GOTO CHECK11 ;No
MOVWF PORTA
GOTO COLUMN2 ;No
MOVWF PORTA
COLUMN2 BSF PORTB,0 ;Set B0
BCF PORTB,1 ;Clear B1
BSF PORTB,2 ;Set B2
GOTO CHECK5 ;No
MOVWF PORTA
GOTO CHECK8 ;No
MOVWF PORTA
GOTO CHECK10 ;No
MOVWF PORTA
GOTO COLUMN3 ;No
MOVWF PORTA
BSF PORTB,1 ;Set B1
GOTO CHECK6 ;No
MOVWF PORTA
GOTO CHECK9 ;No
MOVWF PORTA
Keypad scanning 99

GOTO CHECK12 ;No
MOVWF PORTA
GOTO COLUMN1 ;No
MOVWF PORTA
GOTO COLUMN1 ;Start scanning again.
END
How does the program work?
Port configuration
The first thing to note about the keypad circuit is that the PORTA pins are
being used as outputs. On PORTB, pins B0, B1 and B2 are outputs and B3, B4,
B5 and B6 are inputs. So PORTB is a mixture of inputs and outputs. The
HEADER84.ASM program has to be modified to change to this new
configuration.
To change PORTA to an output port, the following two lines are used in the
Configuration Section:
MOVWF TRISA
To configure PORTB as a mixed input and output port the following two lines
are used in the Configuration Section:
MOVLW B’11111000’
MOVWF TRISB ;PORTB is mixed I/O. B0,B1,B2 are O/P.
Scanning routine
The scanning routine looks at each individual key in turn to see if one is being
pressed. Because it can do this so quickly it will notice we have pressed a key
even if we press it quickly.
The scanning routine first of all looks at the keys in column1 i.e. 1, 4, 7 and *.
It does this by setting B0 low, B1 and B2 high. If a 1 is pressed the B3 will
be low, if a 1 is not pressed then B3 will be high. Because pressing a 1 connects
B0 and B3.
Similarly if 4 is pressed B4 will be low if not B4 will be high.
100 Keypad scanning

If 7 is pressed B5 will be low if not B5 will be high.
If * is pressed B6 will be low if not B6 will be high.
In other words when we set B0 low if any of the keys in column1 are pressed
then the corresponding input to the microcontroller will go low and the
program will output the binary number equivalent of the key that has been
pressed.
If none of the keys in column1 are pressed then we move onto column2.
The code for scanning column1 is as follows:
These 3 lines set up PORTB with B0 ¼ 0, B1 ¼ 1 and B2 ¼ 1.
BSF PORTB,1 ;Set B1
BSF PORTB,2 ;Set B2
These next 4 lines test input B3 to see if it clear if it is then a 1 is placed on
PORTA, then the program continues. If B3 is set then we proceed to check
to see if key 4 has been pressed, with CHECK4.
GOTO CHECK4 ;No
MOVLW .1 ;Yes, output 1
MOVWF PORTA ;to PORTA
These next 4 lines test input B4 to see if it clear if it is then a 4 is placed
on PORTA, then the program continues. If B4 is set then we proceed to check
to see if key 7 has been pressed, with CHECK7.
GOTO CHECK7 ;No
MOVWF PORTA
These next 4 lines test input B5 to see if it clear if it is then a 7 is placed on
PORTA, then the program continues. If B5 is set then we proceed to Check to
see if key * has been pressed, with CHECK11.
GOTO CHECK11 ;No
MOVWF PORTA
Keypad scanning 101

These next 4 lines test input B6 to see if it clear if it is then an 11 is placed on
PORTA, then the program continues. If B5 is set then we proceed to check the
keys in column2, with COLUMN2.
GOTO COLUMN2 ;No
MOVWF PORTA
These 3 lines set up PORTB with B0 ¼ 1, B1 ¼ 0 and B2 ¼ 1.
BSF PORTB,2 ;Set B2
We then check to see if key2 has been pressed by testing to see if B3 is clear,
if it is then a 2 is placed on PORTA and the program continues. If B3 is set
then we proceed with CHECK5. This code is:
GOTO CHECK5 ;No
MOVWF PORTA
The program continues in the same manner checking 5, 8 and 10 (0). Then
moving onto column3 to check for 3, 6, 9 and 12 (#). After completing the scan
the program then goes back to continue the scan again.
It takes about 45 lines of code to complete a scan of the keypad.
With a 32,768Hz crystal the lines of code are executed at ¼ of this speed i.e.
8192 lines per second. So the scan time is 45/8192 ¼ 5.5ms. This is why
no matter how quickly you press the key the microcontroller will be able to
detect it.
Security code
Probably one of the most useful applications of a keypad is to enter a code to
turn something on and off such as a burglar alarm or door entry system.
In the following program KEYS3.ASM the sub-routine SCAN, scans the
keypad, waits for a key to be pressed, waits 0.1 seconds for the bouncing
to stop, waits for the key to be released, waits 0.1 seconds for the bouncing
102 Keypad scanning

to stop and then returns with the key number in W which can then be
transferred into a file.
This is then used as a security code to turn on an LED (PORTA,0) when
3 digits (137) have been pressed and turn the LED off again when the same
3 digits are pressed. You can of course use any 3 digits.
;KEYS3.ASM
;EQUATES SECTION
ZEROBIT EQU 2
TMR0 EQU 1
TRISA EQU 85H
TRISB EQU 86H
OPTION_R EQU 81H
NUM1 EQU 0CH
NUM2 EQU 0DH
NUM3 EQU 0EH
;*********************************************************
;*********************************************************
;SUB-ROUTINE SECTION
SCAN NOP
BSF PORTB,1 ;Set B1
BSF PORTB,2 ;Set B2
GOTO CHECK4 ;No
CALL DELAYP1
CHECK1A BTFSS PORTB,3
GOTO CHECK1A
CALL DELAYP1
RETLW .1
GOTO CHECK7 ;No
CALL DELAYP1
Keypad scanning 103

GOTO CHECK4A
CALL DELAYP1
RETLW .4
GOTO CHECK11 ;No
CALL DELAYP1
GOTO CHECK7A
CALL DELAYP1
RETLW .7
GOTO COLUMN2 ;No
CALL DELAYP1
GOTO CHECK11A
CALL DELAYP1
RETLW .11
BSF PORTB,2 ;Set B2
GOTO CHECK5 ;No
CALL DELAYP1
GOTO CHECK2A
CALL DELAYP1
RETLW .2 ;Yes, output 2.
GOTO CHECK8 ;No
CALL DELAYP1
GOTO CHECK5A
CALL DELAYP1
GOTO CHECK0 ;No
CALL DELAYP1
GOTO CHECK8A
CALL DELAYP1
104 Keypad scanning

GOTO COLUMN3 ;No
CALL DELAYP1
GOTO CHECK0A
CALL DELAYP1
RETLW 0 ;Yes, output 10.
BSF PORTB,1 ;Set B1
GOTO CHECK6 ;No
CALL DELAYP1
GOTO CHECK3A
CALL DELAYP1
GOTO CHECK9 ;No
CALL DELAYP1
GOTO CHECK6A
CALL DELAYP1
GOTO CHECK12 ;No
CALL DELAYP1
GOTO CHECK9A
CALL DELAYP1
GOTO COLUMN1 ;No
CALL DELAYP1
GOTO CHECK12A
CALL DELAYP1
;3/32 second delay.
DELAYP1 CLRF TMR0 ;Start TMR0.
LOOPD MOVF TMR0,W ;Read TMR0 into W.
SUBLW .3 ;TIME–3
Keypad scanning 105

GOTO LOOPD ;Time is not ¼ 3.
;**********************************************************
MOVWF TRISA
MOVLW B’11111000’
MOVWF TRISB ;PORTB is mixed I/O.
MOVLW B’00000111’
MOVWF OPTION_R
;*****************************************************
;Enter 3 digit code here
MOVLW 1 ;First digit
MOVWF NUM1
MOVLW 3 ;Second digit
MOVWF NUM2
MOVLW 7 ;Third digit
MOVWF NUM3
BEGIN CALL SCAN ;Get 1st number
SUBWF NUM1,W
BTFSS STATUS,ZEROBIT ;IS NUMBER ¼ 1?
GOTO BEGIN ;No
CALL SCAN ;Get 2nd number
SUBWF NUM2,W
GOTO BEGIN ;No
CALL SCAN ;Get 3rd number.
SUBWF NUM3,W
GOTO BEGIN ;No
BSF PORTA,0 ;Turn on LED, 137 entered
TURN_OFF CALL SCAN ;Get 1st number again
SUBWF NUM1,W
GOTO TURN_OFF ;No
CALL SCAN ;Get 2nd number
106 Keypad scanning

SUBWF NUM2,W
GOTO TURN_OFF ;No
CALL SCAN ;Get 3rd number.
SUBWF NUM3,W
GOTO TURN_OFF ;No
BCF PORTA,0 ;Turn off LED.
GOTO BEGIN
END
The ports are configured as in the previous code KEYPAD.ASM.
The KEYS3.ASM program looks for the first key press and then it
compares the number pressed with the required number stored in a user
file called NUM1. It then looks for the second key to be pressed. But because
the microcontroller is so quick, the first number could be stored and the
program looks for the second number, but our finger is still pressing the
first number.
Anti-bounce routine
Also when a mechanical key is pressed or released it does not make or break
cleanly, it bounces around. If the micro is allowed too, it is fast enough to see
these bounces as key presses so we must slow it down.
We look first of all for the switch to be pressed.
Then wait 0.1 seconds for the switch to stop bouncing.
We then wait for the switch to be released.
We then wait 0.1 seconds for the bouncing to stop before continuing.
The switch has then been pressed and released indicating one action.
The 0.1 second delay is written in the Header as DELAYP1.
Scan routine
The scan routine used in KEYS3.ASM is written into the subroutine.
When called it waits for a key to be pressed and then returns with the number
just pressed in W. It can be copied and used as a subroutine in any program
using a keypad.
The scan routine checks for key presses as in the previous example
KEYPAD.ASM, Column1 checks for the numbers 1, 4, 7 and 11 being
pressed in turn.
Keypad scanning 107

If the 1 is not pressed then the routine goes on to check for a 4.
If the 1 is pressed then the routine waits 0.1 second for the bouncing to stop.
The program then waits for the key to be released.
Waits again 0.1 seconds for the bouncing to stop,
and then returns with a value of 1 in W.
Code for CHECK1:
CHECK1 BTFSC PORTB,3 ;Is B3 Clear? Pressed?
GOTO CHECK4 ;No
CALL DELAYP1 ;Antibounce delay, B3 clear
CHECK1A BTFSS PORTB,3 ;Is B3 Set? Released?
GOTO CHECK1A ;No
CALL DELAYP1 ;Antibounce delay, B3 Set
RETLW .1 ;Return with 1 in W.
If numbers 4, 7 or 11 are pressed the routine will return with the corresponding
value in W.
If no numbers in column1 are pressed then the scan routine continues on to
column2 and column3. If no keys are pressed then the routine loops back to the
start of the scan routine to continue checking.
Storing the code
The code i.e. 137 is stored in the files NUM1, NUM2, NUM3 with the
following code:
MOVLW 1 ;First digit
MOVWF NUM1
MOVLW 3 ;Second digit
MOVWF NUM2
MOVLW 7 ;Third digit
MOVWF NUM3
Checking for the correct code
We first of all CALL SCAN to collect the first digit, which returns with the
number pressed in W.
We then subtract the value of W from the first digit of our code stored in
NUM1 with:
SUBWF NUM1,W.
This means SUBtract W from the File NUM1. The (,W) stores the result of
the subtraction in W. Without (,W) the result would have been stored
in NUM1 and the value changed!
108 Keypad scanning

We then check to see if NUM1 and W are equal, i.e. a correct match. In this
case the zerobit in the status register would be set. Indicating the result
NUM1W ¼ zero. This is done with:
BTFSS STATUS,ZEROBIT
We skip and carry on if it is set, i.e. a match. If it isn’t we return to BEGIN
to scan again.
With a correct first press we then carry on checking for a second and
if correct a third press to match the correct code.
When the correct code is pressed we turn on our LED with:
BSF PORTA,0
We then run through a similar sequence and wait for the code to turn off
the LED.
Notice that if you enter an incorrect digit you return to BEGIN or
TURN_OFF. If you forget what key you have pressed then press an incorrect
one and start again.
You could of course modify this program by adding a fourth digit to the
program then turn on the LED. In which case you use another user file
called NUM4. You could of course use a different code for switching off
the output.
You can also beep a buzzer for half a second to give yourself an audible
feedback that you had pressed a button.
As an extra security measure you could wait for a couple of seconds if an
incorrect key had been pressed, or wait for 2 minutes if three wrong numbers
had been entered.
The keypad routine opens up many different circuit applications.
The SCAN routine can be copied and then pasted into any program using
the keypad. Then when you CALL SCAN the program will return with the
number pressed in W for you to do with it as you wish.
Keypad scanning 109

8
Program examples
INCF
INCFSZ
DECF
ADDWF
Counting events
Counting of course is a useful feature for any control circuit. We may wish to
count the number of times a door has opened or closed, or count a number of
pulses from a rotating disc. If we count cars into a car park we would
increment a file count every time a car entered, using the instruction INCF
COUNT. If we needed to know how many cars were in the car park we would
have course have to reduce the count by one every time a car left. We would do
this by DECF COUNT. To clear the user file COUNT to start we would
CLRF COUNT. In this way the file count would store the number of cars
in the car park. If you prefer COUNT could be called CARS. It is a user file
call it what you like.
Let’s look at an application.
Design a circuit that will count 10 presses of a switch, then turn an LED on and
reset when the next ten presses are started. The hardware is that of Figure 5.1
with A0 as the switch input and B0 as the output to the LED.
There are two ways to count, UP and DOWN. We usually count up and know
automatically when we have reached 10. A computer however knows when it
reaches a count of 10 by subtracting the count from 10. If the answer is zero,
then bingo. A simpler way however is to start at 10 and count down to zero –
after 10 events we will have reached zero without doing a subtraction. Zero
for the microcontroller is a really useful number.

The initial flowchart for this problem is shown in Figure 8.1.
To ensure that the LED is OFF after the switch is pressed for the eleventh time
put in TURN OFF LED after the switch is pressed, as shown in Figure 8.2.
N.B. The switch will bounce and the micro is fast enough to count these
bounces, thinking that the switch has been pressed several times. A 0.1 second
delay is inserted after each switch operation to allow time for the bounces to
stop.
The final flowchart is shown in Figure 8.2.
Set PORTB as Output.
Set COUNT to 10.
Is
Switch
Pressed?
Decrement COUNT
Is
COUNT
= 0?
Turn on LED
Is
Switch
Released?
N
N
Y
Y
Y
N
Figure 8.1 Initial counting flowchart
Program examples 111

The program for the counting circuit
;COUNT84.ASM using the 16F84 with a 32kHz. crystal
;EQUATES SECTION
Set COUNT to 10.
Is
Switch
Pressed?
Decrement COUNT
Is
COUNT
= 0?
Turn on LED
Wait 0.1 seconds
Turn off LED.
Is
Switch
Released?
Wait 0.1 seconds.
Y
Y
Y
N
N
N
Figure 8.2 Final counting flowchart
112 Program examples

TRISA EQU 85H ;TRISA (the PORTA I/O selection) is
;file 85H
TRISB EQU 86H ;TRISB (the PORTB I/O selection) is
;file 86H
COUNT EQU 0CH ;COUNT is file 0C, a register to count
;events.
;*********************************************************
;*********************************************************
;Configuration Bits
;*********************************************************
;3/32 second delay.
SUBLW .3 ;TIME - 3
BTFSS STATUS, ZEROBIT ;Check TIME-W ¼ 0
;*********************************************************
MOVWF TRISA
MOVLW B’00000000’

;*********************************************************
BEGIN MOVLW .10
MOVWF COUNT ;Put 10 into COUNT.
PRESS BTFSC PORTA,0 ;Check switch is pressed
GOTO PRESS
CALL DELAY ;Wait for 3/32 seconds.
BCF PORTB,0 ;TURN OFF LED.
RELEASE BTFSS PORTA,0 ;Check switch is released.
GOTO RELEASE
CALL DELAY ;WAIT for 3/32 seconds.
DECFSZ COUNT ;Dec COUNT skip if 0.
GOTO PRESS ;Wait for another press.
GOTO BEGIN ;Restart
END
How does it work?
The file COUNT is first loaded with the count i.e. 10 with:
MOVLW .10
MOVWF COUNT ;Put 10 into COUNT.
We then wait for the switch to be pressed, by PORTA,0 going low:
PRESS BTFSC PORTA,0 ;Check switch is pressed
GOTO PRESS
Anti-bounce:
Turn off the LED on B0:
BCF PORTB,0
Wait for switch to be released
GOTO RELEASE
Anti-bounce:

Decrement the file COUNT, if zero turn on LED and return to begin.
If not zero continue pressing the switch.
DECFSZ COUNT ;Dec COUNT skip if 0.
GOTO PRESS ;Wait for another press.
GOTO BEGIN ;Restart
This may appear to be a lot of programming to count presses of a switch,
but once saved as a subroutine it can be reused in any other programs.
Look up table
A look up table is used to change data from one form to another i.e.
pounds to kilograms, 8C to 8F, inches to centimeters etc. The explanation of
the operation of a look up table is best understood by way of an example.
7-Segment display
Design a circuit that will count and display on a 7-segment display, the number
of times a button is pressed, up to 10. The circuit diagram for this is shown in
Figure 8.3.
5v
0v
1K
SW1
17
A0
68p
68p
0v
32kHz 16
15
B0
B1
B2
B3
B4
B5
B6
V+
MCLR
0v
14
4
5v
0v
0.1µ
16F84
5
6
7
8
9
10
11
12
7 × 680R
Figure 8.3 Circuit diagram of the 7-segment display driver

The flowchart for the 7-Segment Display Driver is shown in Figure 8.4.
This is a basic solution that has a few omissions:
The switch bounces when pressed.
Clear the count at the start.
The micro counts in binary, we require a 7-segment decimal display.
So we need to convert the binary count to drive the relevant
segments on the display.
When the switch is released it bounces.
The amended flowchart is shown in Figure 8.5.
Is
Switch
Pressed?
Increment Count
Display Count
Is
Switch
Released?
Y
N
Y
N
Figure 8.4 Initial flowchart for the 7-segment driver

Set PORTB as output.
Clear PORTB.
Clear COUNT.
Is
switch
pressed?
Wait 0.1 seconds
Increment count
Convert binary count
to 7 segment format.
Display Count
Is
switch
released?
Wait 0.1 seconds.
N
Y
N
Y
Figure 8.5 Amended flowchart for 7-segment display

The flowchart is missing just one thing! What happens when the count reaches
10? The counter needs resetting (it would count up to 255 before resetting). The
final flowchart is shown in Figure 8.6.
Now about this look up table:
Table 8.1 shows the configuration of PORTB to drive the 7-segment display.
(Refer also to Figure 8.3).
Set PORTB as output.
Clear PORTB.
Clear COUNT.
Is
switch
pressed?
Wait 0.1 seconds
Increment count
Convert binary count
to 7 segment format.
Display Count
Is
switch
released?
Wait 0.1 seconds.
Is
Count
= 10?
Clear Count
N
Y
N
Y
N
Y
Figure 8.6 Final flowchart for 7-segment display

The look up table for this is:
CONVERT ADDWF PC
RETLW B’01110111’ ;0
RETLW B’01000001’ ;1
RETLW B’00111011’ ;2
RETLW B’01101011’ ;3
RETLW B’01001101’ ;4
RETLW B’01101110’ ;5
RETLW B’01111100’ ;6
RETLW B’01000011’ ;7
RETLW B’01111111’ ;8
RETLW B’01001111’ ;9
How does the look up table work?
Suppose we need to display a 0.
We move 0 into W and CALL the look up table, here it is called CONVERT.
The first line says ADD W to the Program Count, since W ¼ 0 then go to the
next line of the program which will return with the 7-segment value 0.
Suppose we need to display a 6.
Move 6 into W and CALL CONVERT. The first line says ADD W to the
Program Count, since W contains 6 then go to the next line of the program and
move down 6 more lines and return with the code for 6, etc.
Just one more thing: To check that a count has reached 10, subtract 10 from
the count if the answer is 0, bingo!
NUMBER PORTB
B7 B6 B5 B4 B3 B2 B1 B0
0 0 1 1 1 0 1 1 1
1 0 1 0 0 0 0 0 1
2 0 0 1 1 1 0 1 1
3 0 1 1 0 1 0 1 1
4 0 1 0 0 1 1 0 1
5 0 1 1 0 1 1 1 0
6 0 1 1 1 1 1 0 0
7 0 1 0 0 0 0 1 1
8 0 1 1 1 1 1 1 1
9 0 1 0 0 1 1 1 1
Table 8.1 Binary code to drive 7-segment display

The program listing for the complete program is:
;DISPLAY.ASM
;EQUATES SECTION
PC EQU 2 ;means PC is file 2.
;********************************************************
;*********************************************************
;Configuration Bits
;*********************************************************
;3/32 second delay.
SUBLW .3 ;TIME - 3
BTFSS STATUS, ZEROBIT ;Check TIME-W ¼ 0
CONVERT ADDWF PC
RETLW B’01110111’ ;0
RETLW B’01000001’ ;1
RETLW B’00111011’ ;2
RETLW B’01101011’ ;3
RETLW B’01001101’ ;4
RETLW B’01101110’ ;5

RETLW B’01111100’ ;6
RETLW B’01000011’ ;7
RETLW B’01111111’ ;8
RETLW B’01001111’ ;9
;*********************************************************
MOVWF TRISA
MOVLW B’00000000’
;*********************************************************
CLRF COUNT ;Set COUNT to 0.
PRESS BTFSC PORTA,0 ;Test for switch press.
GOTO PRESS ;Not pressed.
CALL DELAY ;Antibounce wait 0.1sec.
INCF COUNT ;Add 1 to COUNT.
MOVF COUNT,W ;Move COUNT to W.
SUBLW .10 ;COUNT-10, W is altered.
BTFSC STATUS,ZEROBIT ;Is COUNT - 10 ¼ 0?
CLRF COUNT ;Count ¼ 10 Make Count ¼ 0
MOVF COUNT,W ;Put Count in W again.
CALL CONVERT ;Count is not 10, carry on.
MOVWF PORTB ;Output number to display.
RELEASE BTFSS PORTA,0 ;Is switch released?
GOTO RELEASE ;Not released.
GOTO PRESS ;Look for another press.
END

The file count is cleared (to zero) and we wait for the switch to be pressed.
CLRF COUNT ;Set COUNT to 0.
PRESS BTFSC PORTA,0 ;Test for switch press.
GOTO PRESS ;Not pressed.
Wait for 0.1 seconds, Anti-bounce.
CALL DELAY
Add 1 to COUNT and check to see if it 10:
INCF COUNT ;Add 1 to COUNT.
MOVF COUNT,W ;Move COUNT to W.
SUBLW .10 ;COUNT-10, W is altered.
BTFSC STATUS,ZEROBIT ;Is COUNT - 10 ¼ 0?
If COUNT is 10, Clear it to 0 and output the count as 0. If the COUNT is
not 10 then output the count.
CLRF COUNT ;Count ¼ 10 Make Count ¼ 0
MOVF COUNT,W ;Put Count in W again.
CALL CONVERT ;Count is not 10, carry on.
MOVWF PORTB ;Output number to display.
Wait for the switch to be released and de-bounce. Then return to monitor
the presses.
RELEASE BTFSS PORTA,0 ;Is switch released?
GOTO RELEASE ;Not released.
GOTO PRESS ;Look for another press.
Test your understanding
Modify the program to Count up to 6 and reset.
Modify the program to Count up to F in HEX and reset.
A look up table to change 8C to 8F is shown below, called DEGREE
DEGREE ADDWF PC ;ADD W to Program Count.
RETLW .32 ;08C ¼ 328F
RETLW .34 ;18C ¼ 348F
RETLW .36 ;28C ¼ 368F
RETLW .37 ;38C ¼ 378F

RETLW .39 ;48C ¼ 398F
RETLW .41 ;58C ¼ 418F
RETLW .43 ;68C ¼ 438F
RETLW .45 ;78C ¼ 458F
RETLW .46 ;88C ¼ 468F
RETLW .48 ;98C ¼ 488F
RETLW .50 ;108C ¼ 508F
RETLW .52 ;118C ¼ 528F
RETLW .54 ;128C ¼ 548F
RETLW .55 ;138C ¼ 558F
RETLW .57 ;148C ¼ 578F
RETLW .59 ;158C ¼ 598F
RETLW .61 ;168C ¼ 618F
RETLW .63 ;178C ¼ 638F
RETLW .64 ;188C ¼ 648F
RETLW .66 ;198C ¼ 668F
RETLW .68 ;208C ¼ 688F
RETLW .70 ;218C ¼ 708F
RETLW .72 ;228C ¼ 728F
RETLW .73 ;238C ¼ 738F
RETLW .75 ;248C ¼ 758F
RETLW .77 ;258C ¼ 778F
RETLW .79 ;268C ¼ 798F
RETLW .81 ;278C ¼ 818F
RETLW .82 ;288C ¼ 828F
RETLW .84 ;298C ¼ 848F
RETLW .86 ;308C ¼ 868F
Another application of the use of the look up table is a solution for a previous
example i.e. the ‘‘Control Application – A Hot Air Blower.’’ Introduced in
Chapter 5.
In this example when PORTA was read the data was treated as a binary
number, but we could just as easily treat the data as decimal number.
i.e. A2 A1 A0 ¼ 000 or 0
¼ 001 or 1
¼ 010 or 2
¼ 011 or 3
¼ 100 or 4
¼ 101 or 5
¼ 110 or 6
¼ 111 or 7

The look up table for this would be:
CONVERT ADDWF PC
RETLW B’00000010’ ;0 on PORTA turns on B1
RETLW B’00000011’ ;2 on PORTA turns on B1,B0
RETLW B’00000000’ ;4 on PORTA turns off B1,B0
The complete program listing for the program DISPLAY2 would be:
;DISPLAY2.ASM
;EQUATES SECTION
PC EQU 2 ;Program Counter is file 2.
TRISA EQU 85H ;TRISA (the PORTA I/O selection) is
;file 85H
TRISB EQU 86H ;TRISB (the PORTB I/O selection) is
;file 86H
COUNT EQU 0CH ;COUNT is file 0C, a register to count
;events.
;*********************************************************
;**********************************************************
;Configuration Bits
;*****************************************************

CONVERT ADDWF PC
RETLW B’00000011’ ;2 on PORTA turns on B1,B0
;******************************************************
MOVWF TRISA
MOVLW B’00000000’
;*********************************************************
BEGIN MOVF PORTA,W ;Read PORTA into W
CALL CONVERT ;Obtain O/Ps from I/Ps.
MOVWF PORTB ;switch on O/Ps
GOTO BEGIN ;repeat
END
The program first of all reads the value of PORTA into the working
register, W:
MOVF PORTA,W

The CONVERT routine is called which returns with the correct setting of
the outputs in W. i.e. If the value of PORTA was 3 then the look up table
would return with 00000001 in W to turn on B0 and turn off B1:
CALL CONVERT ;Obtain O/Ps from I/Ps.
MOVWF PORTB ;switch on O/Ps
The program then returns to check the setting of PORTA again.
Numbers larger than 255
The PIC Microcontrollers are 8 bit devices, this means that they can easily
count up to 255 using one memory location. But to count higher then more
than one memory location has to be used for the count.
Consider counting a switch press up to 1000 and then turn on an LED to show
this count has been achieved. The circuit for this is shown in Figure 8.7.
To count up to 1000 in decimal i.e. 03E8 in hex, files COUNTB and COUNTA
will store the count (a count of 65535 is then possible).
COUNTB will count up to 03H then when COUNTA has reached E8H, LED1
will light indicating the count of 1000 has been reached.
The flowchart for this 1000 count is shown in Figure 8.8.
5v
0v
1K
SW1
17
A0
68p
68p
0v
32kHz16
15
0v
470R
LED1
6
B0
V+
MCLR
0v
14
4
5v
0v
0.1µ
16F84
5
Figure 8.7 Circuit for 1000 count

Set Prescaler to / 256.
Clear PORTB.
Clear COUNTA.
Clear COUNTB.
Is
Switch
Pressed?
Is
COUNTA
= 0?
Wait 0.1 seconds
Is
Switch
Released?
INCREMENT COUNTB
Is
COUNTB
= 03H?
Wait 0.1 seconds.
Increment COUNTA
Y
Y
N
Y
N
Y
N
N
Figure 8.8 Count of 1000 flowchart

Flowchart explanation
The program is waiting for SW1 to be pressed. When it is, there is a delay
of 0.1 seconds to allow the switch bounce to stop.
The program then looks for the switch to be released and waits 0.1 seconds
for the bounce to stop.
Is
Switch
Pressed?
Is
COUNTA
= E8?
Wait 0.1seconds
Is
Switch
Released?
TURN onLED1.
Wait 0.1seconds.
Increment COUNTA
Y
Y
N
Y
N
N

1 is then added to COUNTA and a check is made to see if the count
has overflowed i.e. reached 256. (255 is the maximum it will hold, when it
reaches 256 it will reset to zero just like a two digit counter would reset to
zero going from 99 to 100.)
If COUNTA has overflowed then we increment COUNTB.
A check is made to see if COUNTB has reached 03H, if not we return to
keep counting.
If COUNTB has reached 03H then we count presses until COUNTA reaches
E8H. The count in decimal is then 1000 and the LED is lit.
Any count can be attained by altering the values COUNTB and COUNTA
are allowed to count up to i.e. to count up to 5000 in decimal which is 1388H.
Ask if COUNTB ¼ 13H then count until COUNTA has reached 88H.
The program listing
;CNT1000.ASM
;EQUATES SECTION
TRISA EQU 85H ;TRISA (the PORTA
;I/O selection) is file 85H
COUNTA EQU 0CH ;USER RAM LOCATION.
COUNTB EQU 0DH ;USER RAM LOCATION.
;*********************************************************
;*********************************************************
;Configuration Bits
;*****************************************************

;3/32 second delay.
SUBLW .3 ;TIME - 3
;*********************************************************
MOVWF TRISA
MOVLW B’00000000’
;*********************************************************
CLRF COUNTA
CLRF COUNTB
PRESS BTFSC PORTA,0 ;Check switch pressed
GOTO PRESS
GOTO RELEASE

INCFSZ COUNTA ;Inc. COUNT skip if 0.
GOTO PRESS
INCF COUNTB
MOVLW 03H ;Put 03H in W. *
SUBWF COUNTB,W ;COUNTB - W (i.e. 03)
BTFSS STATUS,ZEROBIT ;IS COUNTB ¼ 03H
GOTO PRESS ;No
PRESS1 BTFSC PORTA,0 ;Check switch pressed.
GOTO PRESS1
RELEASE1BTFSS PORTA,0 ;Check switch released.
GOTO RELEASE1
INCF COUNTA
MOVLW 0E8H ;Put E8 in W. *
SUBWF COUNTA ;COUNTA – E8.
BTFSS STATUS,ZEROBIT ;COUNTA ¼ E8?
GOTO PRESS1 ;No.
BSF PORTB,0 ;Yes, turn on LED1.
STOP GOTO STOP ;stop here
END
The two files used for counting are cleared.
CLRF COUNTA
CLRF COUNTB
As we have done previously we wait for the switch to be pressed and released
and to stop bouncing:
PRESS BTFSC PORTA,0 ;Check switch pressed
GOTO PRESS
GOTO RELEASE

We add1 to file COUNTA and check to see if it zero. If it isn’t then continue
monitoring presses. (The file would be zero when we add 1 to the 8 bit
number 1111 1111, it overflows to 0000 0000):
INCFSZ COUNTA ;Inc. COUNT skip if 0.
GOTO PRESS
If the file COUNTA has overflowed then we add 1 to the file COUNTB, just
like you would do with two columns of numbers. We then need to know if
COUNTB has reached 03H. If COUNTB is not 03H then we return to
PRESS and continue monitoring the presses.
INCF COUNTB
MOVLW 03H ;Put 03H in W.
SUBWF COUNTB,W ;COUNTB - W (i.e. 03)
BTFSS STATUS,ZEROBIT ;IS COUNTB ¼ 03H?
GOTO PRESS ;No
Once COUNTB has reached 03H we need only wait until COUNTA reaches
0E8H and we would have counted up to 03E8H i.e. 5000 in decimal. Then
we turn on the LED.
PRESS1 BTFSC PORTA,0 ;Check switch pressed.
GOTO PRESS1
RELEASE1BTFSS PORTA,0 ;Check switch released.
GOTO RELEASE1
INCF COUNTA
MOVLW 0E8H ;Put E8 in W.
SUBWF COUNTA ;COUNTA – E8.
BTFSS STATUS,ZEROBIT ;COUNTA ¼ E8?
GOTO PRESS1 ;No.
BSF PORTB,0 ;Yes, turn on LED1.
STOP GOTO STOP ;stop here
This listing can be used as a subroutine in your program to count up to
any number to 65535 (or more if you use a COUNTC file). Just alter
COUNTB and COUNTA values to whatever values you wish, in the two
places marked * in the program.

Question. How would you count up to 20,000?
Answer. (Have you tried it first!!).
20,000 ¼ 4E20H so COUNTB would count up to 4EH and COUNTA
would then count to 20H.
Question. How would you count to 100,000?
Answer. 100,000 ¼ 0186A0H, you would use a third file COUNTC to count to
01H, COUNTB would count to 86H and COUNTA would count to A0H.
Programming can be made a lot simpler by keeping a library of subroutines.
Here is another . . . .
Long time intervals
Probably the more frequent use of a large count is to count TMR0 pulses to
generate long time intervals. We have previously seen in the section on delay
that we can slow the internal timer clock down to 1/32 seconds. Counting
a maximum of 255 of these gives a time of 255 1/32 ¼ 8 seconds. Suppose
we want to turn on an LED for 5 minutes when a switch is pressed.
5 minutes ¼ 300 seconds ¼ 300 32 (1/32 seconds) i.e. a TMR0 count of 9600.
This is 2580 in hex. The circuit is the same as Figure 8.7 for the 1000-count
circuit, and the flowchart is shown in Figure 8.9.
Explanation of the flowchart
1. Wait until the switch is pressed, the LED is then turned on.
2. TMR0 is cleared to start the timing interval.
3. TMR0 is moved into W (read) to catch the first count.
4. Then wait for TMR0 to return to zero, (the count will be 256) i.e. 100
in hex.
5. COUNTA is then incremented and steps 3 and 4 repeated until COUNTA
reaches 25H.
6. Wait until TMR0 has reached 80H.
7. The count has reached 2580H i.e. 9600 in decimal. 5 minutes has elapsed
and the LED is turned off.

Is
Switch
Pressed?
Move TMR0 into W.
Is
TMR0
=0?
Is
TMR0
=0?
Is
COUNTA
=25H?
Set PORTB as output
Set Prescalerto /256
Clear PORTB
Clear COUNTA
Increment COUNTA
Is
TMR0
=80H?
Turn off LED
Turn onLED
Clear TMR0
Move TMR0 into W.
Move TMR0 into W.
N
Y
Y
N
N
Y
N
Y
Y
N
Figure 8.9 Flowchart for the 5 minute delay

Program listing for 5 minute delay
;LONGDLY.ASM
;EQUATES SECTION
COUNTA EQU 0CH ;COUNT is file 0C, a register to count events.
;*********************************************************
;*********************************************************
;Configuration Bits
;********************************************************
MOVWF TRISA
MOVLW B’00000000’
;*********************************************************
CLRF COUNTA
PRESS BTFSC PORTA,0 ;Check switch pressed.

GOTO PRESS ;No
BSF PORTB,0 ;Yes, turn on LED
CLRF TMR0 ;Start TMR0.
WAIT0 MOVF TMR0,W ;Move TMR0 into W
BTFSC STATUS,ZEROBIT ;Is TMR0 ¼ 0.
GOTO WAIT0 ;Yes
WAIT1 MOVF TMR0,W ;No, move TMR0 into W.
GOTO WAIT1 ;Wait for TMR0 to overflow
INCF COUNTA ;Increment COUNTA
MOVLW 25H
SUBWF COUNTA,W ;COUNTA - 25H
BTFSS STATUS,ZEROBIT ;Is COUNTA ¼ 25H
GOTO WAIT0 ;COUNTA 5 25H
WAIT2 MOVF TMR0,W ;COUNTA ¼ 25H
MOVLW 80H
SUBWF TMR0,W ;TMR0 - 80H
BTFSS STATUS,ZEROBIT ;Is TMR0 ¼ 80H
GOTO WAIT2 ;TMR0 5 80H
BCF PORTB,0 ;TMR0 ¼ 80H, turn off LED
END
The explanation of this program operation is similar to that of the count to
1000, done earlier in this chapter.
This listing can be used as a subroutine and times upto 65535 1/32 seconds
i.e. 34 minutes can be obtained.
Problem: Change the listing to produce a 30 minute delay.
Hint. 1800sec in hex is 0708H.
One hour delay
Another and probably a simpler way of obtaining a delay of say 1 hour, is
write a delay of 5 seconds,
CALL it 6 times, this gives a delay of 30 seconds,
put this in a loop to repeat 120 times, i.e.120 30 seconds ¼ 1 hour.
This code for the 1 hour subroutine will look like:-
ONEHOUR MOVLW .120 ;put 120 in W
MOVWF COUNT ;load COUNT with 120
LOOP CALL DELAY5 ;Wait 5 seconds
CALL DELAY5 ;Wait 5 seconds

DECFSZ COUNT ;Subtract 1 from COUNT
GOTO LOOP ;Count is not zero.
RETLW 0 ;RETURN to program.
The program for the one-hour delay
;ONEHOUR.ASM for 16F84. This sets PORTA as an INPUT (NB 1
; 1hour and 5 second delays are
;*********************************************************
;EQUATES SECTION
;*********************************************************
;**********************************************************
;Configuration Bits
;*****************************************************
;1 hour delay.
ONEHOUR MOVLW .120 ;put 120 in W

MOVWF COUNT ;load COUNT with 120
LOOP CALL DELAY5 ;Wait 5 seconds
DECFSZ COUNT ;Subtract 1 from COUNT
GOTO LOOP ;Count is not zero.
RETLW 0 ;RETURN to program.
;5 second delay.
;*********************************************************
MOVWF TRISA
MOVLW B’00000000’
;*********************************************************
BSF PORTB,0 ;Turn on B0
CALL ONEHOUR ;Wait 1 Hour.
BCF PORTB,0 ;Turn off B0.
STOP GOTO STOP ;STOP!
END

9
The 16C54 microcontroller
The 16C54 is an example of a one time programmable (OTP) device.
The 16C54 device was brought out before the 16F84.
The main difference between them is that the 16C54 is not electrically erasable,
it has to be erased by UV light for about 15 minutes.
The 16C54 JW version is UV erasable.
The 16C54LP is a one time (only) programmable (OTP), 32 kHz version.
You would use a 16C54 JW for development and then program a OTP device
for your final circuit. The OTP device has to be selected for the correct
oscillator i.e. LP for 32kHz crystal, XT for 4MHz, HS for 20MHz and R-C for
an R-C network.
The header for use with the 16C54 is shown below.
Header for the 16C54
; HEADER54.ASM for 16C54. This sets PORTA as an INPUT (NB 1
; 1 second and 0.5 second delays are
;******************************************************************
; EQUATES SECTION
COUNT EQU 7 ;means COUNT is file 7,

;a register to count events
TIME EQU 8 ;file8 where the time is stored.
;******************************************************************
LIST P¼16C54 ; we are using the 16C54.
ORG 01FFH ;the start address in memory is 1FF at the
;end.
ORG 0
;******************************************************************
; 1 second delay.
LOOPA MOVLW .32
MOVWF TIME ;Time ¼ 32/32 secs.
MOVF TMR0,W ;Read TMR0 into W.
SUBWF TIME,W ;TIME - 32, result in W.
; 0.5 second delay.
LOOPB MOVLW .16
MOVWF TIME ;Time ¼ 16/32 secs.
MOVF TMR0,W ;READ TMR0 INTO W.
SUBWF TIME,W ;TIME - 16
;******************************************************************
START MOVLW B’00001111’ ;4 bits of PORTA are I/P
TRIS PORTA
MOVLW B’00000000’
TRIS PORTB ;PORTB is OUTPUT
140 The 16C54 microcontroller

OPTION ;TIMER is 1/32 secs.
;******************************************************************
This header can now be used to write programs for the 16C54 Microcontroller.
There are a number of differences between the 16F84 and the 16C54 that the
header has taken care of, but be aware of the differences when writing your
program.
The 16C54 does not use Banks so there is no need to change from one to the
other.
There are only 7 Registers on the 16C54 (see 16C54 Memory Map
Table 9.1). So the user files start at number 7. i.e. COUNT EQU 7, TIME
EQU 8.
The 16C54 does not have the instruction SUBLW. So in the DELAY
subroutine the delay is moved into a file called TIME. (NB. TIME
EQUATES TO 8) Then the delay in the file is subtracted from W, giving the
same result as for the 16F84.
Why bother using the 16C54? The reprogrammable 16C54 i.e. 16C54JW is
more expensive than the 16F84. But the one time programmable (OTP)
16C54 i.e. 16C54/04P is cheaper. So when your design is final you can blow
the program into the cheaper 16C54/04P. Why bother with the expensive
16C54JW and not the 16F84 for program development? I don’t know! Only
convenience – not having to change the program.
The 16C54JW has to be erased under an ultra violet lamp for about 15
minutes – this is a bind if you are impatient, you may need a couple.
Pin 3 is only a T0CKI pin it does not double as A4 like the 16F84 and must
be pulled high if the T0CKI is not being used.
The 16C54 microcontroller 141

16C54 memory map
FILE ADDRESS FILENAME
00
01 TMR0
02 PC
03
04
05
06
07
08
09
0A
0B
0C
0D
0E
0F
10
11
12
13
14
15
16
17
18
19
1A
1B
1C
1D
1E
1F USER FILE
USER FILE
USER FILE
USER FILE
USER FILE
USER FILE
USER FILE
USER FILE
USER FILE
USER FILE
USER FILE
USER FILE
USER FILE
USER FILE
USER FILE
USER FILE
USER FILE
USER FILE
USER FILE
USER FILE
USER FILE
USER FILE
USER FILE
USER FILE
USER FILE
PORTB
PORTA
FSR
STATUS
INDIRECT ADDRESS
Table 9.1 16C54 memory map
142 The 16C54 microcontroller

10
Alpha numeric displays
Using an Alpha Numeric Display in a project can bring it alive. Instead
of showing a number on a 7 segment display the Alpha Numeric Display could
indicate ‘The Temperature is 278C’. Instructions can also be given on screen.
This section details the use of a 16 character by 2 line display, which incor-
porates an HITACHI HD44780 Liquid Crystal Display Controller Driver
Chip. The HD44780 is an industry standard also used in displays other than
Hitachi (fortunately). The chip is also used as a driver for other display
configurations i.e. 16 1, 20 2, 20 4, 40 2 etc. It has an on board
character generator ROM which can display 240 character patterns.
The circuit diagram connecting the Alpha Numeric Display to the 16F84
is shown in Figure 10.1. This configuration is for the HD44780 driver and
can be used with any of the displays using this chip.
A0
A1
A2
B0
B1
B2
B3
B4
B5
B6
B7
17
18
1
6
7
8
9
10
11
12
13
5
4
6
7
8
9
10
11
12
13
14
R/W
RS
E
D0
D1
D2
D3
D4
D5
D6
D7
DISPLAY
16F84
16
15
32kHz
68p
68p
0v
MCLR
V+
0v
0v
5v
0.1µ
1 3
0v
Vss Vo
5v
2
Figure 10.1 The 16F84 driving the alpha numeric display

Display pin identification
This display configuration shows 11 outputs from the Microcontroller,
3 control lines and 8 data lines connecting to the display.
R/W is the read/write control line, RS is the register select and E is the
chip enable.
The R/W line tells the display to expect data to be written to it or to have
data read from it. The data that is written to it is the address of the character,
the code for the character or the type of command we require it to perform
such as turn the cursor off.
The R/S line selects either a command to perform (R/S ¼ 0) i.e. clear display,
turn cursor on or off, or selects a data transfer (R/S ¼ 1).
The E line enables, (E ¼ 1) and disables, (E ¼ 0) the display.
There is much more to this display than we are able to look at here. If you
wish to know more about them you will need to consult the manufacturers
data book.
If we use 11 lines to drive the display that would only leave 2 lines for the
rest of our control with the 16F84. We could of course use a micro with
22 or 33 I/O. The display can however be driven with 4 data lines instead
of 8, 4 bits of data are then sent twice. This complicates the program a little –
but since I have done that work in the header it requires no more effort
on your part.
Also the R/W line is used to write commands to the micro and read the
busy line which indicates when the relatively slow display has processed the
data. If we allow the micro enough time to complete its task then we do not
have to read the busy line we can just write to the display. The R/W line can
then be connected to 0v in a permanent write mode and we do not require
a read/write line from the micro.
We will therefore only require 4 data lines and 2 control lines to drive the
display leaving 7 lines available for I/O on the 16F84.
This 6 line control for the display is shown in Figure 10.2.
144 Alpha numeric displays

Configuring the display
Before writing to the display you first of all have to configure it. That means
tell it if you are:
(a) using a 4 bit or 8 bit Microcontroller,
(b) using a 1 or 2 line display,
(c) using a character font size of 5 10 or 5 7 dots,
(d) turning the display on or off,
(e) turning the cursor on or off,
(f) incrementing the cursor or not. The cursor position increments after a
character has been written to the display.
In the program shown below the display has been set up in the Configuration
Section with Function Set at 32H to use a 4 bit Microcontroller with a 2 line
display and Font size of 5 7 dots. The Display is turned on and Cursor
turned off with 0CH and the Cursor set to increment with 06H. This
information was obtained from the display data sheet.
A1
A2
B0
B1
B2
B3
18
1
6
7
8
9
4
6
11
12
13
14
RS
E
D4
D5
D6
D7
DISPLAY
16F84
16
15
32kHz
68p
68p
0v
MCLR
V+
0v
0v
5v
0.1µ
1 3
0v
Vss Vo
5v
2
R/W
5
Figure 10.2 Driving the alpha numeric display with 6 control lines
Alpha numeric displays 145

Writing to the display
To write to the display you first of all set the address of the cursor
(where you want the character to appear). The Cursor address locations
are shown in Figure 10.3 Line1 address starts at 80H. Line2 address starts
at C0H.
Then tell the display what the character code is, e.g. A has the code 41H,
B has the code 42H, C is 43H, 0 is 30H, 1 is 31H, 2 is 32H etc.
To print an A on the screen – first enable the display, send 2 to PORTA,
send the code 41H to PORTB and CLOCK this data.
These instructions have been written in the Subroutine Section so all you
have to do is CALL A.
To write HELLO on the display the program would be:
CALL H
CALL E
CALL L
CALL L
CALL O
Program example
The program below is the listing to spell out MICROCONTROLLERS AT
THE MMU.
Then CONTACT DAVE SMITH. Together with the time delays.
;ANHEAD84.ASM Header for the alpha numeric display using 6 I/O
80 81 82 83 84 87 88 89 8A 8B 8C 8D 8E 8F
C0 C1 C2 C3 C4 C5 C6 C7 C8 C9 CA CB CC CD CE CF
86
85
Figure 10.3 Cursor address location

;*********************************************************
;******************************************************************
;*****************************************************
; SUBROUTINE SECTION.
;3 SECOND DELAY
SUBLW .96 ;TIME - W
GOTO LOOPA
RETLW 0 ;return after TMR0 ¼ 96
;P1 SECOND DELAY
LOOPC MOVF TMR0,W ;Read TMR0 into W
SUBLW .3 ;TIME - W
GOTO LOOPC
RETLW 0 ;return after TMR0 ¼ 3
CLOCK BSF PORTA,2
NOP
BCF PORTA,2
NOP
RETLW 0
;*********************************************************
A MOVLW 2 ;enables the display
MOVWF PORTA
MOVLW 4H

MOVWF PORTB
CALL CLOCK
MOVLW 1H ;41 is code for A
MOVWF PORTB
CALL CLOCK ;clock character onto display.
RETLW 0
BB MOVLW 2 ;enables the display
MOVWF PORTA
MOVLW 4H
MOVWF PORTB
CALL CLOCK
MOVLW 2H ;42 is code for B
MOVWF PORTB
RETLW 0
C MOVLW 2 ;enables the display
MOVWF PORTA
MOVLW 4H
MOVWF PORTB
CALL CLOCK
MOVLW 3H
MOVWF PORTB
RETLW 0
D MOVLW 2 ;enables the display
MOVWF PORTA
MOVLW 4H
MOVWF PORTB
CALL CLOCK
MOVLW 4H
MOVWF PORTB
RETLW 0
E MOVLW 2 ;enables the display
MOVWF PORTA
MOVLW 4H
MOVWF PORTB
CALL CLOCK
MOVLW 5H

MOVWF PORTB
RETLW 0
F MOVLW 2 ;enables the display
MOVWF PORTA
MOVLW 4H
MOVWF PORTB
CALL CLOCK
MOVLW 6H
MOVWF PORTB
RETLW 0
G MOVLW 2 ;enables the display
MOVWF PORTA
MOVLW 4H
MOVWF PORTB
CALL CLOCK
MOVLW 7H
MOVWF PORTB
RETLW 0
H MOVLW 2 ;enables the display
MOVWF PORTA
MOVLW 4H
MOVWF PORTB
CALL CLOCK
MOVLW 8H
MOVWF PORTB
RETLW 0
I MOVLW 2 ;enables the display
MOVWF PORTA
MOVLW 4H
MOVWF PORTB
CALL CLOCK
MOVLW 9H
MOVWF PORTB
RETLW 0

J MOVLW 2 ;enables the display
MOVWF PORTA
MOVLW 4H
MOVWF PORTB
CALL CLOCK
MOVLW 0AH
MOVWF PORTB
RETLW 0
K MOVLW 2 ;enables the display
MOVWF PORTA
MOVLW 4H
MOVWF PORTB
CALL CLOCK
MOVLW 0BH
MOVWF PORTB
RETLW 0
L MOVLW 2 ;enables the display
MOVWF PORTA
MOVLW 4H
MOVWF PORTB
CALL CLOCK
MOVLW 0CH
MOVWF PORTB
RETLW 0
M MOVLW 2 ;enables the display
MOVWF PORTA
MOVLW 4H
MOVWF PORTB
CALL CLOCK
MOVLW 0DH
MOVWF PORTB
RETLW 0
N MOVLW 2 ;enables the display
MOVWF PORTA
MOVLW 4H

MOVWF PORTB
MOVLW 0EH
MOVWF PORTB
RETLW 0
O MOVLW 2 ;enables the display
MOVWF PORTA
MOVLW 4H
MOVWF PORTB
CALL CLOCK
MOVLW 0FH
MOVWF PORTB
RETLW 0
P MOVLW 2
MOVWF PORTA
MOVLW 5H
MOVWF PORTB
CALL CLOCK
MOVLW 0H
MOVWF PORTB
RETLW 0
Q MOVLW 2
MOVWF PORTA
MOVLW 5H
MOVWF PORTB
CALL CLOCK
MOVLW 1H
MOVWF PORTB
RETLW 0
R MOVLW 2
MOVWF PORTA
MOVLW 5H
MOVWF PORTB
CALL CLOCK
MOVLW 2H

MOVWF PORTB
RETLW 0
S MOVLW 2
MOVWF PORTA
MOVLW 5H
MOVWF PORTB
CALL CLOCK
MOVLW 3H
MOVWF PORTB
RETLW 0
T MOVLW 2
MOVWF PORTA
MOVLW 5H
MOVWF PORTB
CALL CLOCK
MOVLW 4H
MOVWF PORTB
RETLW 0
U MOVLW 2
MOVWF PORTA
MOVLW 5H
MOVWF PORTB
CALL CLOCK
MOVLW 5H
MOVWF PORTB
RETLW 0
V MOVLW 2
MOVWF PORTA
MOVLW 5H
MOVWF PORTB
CALL CLOCK
MOVLW 6H
MOVWF PORTB
RETLW 0

WW MOVLW 2
MOVWF PORTA
MOVLW 5H
MOVWF PORTB
CALL CLOCK
MOVLW 7H
MOVWF PORTB
RETLW 0
X MOVLW 2
MOVWF PORTA
MOVLW 5H
MOVWF PORTB
CALL CLOCK
MOVLW 8H
MOVWF PORTB
RETLW 0
Y MOVLW 2
MOVWF PORTA
MOVLW 5H
MOVWF PORTB
CALL CLOCK
MOVLW 9H
MOVWF PORTB
RETLW 0
Z MOVLW 2
MOVWF PORTA
MOVLW 5H
MOVWF PORTB
CALL CLOCK
MOVLW 0AH
MOVWF PORTB
RETLW 0
NUM0 MOVLW 2 ;enables the display
MOVWF PORTA
MOVLW 3H
MOVWF PORTB

CALL CLOCK
MOVLW 0H
MOVWF PORTB
RETLW 0
MOVWF PORTA
MOVLW 3H
MOVWF PORTB
CALL CLOCK
MOVLW 1H
MOVWF PORTB
RETLW 0
MOVWF PORTA
MOVLW 3H
MOVWF PORTB
CALL CLOCK
MOVLW 2H
MOVWF PORTB
RETLW 0
MOVWF PORTA
MOVLW 3H
MOVWF PORTB
CALL CLOCK
MOVLW 3H
MOVWF PORTB
RETLW 0
MOVWF PORTA
MOVLW 3H
MOVWF PORTB
MOVLW 4H

MOVWF PORTB
RETLW 0
MOVWF PORTA
MOVLW 3H
MOVWF PORTB
CALL CLOCK
MOVLW 5H
MOVWF PORTB
RETLW 0
MOVWF PORTA
MOVLW 3H
MOVWF PORTB
CALL CLOCK
MOVLW 6H
MOVWF PORTB
RETLW 0
MOVWF PORTA
MOVLW 3H
MOVWF PORTB
CALL CLOCK
MOVLW 7H
MOVWF PORTB
RETLW 0
MOVWF PORTA
MOVLW 3H
MOVWF PORTB
CALL CLOCK
MOVLW 8H
MOVWF PORTB
RETLW 0

MOVWF PORTA
MOVLW 3H
MOVWF PORTB
CALL CLOCK
MOVLW 9H
MOVWF PORTB
RETLW 0
GAP MOVLW 2
MOVWF PORTA
MOVLW 2H
MOVWF PORTB
CALL CLOCK
MOVLW 0H
MOVWF PORTB
RETLW 0
DOT MOVLW 2
MOVWF PORTA
MOVLW 2H
MOVWF PORTB
CALL CLOCK
MOVLW 0EH
MOVWF PORTB
RETLW 0
CLRDISP CLRF PORTA
MOVLW 0H
MOVWF PORTB
MOVLW 1
MOVWF PORTB
CALL CLOCK
CALL DELAYP1
RETLW 0
;*********************************************************
; CONFIGURATION SECTION.
MOVLW B’00000000’ ;PORTA is O/P
MOVWF TRISA

MOVLW B’00000000’
;Display Configuration
MOVLW 03H ;FUNCTION SET
MOVWF PORTB ;8bit data (default)
CALL CLOCK
CALL DELAYP1 ;wait for display
MOVWF PORTB ;change to 4bit
CALL CLOCK ;clock in data
MOVWF PORTB ;must repeat command
MOVLW 08H ;4 bit micro
MOVWF PORTB ;using 2 line display.
CALL DELAYP1
MOVLW 0H ;Display on, cursor off
MOVWF PORTB ;0CH
CALL CLOCK
MOVLW 0CH
MOVWF PORTB
CALL CLOCK
CALL DELAYP1
MOVLW 0H ;Increment cursor, 06H
MOVWF PORTB
CALL CLOCK
MOVLW 6H
MOVWF PORTB
CALL CLOCK

;********************************************************
BEGIN CALL CLRDISP
CLRF PORTA
MOVLW 8H ;Cursor at top left, 80H
MOVWF PORTB
CALL CLOCK
MOVLW 0H
MOVWF PORTB
CALL CLOCK
CALL M ;display M
CALL DELAYP1 ;wait 0.1 seconds
CALL I ;display I
CALL DELAYP1 ;wait 0.1 seconds
CALL C ;Etc.
CALL DELAYP1
CALL R
CALL DELAYP1
CALL O
CALL DELAYP1
CALL C
CALL DELAYP1
CALL O
CALL DELAYP1
CALL N
CALL DELAYP1
CALL T
CALL DELAYP1
CALL R
CALL DELAYP1
CALL O
CALL DELAYP1
CALL L
CALL DELAYP1
CALL L
CALL DELAYP1
CALL E
CALL DELAYP1
CALL R
CALL DELAYP1
CALL S
CALL DELAYP1

CLRF PORTA
MOVLW 0CH ;Cursor on second line, C3
MOVWF PORTB
CALL CLOCK
MOVLW 3H
MOVWF PORTB
CALL CLOCK
CALL A
CALL DELAYP1
CALL T
CALL DELAYP1
CALL GAP
CALL T
CALL DELAYP1
CALL H
CALL DELAYP1
CALL E
CALL DELAYP1
CALL GAP
CALL M
CALL DELAYP1
CALL M
CALL DELAYP1
CALL U
CALL DELAYP1
CALL DELAY3 ;wait 3 seconds
CALL CLRDISP
MOVWF PORTB
CALL CLOCK
MOVLW 0H
MOVWF PORTB
CALL CLOCK
CALL C
CALL DELAYP1
CALL O
CALL DELAYP1
CALL N
CALL DELAYP1
CALL T

CALL DELAYP1
CALL A
CALL DELAYP1
CALL C
CALL DELAYP1
CALL T
CALL DELAYP1
CLRF PORTA
MOVLW 0CH ;Cursor on 2nd line
MOVWF PORTB
CALL CLOCK
MOVLW 3H
MOVWF PORTB
CALL CLOCK
CALL D
CALL DELAYP1
CALL A
CALL DELAYP1
CALL V
CALL DELAYP1
CALL E
CALL DELAYP1
CALL GAP
CALL DELAYP1
CALL S
CALL DELAYP1
CALL M
CALL DELAYP1
CALL I
CALL DELAYP1
CALL T
CALL DELAYP1
CALL H
GOTO BEGIN
END
Program operation
PORTA and PORTB are configured as outputs in the CONFIGURATION
SECTION.

Display configuration
In the Display Configuration Section, the Register Select (R/S) line, A1on
the microcontroller, is set low by CLRF PORTA in the Configuration
Section.
R/S ¼ 0 ensures that the data to the display will change the registers. Later
R/S ¼ 1 writes the characters to the display.
The display is expecting its data to arrive via 8 lines, but to save I/O lines
we will use 4 and write them twice. The code to do this and also tell the
driver chip the display is a two line display is:
MOVWF PORTB ;8bit data (default)
CALL CLOCK
MOVWF PORTB ;change to 4bit
MOVWF PORTB ;must repeat command
MOVLW 08H ;4 bit micro
MOVWF PORTB ;using 2 line display.
The data is set up on PORTB using B0,1,2 and 3. As in
MOVWF PORTB
This data is then clocked into the display by pulsing the Enable line, (E, A2
on the micro) high and then low with:
CLOCK BSF PORTA,2
NOP
BCF PORTA,2
NOP
RETLW 0

CALL DELAYP1 , waits for 0.1 seconds to give the display time to activate
before continuing.
When the display has been configured to: Turn on, switch the cursor off, and
increment the cursor after every character write. We are then ready to write to
the display.
Writing to the display
The display is cleared if required with:
CALL CLRDISP
The address of the character is first written to the display, say, the 80H
position (top left hand corner).
CLRF PORTA
MOVWF PORTB
CALL CLOCK
MOVLW 0H
MOVWF PORTB
CALL CLOCK
Notice the 8 is sent first followed by the 0.
To write to the position mid-way along the top line the address would be
88H. So the 80H in the code above would be replaced by 88H.
In order to write the letter ‘M’ in the display at the position defined.
We CALL M and use the code 4DH, NB. Send the 4 first followed by
the D. The Register Select Line, R/S, A1 on the micro, is set to 1 for the
character write option. The code is:
M MOVLW 2 ;enables the display
MOVWF PORTA ;sets A1¼1
MOVLW 4H ;send data 4
MOVWF PORTB
CALL CLOCK
MOVLW 0DH ;send data D
MOVWF PORTB
CALL CLOCK ;clock character ‘M’ onto display.
RETLW 0

In this way any one of the 240 characters available can be shown on the
display.
The program continues by printing out the rest of the message. A delay of
0.1 seconds is maintained after printing each character to give the effect
of the message being typed out.
All the Capital Letters and numbers 0 to 9 have been included in the header
so you can easily enter your own message.
The complete character set for the display showing all 240 characters is
illustrated in Figure 10.4.
Displaying a number
Suppose we wish to display a number thrown by a dice, for example a 4. We
could use the instruction CALL NUM4, but we would not have known
previously that the number was going to be a 4. The throw of the dice
would be stored in a user file called, say, THROW and THROW would then
have 4 in it.
Now the code for 0 is 30H
The code for 1 is 31H
The code for 2 is 32H
Etc.
If we wanted to display the number 4 the code is:
MOVWF PORTA
MOVLW 3H ;34H is the code for 4
MOVWF PORTB
CALL CLOCK
MOVLW 4H
MOVWF PORTB
RETLW 0
If the 4 is in the file THROW, we can display this with the code:
MOVLW 2 ;enables the display
MOVWF PORTA
MOVLW 3H
MOVWF PORTB

CALL CLOCK
MOVF THROW,W ;number comes from the file
MOVWF PORTB
RETLW 0
Notice how the value of the number now has come from the file.
This code would then display any number in the file THROW.
If you measured a temperature as 278C, you would probably store the 2 in
a file TEMPTENS (tens of degrees) and the 7 in a file TEMPUNIT (units
of degrees).
You can then modify the code above to display:
THE TEMPERATURE
IS 278C.
The ‘I’ would be located at address C5H on the display. The temperature
would then be written at locations C8H and C9H. There would be no need
to rewrite the message just rewrite the temperature as it changed, after first
moving the cursor to address C8H.

Figure 10.4 Alpha numeric display character set

11
Analogue to digital
conversion
Up to now we have considered inputs as being digital in operation i.e. the input
is either a 0 or 1. But suppose we wish to make temperature measurements,
but not just hot or cold (1 or 0). We may for example require to:
(a) Sound a buzzer if the temperature drops below freezing.
(b) Turn a heater on if the temperature is below 188C.
(c) Turn on a fan if the temperature goes above 258C.
(d) Turn on an alarm if the temperature goes above 308C.
We could of course have separate digital inputs, coming from comparator
circuits for each setting. But a better solution is to use 1 input connected to an
analogue to digital converter and measure the temperature with that.
Figure 11.1 shows a basic circuit for measuring temperature. It consists of
a fixed resistor in series with a thermistor (a temperature sensitive resistor).
The resistance of the thermistor changes with temperature causing a change
in the voltage at point X in Figure 11.1.
5v
X
22k
°C
0v
Thermister
Figure 11.1 Temperature measuring circuit

As the temperature rises the voltage at X rises.
As the temperature decreases the voltage at X reduces.
We need to know the relationship between the temperature of the thermistor
and the voltage at X. A simple way of doing this would be to place the
thermistor in a cup of boiling water (1008C) and measure the voltage at X.
As the water cools corresponding readings of temperature and voltage can be
taken. If needed a graph of these temperature and voltage readings could
be plotted.
Making an A/D reading
In the initial example let us suppose:
08C gave a voltage reading of 0.6v
188C gave a reading of 1.4v
The microcontroller would read these voltages and convert them to an 8-bit
number where 0v is 0 and 5v is 255. I.e. a reading of 51 per volt or a resolution
of 1/51v, i.e. 1 bit is 19.6mv.
So 08C ¼ 0.6v ¼ reading of 31 (0.6 51 ¼ 30.6)
188C ¼ 1.4v ¼ 71 (1.4 51 ¼ 71.4)
258C ¼ 2.4v ¼ 122 (2.4 51 ¼ 122.4)
308C ¼ 3.6v ¼ 184 (3.6 51 ¼ 183.6)
If we want to know when the temperature is above 308C the microcon-
troller looks to see if the A/D reading is above 184. If it is, switch on the
alarm, if not keep the alarm off. In a similar way any other temperature
can be investigated – not just the ones listed. With our 8 bits we have
255 different temperatures we can choose from. The PIC 16C773 and PIC
16C774 have 12-bit A/D converters and can have 4096 different temperature
points.
Analogue to Digital conversion was introduced to the PIC Microcontrollers
with the family called 16C7X devices: 16C71, 16C73 and 16C74. Table 11.1
shows some of the specifications of these devices.
Table 11.1 16C7X Device specifications
Device I/O A/D Channels Program
Memory
Data
Memory
Current
Source/Sink
16C71 13 4 1k 36 25mA
16C73 22 5 4k 192 25mA
16C74 33 8 4k 192 25mA
Analogue to digital conversion 167

This family of devices has now been superceded by the 16F87X devices
shown in Table 11.2.
The device I shall consider in this section is the 16F818. The Device Family
Specifications are shown in Table 11.3.
The 16F818 device needs extra registers that the 16F84 does not have,
to handle the A/D processing.
The 16F818 has 5 Analogue Inputs AN0, AN1, AN2, AN3 and AN4.
Configuring the A/D device
In order to make an analogue measurement we have to configure the device.
HEAD818.ASM has to have the CONFIGURATION SECTION changed
to make some of the PORTA inputs Analogue inputs. PORTB has been set
as an output port.
To configure the 16F818 for A–D measurements three registers need to be
set up.
ADCON0
ADCON1
ADRES
Table 11.2 16F87X Devices
Memory
Data
Memory
Current
Source/Sink
16F870 22 5 2k 128 25mA
16F871 33 8 2k 128 25mA
16F872 22 5 2k 128 25mA
16F873 22 5 4k 192 25mA
16F874 33 8 4k 192 25mA
16F876 22 5 8k 368 25mA
16F877 33 5 8k 368 25mA
Table 11.3 16F818/9 Device specifications
Memory
Data
Memory
Current
Source/Sink
16F818 16 5 1k 128 25mA
16F819 16 5 2k 256 25mA
168 Analogue to digital conversion

ADCON0
The first of the A/D registers, ADCON0 is A to D Control Register 0.
ADCON0 is used to:
Switch the A/D converter on with ADON, bit0. This bit turns the A/D
on when set and off when clear. The A/D once it is turned on can be left
on all of the time but it does draw a current of 90mA, compared to the rest
of the microcontroller which draws a current of 15mA.
Instruct the microcontroller to execute a conversion by setting the
GO/DONE bit, bit2. When the GO/DONE bit is set the micro does an
A/D conversion. When the conversion is complete the hardware clears the
GO/DONE bit. This bit can be read to determine when the result is ready.
Set the particular channel (input) to make the measurement from. This is
done with two Channel Select bits, CHS0, CHS1 and CHS2, bits 3, 4 and 5.
The Register ADCON0 is shown in Figure 11.2.
ADCON1
In ADCON1, A to D Conversion Register 1, only bits 0, 1, 2 and 3 are used.
They are the Port Configuration bits, PCFG0, PCFG1, PCFG2, and PCFG3
that determine which of the pins on PORTA will be analogue inputs and
which will be digital.
Bit7 Bit0
- - CHS2 CHS1 CHS0 GO/DONE - ADON
1=A/D on.
0=A/D off.
1=A/D in progress.
0=A/D finished.
Analogue channel
select.
000=channel 0,AN0
001=channel 1,AN1
010=channel 2,AN2
011=channel3, AN3
100=channel 4,AN4
Figure 11.2 ADCON0 Register

The ADCON1 register is illustrated in Figure 11.3 and the corresponding
Analogue and Digital inputs are shown in Table 11.4.
As mentioned previously the microcontroller will convert an analogue
voltage between 0 and 5v to a digital number between 0 and 255. But suppose
our analogue readings of say, temperature, go from 0.6v representing a
temperature of 08C to 3.6v representing a temperature of 308C. It would
make sense to have our analogue range go from 0.6v to 3.6v. We can set
this by using two reference voltages. One at the low setting of 0.6v called
Vref, connected to AN2. The other setting of 3.6v for Vrefþ, connected
to AN3. The two right hand columns in Table 11.4 show that PCFG Set at 1000
will set the A/D configuration using AN3 and AN2 as the reference voltages.
In this book I have not used any reference voltages but have used 5v, Vdd and
0v. Vss as the references.
Bit7 Bit0
- - - - PCFG3 PCFG2 PCFG1 PCFG0
A/D Port
configuration bits.
Figure 11.3 ADCON1 Register
PCFG AN4 AN3 AN2 AN1 AN0 Vref+ Vref−
0000 A A A A A Vdd Vss
0001 A Vref+ A A A AN3 Vss
0100 D A D A A Vdd Vss
0101 D Vref+ D A A AN3 Vss
011X D D D D D Vdd Vss
1000 A Vref+ Vref− A A AN3 AN2
1101 D Vref+ Vref− A A AN3 AN2
1110 D D D D A Vdd Vss
1111 D Vref+ Vref− D A AN3 AN2
Table 11.4 ADCON1 Port configuration

ADRES
The third register is ADRES, the A to D RESult register. This is the file
where the result of the A/D conversion is stored. If several measurements
require storing then the number in ADRES needs to be transferred to a
user file before it is overwritten with the next measurement. The 16F818
micro is a 10 bit A/D. The top 8 bits are stored in ADRESH and
the lower 2 bits in ADRESL. In this book I am only using 8 bits and have
called the file ADRES.
Analogue header for the 16F818
;HEAD818A.ASM for 16F818. This sets PORTA as analogue/digital
; INPUTs.
; PORTB is an OUTPUT.
; Internal oscillator of 31.25kHz chosen
; The OPTION register is set to /256 giving
; timing pulses 32.768ms.
; 1second and 0.5 second delays are
;*********************************************************
; EQUATES SECTION
;*********************************************************

;*********************************************************
;Configuration Bits
;function on B2,
;*****************************************************
SUBLW .3 ;TIME-3
NOP
;0.5 second delay.
DELAYP5 MOVLW .5
MOVWF COUNT
LOOPC CALL DELAYP1
DECFSZ COUNT
GOTO LOOPC
RETLW 0
;1 second delay.
DELAY1 MOVLW .10
MOVWF COUNT
LOOPA CALL DELAYP1
DECFSZ COUNT
GOTO LOOPA
RETLW 0

;*********************************************************
MOVLW B11111111’ ;8 bits of PORTA are I/P
MOVWF TRISA
MOVLW B’00000100’ ;A0,A1 and A3 are analogue.
MOVWF ADCON1
MOVLW B’00000000’
MOVLW B’00000000’
BSF ADCON0,0 ;Turn ON A/D
;*********************************************************
END
Head818A.ASM explained
HEAD818A.ASM is similar in operation to HEAD818.ASM outlined in
Chapter 6, with the following extras:
The Carry Bit in the status register, that indicates if a calculation is þve
or ve, it is bit 0 and has been equated to 0.

In the Configuration Section A0, A1 and A3 are set as Analogue inputs,
A2, A4, A5, A6 and A7 are set up as digital inputs with:
MOVLW B’00000100’
MOVWF ADCON1
The A/D converter is switched on with:
BSF ADCON0,0
A/D Conversion – example, a temperature sensitive
switch
To introduce the working of the A/D converter we will consider a simple
example. i.e. Turn an LED on when the Temperature is above 258C and turn
the LED off when it is below 258C.
The diagram for this Temperature Switch Circuit is shown in Figure 11.4.
Thermister
22k
17
A0
B0
14
5v
0.1µ
16F818
5
6 680R
0v
LED
0v
0v
5v
Figure 11.4 Temperature switch circuit

Taking the A/D reading
The A/D converter has been switched on in the header and it automatically
looks at Channel 0 unless told otherwise. In order to make the measurement
the GO/DONE bit, bit2 is set and we wait until it is cleared with:
BSF ADCON0,2 ;Take measurement, set GO/DONE
WAIT BTFSC ADCON0,2 ;Wait until GO/DONE is clear
GOTO WAIT
The measurement will then be in the A/D Result register, ADRES.
Determining if the temperature is above or below 258C
Suppose the voltage on the analogue input, Channel 0, A0 is 2.4v when the
temperature is 258C. The required A/D reading for 2.4v is 2.4 51 ¼ 122.
We therefore need to know when the A/D reading is above and below 122,
i.e. above and below 258C.
Previously we have seen how to tell if a value is equal to another by subtracting
and looking at the zerobit in the status register (Chapter 5).
There is another bit, bit 0 in the status register called the Carry Bit, which
indicates if the result of a subtraction is þve or ve. If the Carry Bit is set
the result was þve, if the bit is clear the result was ve. So we can tell if the
number is above or below a defined value.
MOVF ADRES,W ;Move Analogue result into W
SUBLW .122 ;Do 122 – ADRES, i.e. 122-W
BTFSC Status,Carry ;Check the carry bit. Clear if ADRES4122 i.e. ve
GOTO TURNOFF ;Routine to turn off LED
GOTO TURNON ;Routine to turn on LED
The analogue measurement is moved from ADRES into W where we can
subtract it from 122. NB. The subtraction always does, Value W.
The carry bit tells us if the A/D result is above or below 122.
N.B. If the result of the subtraction is zero the carry is also 1. It must be 1 or 0.
Being þve or zero does not matter in this example.
We have then found out if the result is equal to or above 122, or if it is less
than 122.

When the measurement is made we then goto one of two subroutines,
TURNON or TURNOFF. These subroutines are not very grand but they
could easily be more complicated, even hundreds of lines long.
Program code
The full code for this Temperature Sensitive Switch Program is shown below
as TEMPSENS.ASM
;TEMPSENS.ASM. This sets PORTA as analogue/digital INPUTs.
; PORTB is an OUTPUT.
; The OPTION register is set to /256 giving timing
pulses 32.768ms.
; 1second and 0.5 second delays are included in the
subroutine section.
;*********************************************************
;EQUATES SECTION
COUNT EQU 20H ;COUNT a register to count events
;*********************************************************

;*********************************************************
;function on B2,
;*****************************************************
TURNON BSF PORTB,0 ;Turn on LED on B0
GOTO BEGIN ;Return to monitor
TURNOFF BCF PORTB,0 ;Turn off LED on B0
GOTO BEGIN ;Return to monitor
;*********************************************************
MOVWF TRISA
MOVLW B’00000100’ ;A0,A1 and A3 are analogue.
MOVWF ADCON1
MOVLW B’00000000’
MOVLW B’00000000’
BSF ADCON0,0 ;Turn ON A/D

;*********************************************************
BEGIN BSF ADCON0,2 ;Take measurement, set GO/DONE
WAIT BTFSC ADCON0,2 ;Wait until GO/DONE is clear
GOTO WAIT
MOVF ADRES,W ;Move Analogue result into W
SUBLW .122 ;Do 122–ADRES, i.e. 122–W
BTFSC STATUS,
CARRY ; Clear if ADRES4122
GOTO TURNOFF ;Routine to turn off LED
GOTO TURNON ;Routine to turn on LED
END
Another example – a voltage indicator
Previously we have looked at a single input level. But with our 8 bit micro we
could look at 255 different input levels.
Suppose we wish to use the LEDs connected to PORTB to indicate the voltage
on the Analogue Input AN0. So that as the voltage increases then the number
of LEDs lit also increases.
In HEAD818.ASM we have configured the micro so that the voltage reference
is Vdd i.e. the 5v supply. This was done with the instructions:
MOVLW B’00000100’
MOVWF ADCON1
This means that 5v will give a digital reading of 255 in our 8 bit register
ADRES. The resolution of this register is 5v/255 ¼ 19.6mV.
Suppose our LED ladder was to increment in 0.5v steps as indicated below:
Vin ¼ 0–0.5v All LEDs off, 0.5v ¼ 0.5/5 255 ¼ 25.5 ¼ 26
Vin ¼ 0.5–1.0v B0 on, 1.0v ¼ 1/5 255 ¼ 51
Vin ¼ 1.0–1.5v B1 on, 1.5v ¼ 1.5/5 255 ¼ 76.5 ¼ 77
Vin ¼ 1.5–2.0v B2 on, 2.0v ¼ 2/5 255 ¼ 102

Vin ¼ 2.0–2.5v B3 on, 2.5v ¼ 2.5/5 255 ¼ 127.5 ¼ 128
Vin ¼ 2.5–3.0v B4 on, 3.0v ¼ 3/5 255 ¼ 153
Vin ¼ 3.0–3.5v B5 on, 3.5v ¼ 3.5/5 255 ¼ 178.5 ¼ 179
Vin ¼ 3.5–4.0v B6 on, 4.0v ¼ 4/5 255 ¼ 204
Vin ¼ 4.0–5.0v B7 on.
The circuit diagram for this voltage indicator is shown in Figure 11.5 and the
Flowchart is shown in Figure 11.6.
5v
0v
1K
17
A0
B7
B6
B5
B4
V+
0v
14
5v
0v
0.1µ
16F818
5
13
12
11
10
8 × 680R
0v
0v
0v
0v
LED7
LED6
LED5
LED4
B3
B2
B1
B0
9
8
7
6
0v
0v
0v
0v
LED3
LED2
LED1
LED0
Figure 11.5 Circuit for the voltage indicator

CLRF PORTB
Is Vin
1.0v
Is Vin
0.5v
Turn on LED0
Is Vin
1.5v
Turn on LED2
Turn on LED3
Measure Vin
Turn on LED1
Is Vin
2.0v
N
Y
N
Y
N
Y
N
Y
Figure 11.6 Flowchart for the voltage indicator

Is Vin
3.0 v?
Is Vin
2.5 v?
Turn on LED4.
Is Vin
3.5 v?
Turn on LED6.
Turn on LED7.
Turn on LED5.
Is Vin
4.0 v?
N
Y
N
Y
N
Y
N
Y

Voltage indicator, program solution
HEAD818A.ASM is altered to produce the program VOLTIND.ASM for the
Voltage Indicator Circuit.
;VOLTIND.ASM. This sets PORTA as analogue/digital
; INPUTs. PORTB is an OUTPUT.
; The OPTION register is set to /256 giving timing
pulses 32.768ms.
; 1second and 0.5 second delays are included in the
subroutine section.
;*********************************************************
; EQUATES SECTION
COUNT EQU 20H ;COUNT a register to count events
;*********************************************************
;*********************************************************
__CONFIG H’3F10’ ;sets INTRC-A6 is port I/O, WDT off, PUT on,
;MCLR tied to VDD A5 is I/O

;function on B2,
;*********************************************************
MOVWF TRISA
MOVLW B’00000010’ ;A0, A1 are analogue
MOVWF ADCON1 ;A2, A3 are digital I/P.
MOVLW B’00000000’
MOVLW B’00000001’ ;Turns on A/D converter,
MOVWF ADCON0 ;and selects channel AN0
;*********************************************************
BEGIN BSF ADCON0,2 ;Take Measurement.
WAIT BTFSC ADCON0,2 ;Wait until reading done.
GOTO WAIT
MOVF ADRES,W ;Move A/D Result into W
CLRF PORTB ;Clear PortB.
SUBLW .26 ;26-,W. W is altered
BTFSC STATUS,CARRY ;Is W4 or 526
GOTO BEGIN ;W is 526 (0.5v)
BSF PORTB,0 ;Turn on B0.

GOTO BEGIN
END
Operation of the voltage indicator program
The code to make the analogue measurement is the same as in the Temperature
Switch Circuit. Once the measurement has been taken the program checks
to see if the digital value of the input is 426 if it is B0 LED is switched on.
The program then checks to see if the measurement is 451, if so then B1 LED
is lit. If the reading is 477 then B2 LED is lit etc. When the value is less
than the one being checked then the program branches back to the beginning,
makes another measurement and the cycle repeats.

NB. After the A/D reading the LEDs are cleared before being turned on,
in case the voltage has dropped.
To check if a reading (or any number) is 4 say 26.
Put the number into W.
Take W from 26 i.e. 26-W by SUBLW .26
If the result is þve, the number is 526 and the carry bit is set in the Status
Register. If the number is 426 the result is ve and the carry bit is clear.
Problem
To check your understanding of the previous section, try this.
Turn a red LED on only when the input voltage is above 3v and turn a
yellow LED on only when the input voltage is below 1v and turn a green LED
on only when the voltage is between 1v and 3v.
Hint
Check for voltage 43v if true GOTO RED
If not check for voltage 51v if true GOTO YELLOW
If false then GOTO GREEN.

12
Radio transmitters and
receivers
Radio circuits used to frighten me but now with the introduction of low cost
modules the radio novice like myself can transmit data easily.
This section details the use of the 418 MHz Radio Transmitter and Receiver
Modules (RT1-418 and RR3-418). They do not need a license to operate
and there are many varieties available. The transmitters only have 3
connections, 2 power supply and one data input, the transmitting aerial
is incorporated on the unit. The receiver has 4 connections, 2 power supply,
1 aerial input and 1 data output. The receiving aerial only needs to be
a piece of wire about 25cm long.
The basic circuit diagram of the radio system is shown in Figure 12.1.
The microcontroller generates the data and then passes the data pulses to
the transmitter. The receiver receives the data pulses and a microcontroller
decodes the information and processes it.
A microcontroller-radio system could measure the temperature outside and
transmit this temperature to be displayed on a unit inside.
5v
0v
0v
10k
16F84 16F84
Tx Rx
A0
A0
B0
B0
470R
Figure 12.1 Radio data transmission system

How does it work?
The transmitter
Data is generated by the microcontroller say by pressing a switch or from
a temperature sensor via the 16F818 doing an A/D conversion. Suppose this
data is 27H, this would then be stored in a user file, called, say, NUMA.
So file NUMA would appear as shown in Figure 12.2.
The data then needs to be passed from the micro to the data input of the
transmitter. The transmitter output will then be turned on and off by the
data pulses. The length of time the transmitter is on will indicate if the data
was a 1, a 0 or the transmission start pulse.
I have decided to use a start bit that is 7.5ms wide, a 5ms pulse to represent
a logic 1 and a 2.5ms pulse to represent a logic 0. All pulses are separated by
a space of 2.5ms. The pulse train for NUMA is then as shown in Figure 12.3.
In order to generate this train the software turns the output on for the 7.5ms
start pulse, off for 2.5ms, on for 5ms for the first 1, off for 2.5ms, on for
5ms for the next logic 1, off for 2.5ms, on for 5ms for the next logic 1, off
for 2.5ms, on for 2.5ms for the logic 0, etc.
To generate the data each bit in the file NUMA is tested in turn. If the bit is
0 then the output is turned on for 2.5ms, if the bit is 1 then the output
is turned on for 5ms. The code for this data would be:
BSF PORTB,0 ;Transmit start pulse
CALL DELAY3 ;7.5ms Start pulse
BCF PORTB,0 ;Transmit space
CALL DELAY1 :Delay 2.5ms
NUMA,7 NUMA,6 NUMA,5 NUMA,4 NUMA,3 NUMA,2 NUMA,1 NUMA,0
0 0 1 0 0 1 1 1
Figure 12.2 File NUMA containing 27H
0 0 1 0 0 1 1 1 Start
Figure 12.3 NUMA pulse train
Radio transmitters and receivers 187

TESTA0 BTFSC NUMA,0 ;Test NUMA,0
GOTO SETA0 ;If NUMA0 ¼ 1
GOTO CLRA0 ;If NUMA0 ¼ 0
SETA0 BSF PORTB,0 ;Transmit 1
CALL DELAY2 ;Delay 5ms
GOTO TESTA1
CLRA0 BSF PORTB,0 ;Transmit 0
CALL DELAY1 ;Delay 2.5ms
GOTO TESTA1
TEASTA1 BCF PORTB,0 ;Transmit space
CALL DELAY1
BTFSC NUMA,1 ;Test NUMA,1
SETA1 BSF PORTB,0
CALL DELAY2
GOTO TESTA2
CLRA1 BSF PORTB,0
CALL DELAY1
GOTO TESTA2

This bit testing is repeated until all 8 bits are transmitted.
The receiver
The receiver works the opposite way round. The data is received and stored
in a file NUMA. Several data bytes could be transmitted depending on
how many switches are used. Or the data may be continually varying from
a temperature sensor. In this example we are only looking for one byte
i.e. the number 27H which was transmitted. The data is passed from the
receiver to the input A0 of the microcontroller.
We wait to receive the 7.5ms start bit. When this is detected we then measure
the next 8 pulses.
If a pulse is 5ms wide then a one has been transmitted and we SET the
relative bit in the file NUMA. If the pulse is only 2.5ms long then we leave
the bit CLEAR.
188 Radio transmitters and receivers

Measuring the received pulse width
Measuring the width of a pulse is a little more difficult than setting a pulse
width. Consider the pulse in Figure 12.4.
The input is continually tested until it goes high and then the timer, TMR0,
is cleared to start timing. The input is continually tested until it goes low and
then the value of TMR0 is read. This is done by:
MOVF TMR0,W which puts the value of TMR0 into W.
We can then check to see if the pulse is 5ms long i.e. a logic 1, if not then
a shorter pulse means a logic 0 was transmitted. If the pulse is greater than
3.5ms then it must be a logic1, at 5ms. If the pulse is less than 3.5ms then
it must be a logic0. TMR0 will hold a value of 3 after a time of 3.5ms, so we
check to see if the width of the pulse is greater or less than 3.
TESTA0H BTFSS PORTA,0 ;wait for Hi transmission
GOTO TESTA0H
CLRF TMR0 ;start timing
TESTA0L BTFSC PORTA,0 ;wait for Lo transmission
GOTO TESTA0L
MOVF TMR0,W ;read value of TMR0
SUBLW .3 ;3-W or 3-TMR0
BTFSC STATUS,
CARRY ;Is TMR0 4 3 i.e. a logic1
BSF NUMA,0 ;Yes.

Start Finish
CLRF
TMR0
READ
TMR0
Figure 12.4 Measuring the width of a pulse

This measuring of the pulse width continues until all 8 pulses are read and
the relevant bits stored in the file NUMA. A TMR0 value 46 indicates the
pulse was a Start pulse.
We then check to see if the number stored in the file NUMA is 27H. This
is done as we have done before by subtracting 27H from it, if the answer is
zero, i.e. 2727 ¼ 0, then the number transmitted was 27H and we turn on the
LED. It seems such a waste to go to all this trouble to turn an LED on. I hope
you can be a little more imaginative this is only an example.
The complete codes for the transmitter and receiver are shown below as
TX.ASM and RX.ASM.
The OPTION register has been set to produce timing pulses of 1ms.
Transmitter program code
TX.ASM
;tx.asm transmits code from a switch.
;is file 85H
;is file 86H
COUNT EQU 0CH ;COUNT is file 0C, a register to
;count events.
NUMA EQU 0DH
;*********************************************************
LIST P ¼ 16F84 ; we are using the 16F84.
;**********************************************************
;Configuration Bits
; Code Protection disabled.

;*********************************************************
;2.5ms SECOND DELAY
SUBLW .1 ;TIME-W
GOTO LOOPA
;5ms SECOND DELAY
SUBLW .3 ;TIME-W
GOTO LOOPB
;7.5ms SECOND DELAY
SUBLW .6 ;TIME-W
GOTO LOOPC
;**********************************************************
MOVWF TRISA
MOVLW B’00000000’
MOVWF OPTION_R ;PRESCALER is /8,1ms

;*********************************************************
BEGIN BTFSC PORTA,0 ;wait for switch press
GOTO BEGIN
MOVLW 27H ;Put 27H into W
MOVWF NUMA ;PUT 27H into NUMA
BCF PORTB,0
CALL DELAY1
BSF PORTB,0 ;Transmit START
CALL DELAY3 ;wait 7.5ms
TESTA0 BCF PORTB,0 ;Transmit space
BTFSC NUMA,0 ;Test NUMA,0
SETA0 BSF PORTB,0 ;Transmit 1
CALL DELAY2 ;wait 5ms
GOTO TESTA1
CLRA0 BSF PORTB,0 ;Transmit 0
TESTA1 BCF PORTB,0
CALL DELAY1
BTFSC NUMA,1
GOTO SETA1
GOTO CLRA1
SETA1 BSF PORTB,0
CALL DELAY2
GOTO TESTA2
CLRA1 BSF PORTB,0
CALL DELAY1
TESTA2 BCF PORTB,0
CALL DELAY1
BTFSC NUMA,2
GOTO SETA2
GOTO CLRA2

SETA2 BSF PORTB,0
CALL DELAY2
GOTO TESTA3
CLRA2 BSF PORTB,0
CALL DELAY1
TESTA3 BCF PORTB,0
CALL DELAY1
BTFSC NUMA,3
GOTO SETA3
GOTO CLRA3
SETA3 BSF PORTB,0
CALL DELAY2
GOTO TESTA4
CLRA3 BSF PORTB,0
CALL DELAY1
TESTA4 BCF PORTB,0
CALL DELAY1
BTFSC NUMA,4
GOTO SETA4
GOTO CLRA4
SETA4 BSF PORTB,0
CALL DELAY2
GOTO TESTA5
CLRA4 BSF PORTB,0
CALL DELAY1
TESTA5 BCF PORTB,0
CALL DELAY1
BTFSC NUMA,5
GOTO SETA5
GOTO CLRA5
SETA5 BSF PORTB,0
CALL DELAY2
GOTO TESTA6

CLRA5 BSF PORTB,0
CALL DELAY1
TESTA6 BCF PORTB,0
CALL DELAY1
BTFSC NUMA,6
GOTO SETA6
GOTO CLRA6
SETA6 BSF PORTB,0
CALL DELAY2
GOTO TESTA7
CLRA6 BSF PORTB,0
CALL DELAY1
TESTA7 BCF PORTB,0
CALL DELAY1
BTFSC NUMA,7
GOTO SETA7
GOTO CLRA7
SETA7 BSF PORTB,0
CALL DELAY2
CLRF PORTB
GOTO BEGIN
CLRA7 BSF PORTB,0
CALL DELAY1
CLRF PORTB
GOTO BEGIN
END
Receiver program code:
;RX.ASM

CARRY EQU 0
NUMA EQU 0DH
;*********************************************************
;**********************************************************
;Configuration Bits
;**********************************************************
MOVWF TRISA
MOVLW B’00000000’
MOVWF OPTION_R ;PRESCALER is /8,1ms
;*********************************************************
BEGIN CLRF NUMA
WAITHI BTFSS PORTA,0 ;Wait for HI Transmission

GOTO WAITHI
CLRF TMR0
TESTST BTFSC PORTA,0 ;Wait for LOW Transmission
GOTO TESTST ;Test for START PULSE
MOVF TMR0,W
BTFSC STATUS,CARRY ;SKIP IF TIME45
GOTO WAITHI ;NOT START BIT
TESTA0H BTFSS PORTA,0 ;wait for Hi transmission
GOTO TESTA0H
CLRF TMR0 ;start timing
TESTA0L BTFSC PORTA,0 ;wait for Lo transmission
GOTO TESTA0L
NOP
MOVF TMR0,W ;read value of TMR0
BTFSS STATUS,CARRY ;Is TMR043 i.e. a logic1
BSF NUMA,0 ;Yes, 1 was transmitted.
TESTA1H BTFSS PORTA,0 ;Wait for pulse
GOTO TESTA1H
CLRF TMR0
TESTA1L BTFSC PORTA,0 ;Wait for LO.
GOTO TESTA1L
NOP
MOVF TMR0,W
SUBLW .3
BTFSS STATUS,CARRY
BSF NUMA,1 ;1 was transmitted
GOTO TESTA2H
CLRF TMR0
TESTA2L BTFSC PORTA,0 ;Wait for Lo.
GOTO TESTA2L
NOP
MOVF TMR0,W
SUBLW .3
BTFSS STATUS,CARRY
GOTO TESTA3H
CLRF TMR0

TESTA3L BTFSC PORTA,0 ;Wait for Lo
GOTO TESTA3L
NOP
MOVF TMR0,W
SUBLW .3
BTFSS STATUS,CARRY
GOTO TESTA4H
CLRF TMR0
GOTO TESTA4L
NOP
MOVF TMR0,W
SUBLW .3
BTFSS STATUS,CARRY
GOTO TESTA5H
CLRF TMR0
GOTO TESTA5L
NOP
MOVF TMR0,W
SUBLW .3
BTFSS STATUS,CARRY
GOTO TESTA6H
CLRF TMR0
GOTO TESTA6L
NOP
MOVF TMR0,W
SUBLW .3
BTFSS STATUS,CARRY
GOTO TESTA7H
CLRF TMR0

GOTO TESTA7L
NOP
MOVF TMR0,W
SUBLW .3
BTFSS STATUS,CARRY
MOVLW 27H
SUBWF NUMA,W ;NUMA-27
GOTO BEGIN ;If NUMA is not 27
GOTO BEGIN
END
Using the transmit and receive subroutines
The transmit and receive subroutines may seem a little complex, but all you
need to do in your code is call them.
To transmit
Put the data you wish to transmit in the file NUMA then CALL
TRANSMIT. The data in the file NUMA is transmitted.
To receive
CALL RECEIVE, the received data will be present in the file NUMA
for you to use.
These programs have illustrated how to switch an LED on (this could be
a remote control for a car burglar alarm). You may of course want to add
more lines of code to be able to turn the LED off. This could be done in
the receiver section by waiting for say 2 seconds and on the next transmission
turn the LED off, providing of course the code was again 27H. Other codes
could of course be added for other switches or keypad buttons, the possibilities
are endless.
The transmitter and receiver micros could be hard wired together first to
test the software without the radio link. The radio transmitter and receiver
can then replace the wire to give a wireless transmission.

13
EEPROM data memory
One of the special features of the 16F84, the 16F818 and some other micros
is the EEPROM Data Memory. This is a section of Memory not in the
usual program memory space. It is a block of data like the user files, but
unlike the user files the data in the EEPROM Data Memory is saved when
the microcontroller is switched off, i.e. it is non-volatile. Suppose we were
counting cars in and out of a car park and we lost the power to our circuit.
If we stored the count in EEPROM then we could load our count file with
this data and continue without loss of data, when the power returns.
To access the data, i.e. read and write to the EEPROM memory loca-
tions, we must of course instruct the microcontroller. There are 64 bytes of
EEPROM memory on the 16F84, 128 on the 16F818 and 256 on the 16F819.
So we must tell the micro which address we require and if we are reading
or writing to it.
When reading we identify the address from 0 to 3Fh (for the 16F84) using the
address register EEADR. The data is then available in register EEDATA.
When writing to the EEPROM data memory we specify the data in the register
EEDATA and the location in the register EEADR.
Two other files are used to enable the process, they are EECON1 and
EECON2, two EEPROM control registers.
Register EECON1 and EECON2 have addresses 8 and 9 respectively in Bank1.
The Register EECON1 is shown below in Figure 13.1.
Bit 0, RD is set to a 1 to perform a read. It is cleared by the micro when
the read is finished.
Bit 1, WR is set to a 1 to perform a write. It is cleared by the micro when
the write is finished.
Bit 2, WREN, WRite ENable a 1 allows the write cycle, a 0 prohibits it.
bit 7 bit 6 bit 5 bit 4 bit 3 bit 2 bit 1 bit0
EEPGD - - EEIF WRERR WREN WR RD
Figure 13.1 The EECON1 register

Bit 3, WRERR reads a 1 if a write is not completed, reads a 0 if the write
is completed successfully.
Bit 4, EEIF interrupt flag for the EEDATA it is a 1 if the write operation
is completed, it reads 0 if it is not completed or not started (for the
16F84). This bit has another purpose for the 16F818. We have not
used this bit in this book.
Bit 7, EEPGD, Program/Data EEPROM Select Bit. (Not used on 16F84.)
This bit allows either the program memory or the data memory to
be selected. 0 selects Data, 1 selects program memory.
Example using the EEPROM
As usual, I think the best way of understanding how this memory works
is to look at a simple example.
Suppose we wish to count events, people going into a building, cars going
into a carpark etc. So if we loose the power to the circuit the data is still
retained. The circuit for this is shown in Figure 13.2.
Switch 1 is used to simulate the counting process and the 8 LEDs on
PORTB display the count in binary. (This is a good chance to practice
counting in binary.) The switch of course needs de-bouncing.
Remember the idea of this circuit, we are counting events and displaying
the count on PORTB. But if we loose power – when the power is re-applied
we want to continue the count as if nothing had happened.
So when we switch on we must move the previous EEPROM Data into the
COUNT file.
The flowchart is shown below in Figure 13.3.
Just a couple of points before we look at the program:
1. It is a good idea to make sure the EEPROM DATA MEMORY is reset
at the very beginning. This can be done by writing 00h to EEPROM
DATA address 00h when we blow the program into the chip – this is done
with the following lines of code.
ORG 2100H
DE 00H
2100H is the address of the first EEPROM data memory file i.e. 00h.
200 EEPROM data memory

DE is Define EEPROM data memory, so we are initializing it with 00h,
and of course 2101H is EEPROM address1 etc.
Data can also be written into the EEPROM using MPLAB, with VIEW,
EEPROM and writing the data in the EEPROM box as shown in
Figure 13.4.
2. Reading and Writing to EEPROM data is not as straightforward as with
user files, you probably suspected that! There is a block of code you need
to use – just add it to your program as required.
When reading EEPROM data at address 0 to the file COUNT then
CALL READ. The subroutine written in the header.
When writing the file COUNT to EEPROM data address 0, CALL
WRITE.
8 × 680R
A0
0v
5v
17
1k
16F84
B5
B4
V+
MCLR
0v
14
4
5v
0.1µ
11
10
9
12
B6
0v
B7
0v
0v
0v
0v
0v
B0
6
0v
0v
7
8
B3
B1
B2
5
15
1
32kHz
0v 68p
68p
13
0v
Figure 13.2 Switch press counting circuit
EEPROM data memory 201

Y
N
Move EEPROM DATA to COUNT
Move Count to EEPROM Data.
Move Count to PORTB
INCF COUNT
Is Sw.
Pressed?
Figure 13.3 The switch press count flowchart
Figure 13.4 Writing EEPROM data

EEPROM program code
The complete program EEDATAWR.ASM is shown below:
;EEDATAWR.ASM This program will count and display switch
; presses.
; The count is saved when the power is removed
; and continues when the
; power is re-applied.
EEADR EQU 9 ;EEPROM address register
EEDATA EQU 8 ;EEPROM data register
EECON1 EQU 8 ;EEPROM control register1
EECON2 EQU 9 ;EEPROM control register2
RD EQU 0 ;read bit in EECON1
WR EQU 1 ;Write bit in EECON1
WREN EQU 2 ;Write enable bit in EECON1
;**********************************************************
LIST P¼16F84 ;We are using the 16F84.
ORG 2100H ;ADDRESS EEADR 0
DE 00H ;put 00H in EEADR 0
;**********************************************************
;Configuration Bits
;*****************************************************

;0.1 SECOND DELAY
SUBLW .3 ;TIME - W
GOTO LOOPA
;Put EEDATA 0 into COUNT
READ MOVLW 0 ;read EEDATA from EEADR
0 into W
MOVWF EEADR
BSF STATUS,5 ;BANK1
BSF EECON1,RD
BCF STATUS,5 ;BANK0
MOVF EEDATA,W
MOVWF COUNT
RETLW 0
;WRITE COUNT INTO EEDATA 0
WRITE BSF STATUS,5 ;BANK1
BSF EECON1,WREN ;set WRITE ENABLE
BCF STATUS,5 ;BANK0
MOVF COUNT,W ;move COUNT to EEDATA
MOVWF EEDATA
MOVLW 0 ;set EEADR 0 to receive
EEDATA
MOVWF EEADR
BSF STATUS,5 ;BANK1
MOVLW 55H ;55 and AA initiates write cycle
MOVWF EECON2
MOVLW 0AAH
MOVWF EECON2
BSF EECON1,WR ;WRITE data to EEADR 0
WRDONE BTFSC EECON1,WR
GOTO WRDONE ;wait for write cycle to complete

BCF EECON1,WREN
BCF STATUS,5 ;BANK0
RETLW 0
;**********************************************************
MOVWF TRISA
MOVLW 0
MOVLW B’00000111’
CLRF COUNT
;**********************************************************
CALL READ ;read EEPROM data into COUNT
MOVF COUNT,W
MOVWF PORTB ;Display previous COUNT (if any)
PRESS BTFSC PORTA,0 ;wait for switch press
GOTO PRESS
CALL DELAYP1 ;antibounce
RELEASE BTFSS PORTA,0 ;wait for switch release
GOTO RELEASE
INCF COUNT ;add 1 to COUNT
MOVF COUNT,W ;put COUNT into W
MOVWF PORTB ;move W (COUNT) to PORTB to
display
CALL WRITE ;write COUNT to EEPROM
address 0
GOTO PRESS ;return and wait for press
END

Microchip are continually expanding their range of microcontrollers and
a new series of flash micros have been introduced, namely the 16F87X series
which include 8k of program memory, 368 bytes of user RAM, 256 bytes of
EEPROM data memory and an 8 channel 10 bit A/D converter. So now
analogue measurements can be stored and saved in EEPROM Data!

14
Interrupts
RETFIE
We all know what interrupts are and we don’t like being interrupted.
We are busy doing something and the phone rings or someone arrives at
the door.
If we are expecting someone, we could look out of the window every now
and again to see if they had arrived or we could carry on with what we are
doing until the doorbell rings. These are two ways of receiving an interrupt.
The first when we keep checking in software terms is called polling, the second
when the bell rings is equivalent to the hardware interrupt.
We have looked at polling when we used the keypad to see if any keys
had been pressed. We will now look at the interrupt generated by the hardware.
Before moving onto an example of an interrupt consider the action of the
door in a washing machine. The washing cycle does not start until the door
is closed, but after that the door does not take any part in the program.
But what if a child opens the door, water could spill out or worse!! We
need to switch off the outputs if the door is opened. To keep looking at
the door at frequent intervals in the program (software polling) would be
very tedious indeed, so we use a hardware interrupt. We carry on with the
program and ignore the door. But if the door is opened the interrupt
switches off the outputs – spin motor etc. If the door had been opened
accidentally then closing the door would return back to the program for
the cycle to continue.
This suggests that when an interrupt occurs we need to remember what the
contents of the files were. i.e. the STATUS register, W register, TMR0 and
PORT settings so that when we return from the interrupt the settings are
restored. If we did not remember the settings, we could not continue where we
left off, because the interrupt switches off all the outputs and the W register
would also be altered, at the very least.

Interrupt sources
The 16F84 has 4 interrupt sources.
Change of rising or falling edge of PORTB,0.
TMR0 overflowing from FFh to 00h.
PORTB bits 4–7 changing.
DATA EEPROM write complete.
The 16F818/9 has 9 interrupt sources, and of course need extra bits in
the interrupt registers to handle them. The additional interrups used in the
16F818/9 are
A/D conversion complete
Synchronous Serial Port Interrupt
TMR1 overflowing
TMR2 overflowing
Capture Compare Pulse Width Modulator Interrupt.
These interrupts can be enabled or disabled as required by their own
interrupt enable/disable bits. These bits can be found in the interrupt control
register INTCON for the 16F84 and also on the Peripheral Interrupt Enable
Register1, PIE1 on the 16F818/9.
In this section we will be looking at the interrupt caused by a rising or falling
edge on PORTB,0.
Interrupt control register
The Interrupt Control Register INTCON, file 0Bh is shown in Figure 14.1.
Bit 6 in this register is designated as the Peripheral Interrupt Enable Bit,
PEIE for the 16F818/9.
Before any of the individual enable bits can be switched ON, the Global
Interrupt Enable (GIE) bit 7 must be set, i.e. a 1 enables all unmasked
interrupts and a 0 disables all interrupts.
Bit 6 EEIE (16F84) is an EEPROM data write complete interrupt enable
bit, a 1 enables this interrupt and a 0 disables it.
Bit 6 PEIE (16F818/9) is the bit that permits enabling of the extra,
peripheral bits.
GIE EEIE T0IE INTE RBIE T0IF INTF RBIF
Figure 14.1 The interrupt control register, INTCON of 16F84
208 Interrupts

Bit 5 T0IE is the TMR0 overflow interrupt enable bit, a 1 enables this
interrupt and a 0 disables it.
Bit 4 INTE is the RB0/INT Interrupt Enable bit, a 1 enables this interrupt
and a 0 disables it.
Bit 3 RBIE is the RB Port change (B4-B7) Interrupt enable bit, a 1 enables
it and a 0 disables it.
Bit 2 T0IF is the flag, which indicates TMR0 has overflowed to generate
the interrupt. 1 indicates TMR0 has overflowed, 0 indicates it hasn’t.
This bit must be cleared in software.
Bit 1 INTF is the RB0/INT Interrupt flag bit which indicates a change on
PORTB,0. A 1indicates a change has occurred, a 0 indicates it hasn’t.
Bit 0 RBIF is the RB PORT Change Interrupt flag bit. A 1 indicates
that one of the inputs PORTB,4–7 has changed state. This bit must be
cleared in software. A 0 indicates that none of the PORTB,4-7 bits
have changed.
Program using an interrupt
As an example of how an interrupt works consider the following example:
Suppose we have 4 lights flashing consecutively for 5 seconds each. A switch
connected to B0 acts as an interrupt so that when B0 is at a logic 0 an interrupt
routine is called. This interrupt routine flashes all 4 lights ON and OFF
twice at 1 second intervals and then returns back to the program providing
the switch on B0 is at a logic1.
I have used the 16F818 for this application.
The circuit diagram for this application is shown in Figure 14.2.
One thing to note from the circuit the 16F818 chip has internal pull-up
resistors on PORTB so B0 does not need a pull up resistor on the switch.
The interrupt we are using is a change on B0, we are therefore concerned
with the following bits in the INTCON register, i.e. INTE bit4 the enable
bit and INTF bit1 the flag showing B0 has changed, and of course GIE
bit7 the Global Interrupt Enable Bit.
Program operation
When B0 generates an interrupt the program branches to the interrupt service
routine. Where? Program memory location 4 tells the Microcontroller where
to go to find the interrupt service routine.
Interrupts 209

Program memory location 4 is then programmed using the org statement as:
ORG 4 ;write next instruction in program memory location 4
GOTO ISR ;jump to the Interrupt Service Routine.
The interrupt service routine
The Interrupt Service Routine, ISR, is written like a subroutine and is
shown below:
;Interrupt Service Routine
MOVWF W_TEMP ;Save W
SWAPF STATUS,W
MOVWF STATUS_T ;Save STATUS
MOVF TMR0,W
MOVWF TMR0_T ;Save TMR0
MOVF PORTB,W
MOVWF PORTB_T ;Save PORTB
MOVLW 0FFH
MOVWF PORTB ;turn on all outputs.
CALL DELAPY1 ;1 second delay
0v
SW
6
B0
B1
B2
B3
B4
0v
14
5v
0v
0.1µ
5
7
8
9
10
4 × 680R
0v
0v
0v
0v
LED0
LED1
LED2
LED3
16F818
Figure 14.2 Interrupt demonstration circuit
210 Interrupts

MOVLW 0
MOVWF PORTB ;turn off all outputs
MOVLW 0FFH
MOVLW 0
SW_HI BTFSS PORTB,0
GOTO SW_HI ;wait for switch to be HI.
SWAPF STATUS_T,W
MOVWF STATUS ;Restore STATUS
MOVF TMR0_T,W
MOVWF TMR0 ;Restore TMR0
MOVF PORTB_T,W
MOVWF PORTB ;Restore PORTB
MOVF W_TEMP,W ;Restore W
BCF INTCON, INTF ;Reset Interrupt Flag
RETFIE ;Return from the interrupt
Operation of the interrupt service routine
The interrupt service routine operates in the following way.
When an interrupt is made the Global Interrupt Enable is cleared
automatically (disabled) to switch off all further interrupts. We would
not wish to be interrupted while we are being interrupted.
The registers W, STATUS, TMR0 and PORTB are saved in temporary
locations W_TEMP, STATUS_T, TMR0_T and PORTB_T.
The interrupt routine is executed, the lights flash on and off twice. This
is a separate sequence than before to show the interrupt has interrupted
the normal flow of the program. NB. The program has not been looking
at the switch that generated the interrupt.
We then wait until the switch returns HI.
The temporary files W_TEMP, STATUS_T, TMR0_T and PORTB_T are
restored back into W, STATUS, TMR0 and PORTB.
The PORTB,0 interrupt flag INTCON,INTF is cleared ready to indicate
further interrupts.
We return from the interrupt, and the Global Interrupt Enable bit is
automatically set to enable further interrupts.
Interrupts 211

Program of the interrupt demonstration
The complete code for this program is shown below as INTFLASH.ASM.
;INTFLASH.ASM Flashing lights being interrupted by a switch on B0.
;Using 16F818
;EQUATES SECTION
INTCON EQU 0BH ;Interrupt Control Register
GIE EQU 7 ;Global Interrupt bit
INTE EQU 4 ;B0 interrupt enable bit.
INTF EQU 1 ;B0 interrupt flag
OPTION_R EQU 81H
;a register to count events
TMR0_T EQU 21H ;TMR0 temporary file
W_TEMP EQU 22H ;W temporary file
STATUS_T EQU 23H ;STATUS temporary file
PORTB_T EQU 24H ;PORTB temporary file
COUNTA EQU 25H
;*********************************************************
ORG 4 ;write to memory location 4
GOTO ISR ;location4 jumps to ISR
;*********************************************************
;Configuration Bits
212 Interrupts

;function on B2,
;*********************************************************
;SUBROUTINE SECTION
SUBLW .3 ;TIME-3
NOP
;5 second delay.
DELAY5 MOVLW .50
MOVWF COUNTA
LOOPC CALL DELAYP1
DECFSZ COUNTA
GOTO LOOPC
RETLW 0
;1 second delay.
DELAY1 MOVLW .10
MOVWF COUNT
LOOPA CALL DELAYP1
DECFSZ COUNT
GOTO LOOPA
RETLW 0
;Interrupt Service Routine.
ISR MOVWF W_TEMP ;Save W
SWAPF STATUS,W
MOVWF STATUS_T ;Save STATUS
MOVF TMR0,W
MOVWF TMR0_T ;Save TMR0
MOVF PORTB,W
MOVWF PORTB_T ;Save PORTB
Interrupts 213

MOVLW 0FFH
CALL DELAY1 ;1 second delay
MOVLW 0
MOVLW 0FFH
MOVLW 0
SW_HI BTFSS PORTB,0
GOTO SW_HI ;wait for switch to be HI.
SWAPF STATUS_T,W
MOVWF STATUS ;Restore STATUS
MOVF TMR0_T,W
MOVWF TMR0 ;Restore TMR0
MOVF PORTB_T,W
MOVWF PORTB ;Restore PORTB
MOVF W_TEMP,W ;Restore W
BCF INTCON,INTF ;Reset Interrupt Flag
RETFIE ;Return from the interrupt
;*********************************************************
MOVWF TRISA
MOVWF ADCON1
MOVLW B’00000001’
MOVWF TRISB ;PORTB,0 is I/P
MOVLW B’00000000’
214 Interrupts

BSF INTCON,GIE ;Enable Global Interrupt
BSF INTCON,INTE ;Enable B0 interrupt
;*********************************************************
BEGIN MOVLW B’00000010’ ;Turn on B1
MOVWF PORTB
MOVLW B’00000100’ ;Turn on B2
MOVWF PORTB
MOVWF PORTB
MOVWF PORTB
GOTO BEGIN
END
The 4 lights are flashing on and off slowly enough (5 second intervals)
so that you can interrupt part way through taking B0 low via the switch,
(make sure B0 is hi when starting). The interrupt service routine then flashes
all the lights on and off twice at 1 second intervals.
When returning from the interrupt with B0 hi again, the program resumes
from where it left off, i.e. if the 2nd LED had been on for 3 seconds it
would come back on for the remaining 2 seconds and the sequence would
continue.
Interrupts 215

15
The 12 series 8 pin
microcontroller
Arizona Microchip have a range of microcontrollers with 8 pins. They include
types with Data EEPROM and A/D converters. In this section we will cover
the 12C508 and 12C509, which are one time programmable devices and the
flash 12F629 and 12F675 (electronically) reprogrammable devices.
The device memory specifications are shown in Table 15.1.
Pin diagram of the 12C508/509
Pin diagram of the 12F629 and 12F675
Table 15.1 12C508/509, 12F629 and 12F675 memory specifications
Device EEPROM User Files Registers
12C508 512 12 25 7
12C509 1024 12 41 7
12F629 1024 14 64 29
12F675 1024 14 64 33
VDD VSS
GP0
GP1
GP2/T0CKI
GP5/OSC1/CLKIN
GP4/OSC2
GP4/MCLR/VPP
1
2
3
4
8
7
6
5
PIC12C509
PIC12C508
Figure 15.1 Pin diagram of the 12C508/9
VDD
GP5/T1CkI/OSC1/CLKIN
GP4/AN3/T1G/OSC2/CLOUT
GP3/MCLR/VPP
VSS
GP0/AN0/CIN+/ICSPDAT
GP1/AN1/CIN−/VREF/ICSPCLK
GP2/AN2/T0CKI/INT/COUT
1
2
3
4
8
7
6
5
PIC12F675
Figure 15.2 Pin diagram of the 12F629 and 12F675

Features of these 12 series
One of the special features of this Micro is that it has 8 pins, but 6 of them can
be used as I/O pins, the remaining 2 pins being used for the power supply.
There is no need to add a crystal and capacitors, because a 4MHz oscillator
is built on board! If you wish to use a clock other than the 4MHz provided,
then you can connect an oscillator circuit to pins 2 and 3 (as in the 16F84).
That leaves you with of course only 4 I/O.
Being an 8 pin device means of course it is smaller than an 18 pin device
and cheaper. The on board oscillator means that the crystal and timing
capacitors are not required, reducing the component count, size and cost even
further. So if your application requires no more than 6 I/O these are devices
to use. They have useful applications in burglar alarm circuits and the radio
transmitter circuits we have looked at previously.
The memory maps of the 12C508 and 12F629/675
The memory map of the 12C508 is shown in Figure 15.3, showing the 7 registers
and 25 user files. Figure 15.4 shows the 12F629/675 map.
The 12C509 has 16 extra user files mapped in Bank1.
There is no longer a PORTA or PORTB because we only have 6 I/O, they
are in a port called GPIO (General Purpose Input Output), File 6.
Address File
01h TMR0
02h PCL
03h STATUS
04h FSR
05h OSCCAL
06h GPIO
07h
General
Purpose
Registers
(User files)
1Fh
Figure 15.3 12C508 Memory map
The 12 series 8 pin microcontroller 217

Oscillator calibration
Apart from the small size of this device an appealing feature is that the
oscillator is on board. The file OSCCAL is an oscillator calibration file used
to trim the 4MHz oscillator.
The 4MHz oscillator takes its timing from an on board R-C network, which is
not very precise. So these chips have a value that can be put into OSCCAL
Address Register
00H INDADRESS
01H TMR0
02H PCL
03H STATUS
04H FSR
05H GPIO
06H
07H
08H
09H
0AH PCLATH
0BH INTCON
0CH PIR1
0DH
0EH TMR1L
0FH TMR1H
10H T1CON
11H
12H
13H
14H
15H
16H
17H
18H
19H CMCON
1AH
1BH
1CH
1DH
1EH ADRESH
1FH ADRESL
20H
5FH
General
Purpose
Register
64 bytes
BANK 0
Address Register
80H INDADRR
81H OPTION REG
82H PCL
83H STATUS
84H FSR
85H TRISIO
86H
87H
88H
89H
8AH PCLATH
8BH INTCON
8CH PIE1
8DH
8EH PCON
8FH
90H OSCCAL
91H
92H
93H
94H
95H WPU
96H IOCB
97H
98H
99H VRCON
9AH EEDATA
9BH EEADR
9CH EECON1
9DH EECON2
9EH ADRESL
9FH ANSEL
BANK 1
Figure 15.4 12F629/675 Memory map
218 The 12 series 8 pin microcontroller

to trim it. This value is stored in the last memory address i.e. 01FFh for the
12C508 and 03FFh for the 12C509 and 12F629/675.
Trimming the 12C508/9
The code, which is loaded by the manufacturer in the last memory location
for the 12C508/9, is MOVLW XX where XX is the trimming value. The last
memory location is the reset vector i.e. when switched on the micro goes to this
location first, it loads the calibration value into W and the program counter
overflows to 000h and continues executing the code. To use the calibration
value, in the Configuration Section write the instruction MOVWF OSCCAL,
which then moves the manufacturers calibration value into the timing circuit.
There is one point to remember – if you are using a windowed device then the
calibration value will be erased when the memory is erased. So make a note
of the MOVLW XX code by looking in MPLAB with: VIEW-PROGRAM
MEMORY and program it back in by ORG 01FFH MOVLW XX.
Trimming the 12F629/675
A calibration instruction is programmed into the last location of program
memory, i.e. 3FFH. The instruction is RETLW XX, where XX is the calibra-
tion value. This value is placed in the OSCCAL register to set the calibration
value of the internal oscillator. This is done in the 12F629 header as
CALL 3FFH ;call instruction at location 3FFH
MOVWF OSCCAL ;move calibration value to OSCCAL
The trimming can be ignored if required – but it only requires 1 or 2 lines
of code, so why not use it.
I/O PORT, GPIO
The GPIO, General Purpose Input/Output, is an 8 bit I/O register, it has 6 I/O
lines available so bits GPIO 0 to 5 are used, bits 6 and 7 are not.
N.B. GPIO bit3 is an input only pin so there is a maximum of 5 outputs.
For the 12C508 GPIO pins 0,1 and 3 can be configured with weak pull ups
by writing 0 to OPTION,6 (bit 6 in the OPTION register).
For the 12F629/675 all GPIO pins except GPIO3 can be configured with
weak pull ups. This is done by setting the relevant bits in the Weak Pull Up
Register, WPU. When in
Bank1 MOVLW B’00110111’
MOVWF WPU
Will turn on all the weak pull ups.

Delays with the 12 series
We have previously used a 32kHz. Crystal with the 16F84 device, but now
we are going to use the internal 4MHz clock.
A 4MHz clock means that the basic timing is ¼ of this i.e. 1MHz. If we
set the OPTION register to divide by 256 this gives a timing frequency of
3906Hz. In the headers for the 12C508/9, 12F629 and 12F675 I have (as with
the 16F84) included a one second and a 0.5 second delay. In order to achieve
a one second delay from a frequency of 3906Hz I first of all produced a delay of
1/100 second by counting 39 timing pulses i.e. 3906Hz/39 ¼ 100.15 ¼ 100Hz
approx., called DELAY. A one second delay, subroutine DELAY1 then counts
100 of these DELAY times (i.e. 100 1/100 second), and of course a delay of
0.5 seconds would count 50.
Just before we look at the headers – we do not have an instruction SUBLW
on the 12C508. I have therefore set up a file called TIME that I have written
39 into. I then move TMR0 into W and subtract the file TIME (39d) from
it to see if TMR0 ¼ 39 i.e. 1/100 of a second has elapsed.
WARNING: The 12C508 and 509 micros only have a two level deep stack.
Which means when you do e.g. a one second delay, CALL DELAY1 this
then calls another subroutine, i.e. CALL DELAY. You have used your two
levels and cannot do any further calls without returning from one at least
one of those subroutines. If you did make a third CALL the program would
not be able to find its way back!
Header for 12C508/9
;HEAD12C508.ASM FOR 12C508/9.
OSCCAL EQU 5 ;Oscillator calibration
;**********************************************************
bit 7 bit 0
WPU0
WPU1
WPU2
WPU4
WPU5 ––
––
––
Figure 15.5 Weak pull up register

LIST P¼12C508 ;We are using the 12C508.
;**********************************************************
Configuration Bits
__CONFIG H’0FEA’ ;selects internal RC oscillator, WDT off,
;code protection disabled
;************************************************************
;1/100 SECOND DELAY
DELAY CLRF TMR0 ;Start TMR0
SUBWF TIME,W ;TIME-W
BTFSS STATUS,ZEROBIT ;Check TIME-W¼0
GOTO LOOPA
;1 SECOND DELAY
DELAY1 MOVLW .100
MOVWF COUNT
TIMEA CALL DELAY
DECFSZ COUNT
GOTO TIMEA
RETLW 0
;1/2 SECOND DELAY
DELAYP5 MOVLW .50
MOVWF COUNT
TIMEB CALL DELAY
DECFSZ COUNT
GOTO TIMEB
RETLW 0
;**********************************************************
START MOVWF OSCCAL ;Calibrate oscillator.
TRIS GPIO ;Bit3 is Input
MOVLW B’00000111’

CLRF GPIO ;Clear GPIO
MOVLW .39
MOVWF TIME ;TIME ¼ 39
;**********************************************************
Program application for 12C508
There are 5 I/O on the 12C508 i.e. GPIO bits 0,1,2,4 and 5. Bit3 is an input
only. For our application we will chase 5 LEDs on our outputs backwards and
forwards at 0.5 second intervals.
The Circuit diagram is shown in Figure 15.6.
Vdd
0v
1
5v
0v
0.1µ
8
5
3
9
5 x 680R
0v
0v
0v
0v
GPIO0
0v
6
7
GPIO1
GPIO2
GPIO4
GPIO5
Figure 15.6 LED chasing circuit for the 12C508

Notice that the only other component required is the power supply decoupling
capacitor, 0.1mF, no oscillator circuit is required.
The program for the LED Chasing Project, LED_CH12.ASM is shown below.
;LED_CH12.ASM Program to chase 5 LEDs with the 12C508
OSCCAL EQU 5
;**********************************************************
;**********************************************************
;Configuration Bits
_CONFIG H’0FEA’ ;selects Internal RC oscillator, WDT off,
;**********************************************************
DELAY CLRF TMR0 ;Start TMR0
SUBWF TIME,W ;TIME - W
GOTO LOOPA
;1 SECOND DELAY
DELAY1 MOVLW .100
MOVWF COUNT
TIMEA CALL DELAY
DECFSZ COUNT
GOTO TIMEA
RETLW 0
;1/2 SECOND DELAY
DELAYP5 MOVLW .50
MOVWF COUNT

TIMEB CALL DELAY
DECFSZ COUNT
GOTO TIMEB
RETLW 0
;**********************************************************
START MOVWF OSCCAL ;Calibrate oscillator.
TRIS GPIO ;Bit3 is Input
MOVLW B’00000111’
CLRF GPIO ;Clear GPIO
MOVLW .39
MOVWF TIME ;TIME ¼ 39
;**********************************************************
BEGIN MOVLW B’00000001’ ;turn on LED0
MOVWF GPIO
CALL DELAYP5
MOVLW B’00000010’ ;turn on LED1
MOVWF GPIO
CALL DELAYP5
MOVWF GPIO
CALL DELAYP5
MOVWF GPIO
CALL DELAYP5
MOVWF GPIO
CALL DELAYP5
MOVWF GPIO
CALL DELAYP5
MOVWF GPIO
CALL DELAYP5

MOVWF GPIO
CALL DELAYP5
GOTO BEGIN
END
The program is similar in content to the 16F84 programs used previously, but
with the following exceptions:
A file TIME, file 8, has been set up which has had 39 loaded into it, in the
Configuration Section. This is used to determine when TMR0 has reached
a count of 39, time of 0.01 seconds, which is then used in the timing
subroutines.
In the Configuration Section the first instruction the program encounters
is MOVWF OSCCAL. This moves the calibration value which has just
been read by MOVLW XX, from location 1FFH, the first instruction, into
the calibration file OSCCAL.
GPIO is used in the program instead of the usual PORTA and PORTB.
Program application using the 12F629/675
To perform the LED chasing action of the previous example in Figure 15.6
using the 12F675 the following code would be required.
;LED_CH675.ASM FOR 12F675 using 4MHz internal RC.
TRISIO EQU 85H
GO EQU 1
ADSEL EQU 9EH
ADCON0 EQU 1FH
ADRESH EQU 1EH
OPTION_R EQU 81H
CMCON EQU 19H
OSCCAL EQU 90H
;**********************************************************
LIST P¼12F675 ;We are using the 12F675.

;***************************************************
;Configuration Bits
;**********************************************************
;1/100 SECOND DELAY
SUBLW .39 ;TIME-W
BTFSS STATUS,ZEROBIT ;CHECK TIME-W¼0
GOTO LOOPA
;P1 SECOND DELAY
DELAYP1 MOVLW .10
MOVWF COUNT
TIMEC CALL DELAY
DECFSZ COUNT
GOTO TIMEC
RETLW 0
;P5 SECOND DELAY
DELAYP5 MOVLW .50
MOVWF COUNT
TIMED CALL DELAY
DECFSZ COUNT
GOTO TIMED
RETLW 0
;*******************************************************
MOVLW B’00010000’ ;All I/O are digital (12F675 only)
MOVWF ADSEL
MOVLW B’00001000’ ;Bit3 is IP
MOVWF TRISIO
MOVLW B’00000111’

CALL 3FFH
BCF STATUS,5 ;BANK0
MOVLW 7H
BSF ADCON0,0 ;Turns on A/D converter.
;**********************************************************
BEGIN MOVLW B’00000001’ ;turn on LED0
MOVWF GPIO
CALL DELAYP5
MOVWF GPIO
CALL DELAYP5
MOVWF GPIO
CALL DELAYP5
MOVWF GPIO
CALL DELAYP5
MOVWF GPIO
CALL DELAYP5
MOVWF GPIO
CALL DELAYP5
MOVWF GPIO
CALL DELAYP5
MOVWF GPIO
CALL DELAYP5
GOTO BEGIN
END
The differences in the code between the 12C508 and 12F675 are:
MOVLW B’00010000’ ;All I/O are digital (12F675 only)
MOVWF ADSEL

These two lines are used to inform the 12F675 that the inputs are all
digital. Change the data to make the inputs analogue – refer to manufacturers
data. These two lines are not required for the 12F629 which does not have
any A/D.
CALL 3FFH
These lines are used to calibrate the internal 4MHz oscillator.
MOVLW 7H
The 12F629/675 have analogue comparators, which we have not looked at.
They need to be turned off to use the I/O pins. The default is that the
comparators are on!
There are numerous other 12 series microcontrollers but once you have
understood how to move from the 12C508/9 to the 12F629/675 you will be
able to migrate to the rest.

16
The 16F87X microcontroller
The 16F87X range includes the devices, 16F870, 16F871, 16F872, 16F873,
16F874, 16F876 and 16F877. They are basically the same device but differ in
the amounts of I/O, analogue inputs, program memory, data memory (RAM)
and EEPROM data memory that they have.
The 16F87X have more I/O, program memory, data memory, EEPROM data
memory and analogue inputs than the 16F818.
16F87X family specification
Device
Program
Memory
EEPROM
Data Memory
(bytes)
RAMBytes Pins I/O
10 bit A/D
Channels
16F870 2k 64 128 28 22 5
16F871 2k 64 128 40 33 8
16F872 2k 64 128 28 22 5
16F873 4k 128 192 28 22 5
16F874 4k 128 192 40 33 8
16F876 8k 256 368 28 22 5
16F877 8k 256 368 40 33 8
16F87X memory map
The 16F87X devices have more functions than we have seen previously.
These functions of course need registers in order to make the various selections.
The memory map of the 16F87X showing these registers is shown in
Figure 16.3.
The 16F87X devices have a number of extra registers that are not required
in the applications we have looked at. For an explanation of these registers
please see Microchip’s website @ www.microchip.com, where you can
download the data sheet as a pdf (portable document file), which can be read
using Adobe Acrobat Reader.

The 16F872 microcontroller
In order to demonstrate the operation of the 16F87X series we will consider
the 16F872 device. This is a 28pin device with 22 I/O available on 3 ports.
PortA has 6 I/O, PortB has 8 I/O and PORTC has 8 I/O. Of the 6 I/O available
on PortA 5 of them can be analogue inputs. The header for the 16F872,
HEAD872.ASM, configures the device with 5 analogue inputs on PortA,
8 digital inputs on PortC and 8 outputs on PortB. The port configuration for
the device is shown in Figure 16.4.
The 16F872 has been configured in HEAD872.ASM, using a 32 kHz crystal,
to allow all the programs used previously to be copied over with as little
alteration as possible.
1
2
3
4
5
6
7
8
9
10
11
12
15
16
17
18
13
14
19
20
21
22
23
24
25
26
27
MCLR/Vpp/THV
A0/AN0
A1/AN1
A2/AN2/Vref-
A3/AN3/Vref+
A4/T0CKI
A5/AN4/SS
Vdd
Vss
OSC1/CLKIN
OSC2/CLKOUT
C0/T1OSO/T1CLKI
C1/T1OSI/CCP2
C2/CCP1
C3/SCK/SCL C4/SD1/SDA
C5/SDO
C6/TX/CK
C7/RX/DT
Vss
B0/INT
B1
B2
B3/PGM
B4
B5
B6/PGC
B7/PGD
28
Figure 16.1 The 16F870/2/3/6 pinout
230 The 16F87X microcontroller

Devices included in this Data Sheet:
· PIC16F873 · PIC16F876
· PIC16F874 · PIC16F877
Microcontroller Core Features:
· High performance RISC CPU
· Only 35 single word instructions to learn
· All single cycle instructions except for program
branches which are two cycle
· Operating speed: DC - 20 MHz clock input
DC - 200 ns instruction cycle
· Up to 8K × 14 words of FLASH Program Memory,
Up to 368 × 8 bytes of Data Memory (RAM)
Up to 256 × 8 bytes of EEPROM Data Memory
· Pinout compatible to the PIC16C73B/74B/76/77
· Interrupt capability (up to 14 sources)
· Eight level deep hardware stack
· Direct, indirect and relative addressing modes
· Power-on Reset (POR)
· Power-up Timer (PWRT) and
Oscillatior Start-up Times (OST)
· Watchdog Timer (WDT) with its own on-chip RC
oscillator for reliable operation
· Programmable code-protection
· Power saving SLEEP mode
· Selectable oscillator options
· Low power, high speed CMOS FLASH/EEPROM
technology
· Fully static design
· In-Circuit Serial Programming (ICSP) via two
pins
· Single 5V In-Circuit Serial Programming capability
· In-Circuit Debugging via two pins
· Processor read/write access to program memory
· Wide operating voltage range: 2.0V to 5.5V
· High Sink/Source Current: 25 mA
· Commercial and Industrial and Extended temperature
ranges
· Low-power consumption:
- 2 mA typical @ 3V, 4 MHz
- 20 µA typical @3V, 32 kHz
- 1 µA typical standby current
· Timer0: 8-bit timer/counter with 8-bit prescaler
· Timer1: 16-bit timer/counter with prescaler,
can be incremented during SLEEP via external
crystal/clock
· Timer2: 8-bit timer/counter with 8-bit period
register, rescaler and postscaler
· Two Capture, Compare, PWM modules
- Capture is 16-bit, max. resolution is 12.5 ns
- Compare is 16-bit, max. resolution is 200 ns
- PWM max. resolution is 10-bit
· 10-bit multi-channel Analog-to-Digital converter
· Synchronous Serial Port (SSP) with SPI (Master
mode) and I2C (Master/Slave)
· Universal Synchronous Asychronous Receiver
Transmitter (USART/SCI) with 9-bit address detection
· Parallel Slave Port (PSP) 8-bits wide, with
external RD, WR and CS controls (40/44-pin only)
· Brown-out detection circuitry for
Brown-out Reset (BOR)
Pin Diagram
PDIP
PIC16F877/874
1
MCLR/VPP
RA0/AN0
RA1/AN1
RA2/AN2/VREF-
RA3/AN3/VREF+
RA4/T0CKI
RA5/AN4/SS
RE0/RD/AN5
RE1/WR/AN6
RE2/CS/AN7
VDD
VSS
OSC1/CLKIN
OSC2/CLKOUT
RC0/T1OSO/T1CKI
RC1/T1OSI/CCP2
RC2/CCP1
RC3/SCK/SCL
RD0/PSP0
RD1/PSP1
RB7/PGD
RB6/PGC
RB5
RB4
RB3/PGM
RB2
RB1
RB0/INT
VDD
VSS
RD7/PSP7
RD6/PSP6
RD5/PSP5
RD4/PSP4
RC7/RX/DT
RC6/TX/CK
RC5/SDO
RC4/SDI/SDA
RD3/PSP3
RD2/PSP2
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20 21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
Figure 16.2 The 16F87X data sheet
The 16F87X microcontroller 231

Address File Name
Bank0
File Name
Bank1
File Name
Bank2
File Name
Bank3
00h Ind.Add Ind.Add Ind.Add Ind.Add
01h TMR0 Option TMR0 Option
02h PCL PCL PCL PCL
03h Status Status Status Status
04h FSR FSR FSR FSR
05h PORTA TRISA
06h PORTB TRISB PORTB TRISB
07h PORTC TRISC
08h PORTD TRISD
09h PORTE TRISE
0Ah PCLATH PCLATH PCLATH PCLATH
0Bh INTCON INTCON INTCON INTCON
0Ch PIR1 PIE1 EEDATA EECON1
0Dh PIR2 PIE2 EEADR EECON2
0Eh TMR1L PCON EEDATH
0Fh TMR1H EEADRH
10h T1CON
11h TMR2 SSPCON2
12h T2CON PR2
13h SSPBUF SSPADD
14h SSPCON SSPSTAT
15h CCPR1L
16h CCPR1H
17h CCP1CON General
Purpose
Register
96 bytes
General
Purpose
Register
96 bytes
18h RCSTA TXSTA
19h TXREG SPBRG
1Ah RCREG
1Bh CCPR2L
1Ch CCPR2H
1Dh CCP2CON
1Eh ADRESH ADRESL
1Fh ADCON0 ADCON1
.
General
Purpose
Register
General
Purpose
Register
6Fh 96 bytes 80 bytes
7FH
Figure 16.3 The 16F87X memory map

The 16F872 header
HEAD872.ASM
;EQUATES SECTION
TMR0 EQU 1
OPTION_R EQU 1
PORTA EQU 5
PORTB EQU 6
PORTC EQU 7
TRISA EQU 5
TRISB EQU 6
TRISC EQU 7
STATUS EQU 3
ZEROBIT EQU 2
CARRY EQU 0
EEADR EQU 0DH
EEDATA EQU 0CH
EECON1 EQU 0CH
EECON2 EQU 0DH
RD EQU 0
WR EQU 1
WREN EQU 2
ADCON0 EQU 1FH
ADCON1 EQU 1FH
ADRES EQU 1EH
A4
1 Digital Input
B0
B1
B2
B3
B4
B5
B6
B7
8 Outputs
8 Digital Inputs
5 Analogue Inputs
C0
C1
C2
C3
C4
C5
C6
C7
AN0(A0)
AN1(A1)
AN2(A2)
AN3(A3)
AN4(A5)
Figure 16.4 Port configuration of the 16F872

CHS0 EQU 3
GODONE EQU 2
COUNT EQU 20H
;*****************************************************
LIST P¼16F872
ORG 0
GOTO START
;*******************************************************
; SUBROUTINE SECTION.
;1 SECOND DELAY
SUBLW .32 ;TIME-W
GOTO LOOPA
;0.5 SECOND DELAY
SUBLW .16 ;TIME-W
GOTO LOOPB
;******************************************************************
MOVLW B’11111111’
MOVLW B’00000000’
MOVLW B’11111111’
MOVLW B’00000111’
MOVWF OPTION_R ;Option Register, TMR0 / 256

MOVLW B’00000000’
MOVWF ADCON1 ;PortA bits 0, 1, 2, 3, 5 are analogue
BSF STATUS,6 ;BANK3
BCF STATUS,5
BSF ADCON0,0 ;turn on A/D.
CLRF PORTA
CLRF PORTB
CLRF PORTC
;*********************************************************
Explanation of HEAD872.ASM
Equates Section
We have a third port, PORTC file 7 and its corresponding TRIS file,
TRISC file 7 on Bank1. The TRIS file sets the I/O direction of the port bits.
The EEPROM data file addresses have been included. EEADR is file 0Dh
in Bank2, EEDATA is file 0Ch in Bank2, EECON is file 0Ch in Bank3
and EECON2 is file 0Dh in Bank3.
The EEPROM data bits have been added. RD the read bit is bit 0, WR the
write bit is bit 1, WREN the write enable bit is bit 2.
The Analogue files ADRES, ADCON1 and ADCON2 have been included
as have the associated bits CHS0 channel 0 select bit 3 and the GODONE
bit, bit 2.
List Section
This of course indicates the microcontroller being used, the 16F872 and
that the first memory location is 0. In address 0 is the instruction GOTO
START that instructs the micro to bypass the subroutine section and goto
the configuration section at the label START.
Subroutine Section
This includes the 2 delays DELAY1 and DELAYP5 as before.
Configuration Section
As before we need to switch to Bank1 to address the TRIS files to configure
the I/O. PORTA is set as an input port with the two instructions

MOVLW B’00000111’
MOVWF TRISA
PORTB and PORTC are configured in a similar manner using TRISB and
TRISC.
The Option register is configured with the instructions
MOVLW B’00000111’
MOVWF OPTION_R
The A/D register is configured with the instructions
MOVLW B’00000000’
MOVWF ADCON1
Setting PORTA bits 0, 1, 2, 3 and 5 as analogue inputs.
We turn to Bank3 by setting Bank select bit, STATUS,6 (bit 5 is still set)
so that we can address EECON1, the EEPROM data control register. BSF
EECON1 then enables access to the EEPROM program memory when
required.
We then turn back to Bank0 by clearing bits 5 and 6 of the Status register
and clear the files PortA, PortB and PortC.
16F872 Application – a greenhouse control
In order to demonstrate the operation of the 16F872 and to develop our
programming skills a little further consider the following application.
A greenhouse has its temperature monitored so that a heater is turned
on when the temperature drops below 158C and turns the heater off when
the temperature is above 178C.
A probe in the soil monitors the soil moisture so that a water valve will
open for 5 seconds to irrigate the soil if it dries out. The valve is closed and
will not be active for a further 5 seconds to give the water time to drain into
the soil.
A float switch monitors the level of the water and sounds an alarm if
the water drops below a minimum level.
The circuit diagram for the greenhouse control is shown in Figure 16.5 and
the flowchart is drawn in Figure 16.6.
Greenhouse program
In order to program the analogue/digital settings consider the NTC
Thermister. As the temperature increases the resistance of the thermister
will decrease and so the voltage presented to AN0 will increase.

Let us assume the voltage is 2.9v at 158C and 3.2v at 178C they correspond
to digital readings of 2.9 51 ¼ 147.9 i.e. 148 and 3.2 51 ¼ 163.2 i.e. 163.
(N.B. 5v ¼ 255, so 1v ¼ 51 we are using an 8 bit A/D.)
Our program then needs to check when AN0 goes above 163 and below 148.
As the soil dries out its resistance will increase. Let us assume in our
application dry soil will give a reading of 2.6v, (on AN1), i.e. 2.6 51 ¼ 132.6
i.e. 133. So any reading above 133 is considered dry.
The float switch is a digital input showing 1 if the water level is above
the minimum required and a 0 if it is below the minimum.
Greenhouse code
The code for the greenhouse uses HEAD872.ASM with the program
instuctions added and saved as GREENHO.ASM.
5v
Thermistor °C
19
8
20
1
Alarm
B2
10
9
68p
32kHz
68p
0.1µF
0v
5v
B1
Water Valve
B0 Heater
C0
AN0
AN1
0v
Soil Moisture Probe
5v
10k
68k
22k
5v
0v
0v
0v
Float Switch
Figure 16.5 Greenhouse control circuit

Turn on water valve
Wait 5 seconds
Turn off water valve
Wait 5 seconds
Turn on heater
Turn off heater
Y
Y
N
N
N
Y
Y
Is Temp
17°C?
Is Temp
15°C?
Is
Soil
Dry?
Is
Water
Empty?
Turn on Alarm
N
Figure 16.6 Greenhouse control flowchart

;GREENHO.ASM
;EQUATES SECTION
TMR0 EQU 1
OPTION_R EQU 1
PORTA EQU 5
PORTB EQU 6
PORTC EQU 7
TRISA EQU 5
TRISB EQU 6
TRISC EQU 7
STATUS EQU 3
ZEROBIT EQU 2
CARRY EQU 0
EEADR EQU 0DH
EEDATA EQU 0CH
EECON1 EQU 0CH
EECON2 EQU 0DH
RD EQU 0
WR EQU 1
WREN EQU 2
ADCON0 EQU 1FH
ADCON1 EQU 1FH
ADRES EQU 1EH
CHS0 EQU 3
GODONE EQU 2
COUNT EQU 20H
;*****************************************************
LIST P¼16F872
ORG 0
GOTO START
;*******************************************************
;Configuration Bits
__CONFIG H’3F30’ ;selects LP oscillator, WDT off, PUT on,
;*******************************************************
;1 SECOND DELAY
SUBLW .32 ;TIME-W

GOTO LOOPA
;0.5 SECOND DELAY
SUBLW .16 ;TIME-W
GOTO LOOPB
;5 SECOND DELAY
SUBLW .160 ;TIME-W
GOTO LOOPC
HEAT_ON BSF PORTB,0 ;Turn heater on
GOTO SOIL ;Check soil moisture
HEAT_OFF BCF PORTB,0 ;Turn heater off
GOTO SOIL ;Check soil moisture
WATER_ON BSF PORTB,1 ;Turn water on
CALL DELAY5
BCF PORTB,1 ;Turn water off
CALL DELAY5
GOTO WATER ;Check water level
ALARM_ON BSF PORTB,2 ;Turn alarm on
GOTO BEGIN ;Repeat the process
ALARM_OFF BCF PORTB,2 ;Turn alarm off
GOTO BEGIN ;Repeat the process
;******************************************************************
MOVLW B’11111111’
MOVLW B’00000000’

MOVLW B’11111111’
MOVLW B’00000111’
MOVLW B’00000000’
MOVWF ADCON1 ;PortA bits 0, 1, 2, 3, 5 are
;analogue
BSF STATUS,6 ;BANK3
BCF STATUS,5
BSF ADCON0,0 ;turn on A/D.
CLRF PORTA
CLRF PORTB
CLRF PORTC
;*********************************************************
;Check the temperature on AN0
BEGIN BCF ADCON0,CHS0 ;C to select AN0
BSF ADCON0,GODONE
WAIT1 BTFSC ADCON0,GODONE
GOTO WAIT1
MOVF ADRES,W
SUBLW .163 ;163 – W
BTFSS STATUS,CARRY ;C if W 4 163 i.e. hot
;(above 178C)
GOTO HEAT_OFF
MOVF ADRES,W
SUBLW .148 ;148 – W
BTFSC STATUS,CARRY ;S if W 5 148 i.e. cold
;(below 158C)
GOTO HEAT_ON
;Check the soil moisture on AN1
SOIL BSF ADCON0,CHS0 ;S to select AN1
BSF ADCON0,GODONE
WAIT2 BTFSC ADCON0,GODONE
GOTO WAIT2

MOVF ADRES,W
SUBLW .133 ;133 – W
BTFSS STATUS,CARRY ;C if W 4 133 i.e. dry
GOTO WATER_ON
;Check water is above minimum
WATER BTFSC PORTC,0 ;C if below minimum
GOTO ALARM_OFF
GOTO ALARM_ON
END
Explanation of code
In the previous analogue circuits in Chapter 11 we only used 1 analogue
input on AN0. We now have two analogue inputs on AN0 and AN1. When
making an analogue measurement we must specify which analogue channel
we wish to measure. The default is AN0 when moving to AN1 we select AN1
by setting channel select bit0 i.e. BSF ADCON0,CHS0.
When moving back to AN0 clear the channel select bit. The 8 channels, AN0 to
AN7 are seclected using bits, CHS2, CHS1, CHS0.
The temperature is read on AN0 with and then checked to see if it is
greater than 178C, by subtracting the A/D reading from 163 (the reading
equating to 178C). The carry bit in the status register indicates if the result
is þve or ve being set or clear. We then go to turn off the heater if the
temperature is above 178C or check if the temperature is below 158C.
In which case we turn on the heater.
The soil moisture is checked next. AN1 is selected and the reading compared
this time to 133 indicating dry soil. The program either goes to turn on
the water valve if the soil is dry or continues to check the water level if the
soil is wet.
If the water level is below minimum then the alarm sounds, if above
minimum the alarm is turned off. The program then repeats the checking
of the inputs and reacts to them accordingly.
Programming the 16F872 microcontroller
using PICSTART PLUS
Once the pogram GREENHO.ASM has been saved it is then assembled using
MPASMWIN. The next step as previously is to program GREENHO.HEX
into the micro using PICSTART PLUS.
This process has been outlined in Chapter 2, but there are a few more selections
to attend to in the ‘Device Specification’ Section.

Select the device 16F872, if this device is not available you will require a later
version of MPLAB, obtainable from www.microchip.com.
Set the fuses.
Configuration bits
The configuration bit settings when programming the 16F872 for the
Greenhouse program are shown in Figure 16.7.
Reconfiguring the 16F872 header
The port settings are changed as they were for the 16F84 i.e. a 1 means
the pin is an input and a 0 means an output.
The Option Register is configured as in the 16F84 see also Chapter 19.
The A/D convertor configuration is adjusted using A/D configuration
register 1, i.e. ADCON1 shown in Figure 16.8.
Bit7 is the A/D Format Select bit, which selects which bits of the A/D result
registers are used. I.e. the A/D can use 10 bits which requires two result
registers, ADRESH and ADRESL. Two formats are available.
(a) the most significant bits of ADRESH read as 0, with ADFM ¼ 1
Figure 16.7 Greenhouse program configuration bits
ADFM PCFG3 PCFG2 PCFG1 PCFG0
bit7 bit0
Figure 16.8 ADCON1, A/D port configuration register 1
ADRERSH ADRESL
0 0 0 0 0 0

Or (b) the least significant bits of ADRESL read as 0, with ADFM ¼ 0
For 8 bit operation condition (b) is used with ADRESH as the 8 most
significant bits of the A/D result. This is the default configuration used in
HEADER872.ASM where ADRESH (ADRES in the equates) is register 1Eh
in Bank0.
Table 16.1 shows the A/D Port Configuration settings for PCFG3, PCFG2,
PCFG1 and PCFG0.
A ¼ Analogue Input, D ¼ Digital input.
Vdd ¼ þve supply, Vss ¼ ve supply.
Vrefþ ¼ high voltage reference.
Vref ¼ low voltage reference.
A3 ¼ PortA,3 A2 ¼ PortA,2 etc.
N.B. AN7, AN6 and AN5 are only available on the 40 pin devices 16F871,
16F874 and 16F877.
PCFG3:
PCFG0
AN7
E2
AN6
E1
AN5
E0
AN4
A5
AN3
A3
AN2
A2
AN1
A1
AN0
A0
Vref+ Vref−
0000 A A A A A A A A Vdd Vss
0001 A A A A Vref+ A A A A3 Vss
0010 D D D A A A A A Vdd Vss
0011 D D D A Vref+ A A A A3 Vss
0100 D D D D A D A A Vdd Vss
0101 D D D D Vref+ D A A A3 Vss
011X D D D D D D D D Vdd Vss
1000 A A A A Vref+ Vref− A A A3 A2
1001 D D A A A A A A Vdd Vss
1010 D D A A Vref+ A A A A3 Vss
1011 D D A A Vref+ Vref− A A A3 A2
1100 D D D A Vref+ Vref− A A A3 A2
1101 D D D D Vref+ Vref− A A A3 A2
1110 D D D D D D D A Vdd Vss
1111 D D D D Vref+ Vref− D A A3 A2
Table 16.1 A/D Port configuration
ADRESH ADRESL
0 0 0 0 0 0

17
The 16F62X microcontroller
The 16F62X family of microcontrollers includes the two devices 16F627 and
16F628.
The 16F62X microcontrollers are flash devices and have 18 pins and data
EEPROM just like the 16F84, but they have more functions. Notably there is
an on board oscillator so an external crystal is not required. This frees up two
pins for extra I/O. The 16F62X in fact can use 16 of its 18 pins as I/O.
Table 17.1 shows the specification of the 16F62X devices and the 16F84 for
comparison.
16F62X oscillator modes
The 16F62X can be operated in 8 different oscillator modes. They are selected
when programming the device just like the 16F84, or by inserting the
configuration bits in the header.
Device
Flash
Program
Memory
(bytes)
RAM
Data
Memory
(bytes)
EEPROM
Data
Memory
(bytes)
Timer
Modules
I/O
Pins
16F627 1024 224 128 3 16
16F628 2048 224 128 3 16
16F84 1024 68 64 1 13
Table 17.1 The 16F62X specification

The options are:
LP Low Power Crystal, 32.768kHz
XT 4MHz Crystal
HS High Speed Crystal, 20MHz
ER External Resistor (2 modes)
INTRC Internal Resistor/Capacitor (2 modes)
EC External Clock in
The two modes for the internal resistor/capacitor configuration are 4MHz and
37kHz. The default setting is 4MHz. The 16F627 header, HEAD62RC.ASM,
selects the 37kHz oscillator by clearing the OSCF (oscillator frequency) bit,
bit3 in the Peripheral Control Register, PCON with BCF PCON,3.
There was obviously a good reason for Microchip choosing 37kHz for the
oscillator instead of 32.768kHz, I only wish I knew what it was! 32.768kHz
as we have seen before (HEADER84.ASM) can give us TMR0 pulses of 32
a second when setting the option register to divide the program timing pulses
by 256.
The most attractive proposition I can see using 37kHz is:
Clock frequency ¼ 37kHz,
Program execution frequency is 37kHz/4 ¼ 9250Hz.
Setting the prescaler to /32 gives TMR0 pulses of 9250 / 32 ¼ 289.0625Hz ¼
0.03459459s for each pulse.
Counting 29 TMR0 pulses gives a time of 0.100324324s i.e. 0.1s þ 0.3%
error. If this error, about 4.5 minutes a day, is unacceptable then
a 32.768kHz crystal can be used as we did with the 16F84.
Since the programs used previously on the 16F84 did not require any accurate
timing our 16F62X header will set the prescaler to divide by 32 and use
a subroutine to count 29 TMR0 pulses to give a time of 0.1s.
All of the 16F84 programs can then be transferred to the 16F62X header.
The choice of a 32.768kHz crystal or the 37kHz internal RC will obviously
make a difference to the timing routines in the header. I have therefore
included two headers for the 16F62X devices. HEAD62LP.ASM for use with
the 32kHz crystal and HEAD62RC.ASM for use with the 37kHz internal RC
oscillator.

16F62X and 16F84 Pinouts
16F62X Port configuration
The header (HEAD62RC.ASM) will configure the 16F62X I/O as shown in
Figure 17.1.
The header (HEAD62LP.ASM) will configure the 16F62X I/O as shown in
Figure 17.2.
16F84 Pinout
16F62X Pinout
1
2
3
4
5
6
7
8
9 10
11
12
15
16
17
18
13
14
A2
A3
MCLR
Vss
B0
B1
B2
B3
9 10
A1
A0
1
2
3
4
5
6
7
8 11
12
15
16
17
18
13
14
A4/T0CLKIN A7/OSC1/CLKIN A4/T0CLKIN
A6/OSC2/CLKOUT
Vdd
B7
B6
B5
B4
B3
B2
B1
B0
Vss
A5/MCLR
A2
A3
A1
A0
OSC1/CLKIN
OSC2/CLKOUT
Vd
B7
B6
B5
B4
0.1µF
0v
V+
B0
B1
B2
B3
B4
B5
B6
B7
8 Outputs
8 Inputs
A0
A1
A2
A3
A4
A5
A6
A7
Figure 17.1 The 16F62X port configuration in HEAD62RC.ASM

16F62X Memory map
The 16F62X Memory Map at the end of the chapter (page 256).
The 16F62X headers
HEAD62LP.ASM
;HEAD62LP.ASM using the 32kHz crystal
;Prescaler / 256
;********************************************
;EQUATES SECTION
TMR0 EQU 1
OPTION_R EQU 1
PORTA EQU 5
PORTB EQU 6
TRISA EQU 5
TRISB EQU 6
STATUS EQU 3
ZEROBIT EQU 2
CARRY EQU 0
EEADR EQU 1BH
EEDATA EQU 1AH
0v
OSC2
OSC1
68p
32kHz
68p
0.1µF
0v
V+
B0
B1
B2
B3
B4
B5
B6
B7
8 Outputs
6 Inputs
A0
A1
A2
A3
A4
A5
Figure 17.2 The 16F62X port configuration in HEAD62LP.ASM

EECON1 EQU 1CH
EECON2 EQU 1DH
RD EQU 0
WR EQU 1
WREN EQU 2
COUNT EQU 20H
;*****************************************************
LIST P¼16F627 ;using the 627
ORG 0
GOTO START
;*******************************************************
Configuration Bits
__CONFIG H’3F00’ ;selects LP oscillator, WDT off,
;*******************************************************
;1 SECOND DELAY
SUBLW .32 ;TIME-W
GOTO LOOPA
;0.5 SECOND DELAY
SUBLW .16 ;TIME-W
GOTO LOOPB
;**********************************************************
MOVLW B’11111111’

MOVLW B’00000000’
MOVLW B’00000111’
BCF STATUS,5 ;Bank0
CLRF PORTA
CLRF PORTB
MOVLW .7
;*********************************************************
END
HEAD62RC.ASM
;HEAD62RC.ASM using the 37kHz internal RC
;Prescaler/32
;********************************************
;EQUATES SECTION
TMR0 EQU 1
OPTION_R EQU 1
PORTA EQU 5
PORTB EQU 6
TRISA EQU 5
TRISB EQU 6
STATUS EQU 3
ZEROBIT EQU 2
CARRY EQU 0
EEADR EQU 1BH
EEDATA EQU 1AH
EECON1 EQU 1CH
EECON2 EQU 1DH
RD EQU 0
WR EQU 1
WREN EQU 2
PCON EQU 0EH
COUNT EQU 20H

;*****************************************************
ORG 0
GOTO START
;*******************************************************
Configuration Bits
;*******************************************************
;0.1 SECOND DELAY
SUBLW .29 ;TIME-W
GOTO LOOPA
;0.5 SECOND DELAY
DELAYP5 MOVLW 5
MOVWF COUNT
DECFSZ COUNT
GOTO LOOPB
;1 SECOND DELAY
DELAY1 MOVLW 10
MOVWF COUNT
LOOPC CALL DELAYP1 ;0.1s delay
DECFSZ COUNT
GOTO LOOPC
;************************************************************
MOVLW B’11111111’

MOVLW B’00000000’
MOVLW B’00000100’
CLRF PCON ;Select 37kHz oscillator.
BCF STATUS,5 ;Bank0
CLRF PORTA
CLRF PORTB
MOVLW .7
;*********************************************************
A 16F627 application – flashing an LED on and off
In order to introduce the operation of the 16F672 device we will consider the
simple example of the single LED flashing on and off, which was introduced in
Chapter 2.
The 16F627 will be operated in the INTRC mode using the internal 37kHz
oscillator.
The circuit diagram for this is shown in Figure 17.3.
0v
470R
LED1
6
B0
V+
0v
14
5v
0v
0.1µ
16F627
5
Figure 17.3 The 16F627 LED flashing circuit

The 16F627 LED flasher code
;FLASH_RC.ASM using the 37kHz internal RC
;Prescaler/32
;********************************************
;EQUATES SECTION
TMR0 EQU 1
OPTION_R EQU 1
PORTA EQU 5
PORTB EQU 6
TRISA EQU 5
TRISB EQU 6
STATUS EQU 3
ZEROBIT EQU 2
CARRY EQU 0
EEADR EQU 1BH
EEDATA EQU 1AH
EECON1 EQU 1CH
EECON2 EQU 1DH
RD EQU 0
WR EQU 1
WREN EQU 2
PCON EQU 0EH
COUNT EQU 20H
;*****************************************************
ORG 0
GOTO START
;*******************************************************
;Configuration Bits
;*******************************************************
;0.1 SECOND DELAY

SUBLW .29 ;TIME-W
GOTO LOOPA
;0.5 SECOND DELAY
DELAYP5 MOVLW 5
MOVWF COUNT
DECFSZ COUNT
GOTO LOOPB
;******************************************************************
MOVLW B’11111111’
MOVLW B’00000000’
MOVLW B’00000100’
CLRF PCON ;Selects 37kHz oscillator.
BCF STATUS,5 ;Bank0
CLRF PORTA
CLRF PORTB
MOVLW .7
;*********************************************************
BEGIN BSF PORTB,0 ;Turn on LED
CALL DELAYP5 ;Wait 0.5s
BCF PORTB,0 ;Turn off LED
CALL DELAYP5 ;Wait 0.5s
GOTO BEGIN ;Repeat
END

The operation of the program after ‘Program starts now’, is exactly the same as
in FLASHER.ASM in Chapter 2, using the 16F84.
All of the programs using the 16F84 can be transferred by copying the code
starting at ‘Program starts now’ and pasting into HEAD62RC.ASM or
HEAD62LP.ASM as required.
Configuration settings for the 16F627
When programming the Code FLASH_RC.HEX into the 16F627 use the
configuration settings shown in Figure 17.4. This setting equates to H’3F10’
which can be written into the Configuration Bits setting in your code.
Other features of the 16F62X
The 16F62X also includes,
An analogue comparator module with 2 analogue comparators and an on-
chip voltage reference module.
Timer1 a 16 bit timer/counter module with external crystal/clock capability
and Timer2 an 8 bit timer/counter with prescaler and postscaler.
A Capture, Compare and Pulse Width Modulation modes.
Please refer to the 16F62X data sheet for operation of these other features.
Figure 17.4 Configuration settings for FLASH_RC.HEX

Address File Name File Name File Name File Name
00h Ind.Add Ind.Add Ind.Add Ind.Add
01h TMR0 Option TMR0 Option
02h PCL PCL PCL PCL
03h Status Status Status Status
04h FSR FSR FSR FSR
05h PORTA TRISA
06h PORTB TRISB PORTB TRISB
07h
08h
09h
0Ah PCLATH PCLATH PCLATH PCLATH
0Bh INTCON INTCON INTCON INTCON
0Ch PIR1 PIE1
0Dh
0Eh TMR1L PCON
0Fh TMR1H
10h T1CON
11h TMR2
12h T2CON PR2
13h
14h
15h CCPR1L
16h CCPR1H
17h CCP1CON
18h RCSTA TXSTA
19h TXREG SPBRG
1Ah RCREG EEDATA
1Bh EEADR
1Ch EECON1
1Dh EECON2
1Eh
1Fh CMCON VRCON
.
General
Purpose
Register
96 bytes
General
Purpose
Register
80 bytes
General
Purpose
Register
48 bytes
6Fh
7F h
Bank0 Bank1 Bank2 Bank3
The 16F62X memory map

18
Projects
Project 1 Electronic dice
When using a Microcontroller in a control system the place to start is to decide
what hardware you are controlling. In the Electronic Dice we will use 7 LEDs
for the display and a push button to make the ‘‘throw’’. Just to make the dice a
little more interesting we will use a buzzer to give an audible indication of the
number thrown.
The circuit for the Dice is shown in Figure 18.1, using the 16F818 with its
internal 31.25kHz clock. The push button is an input connected to PortA,2.
The 7 LEDs are connected to PortB and the buzzer is on A1.
The truth table for the dice is shown in Table 18.1.
How does it work?
The dice has an input – the ‘‘throw’’ button. When it is pressed the internal
count repeatedly runs through from 1 to 6 changing some 8000 times a second
and stops on a number when the button is released.
This would be a complicated circuit to design with a timer, counter and
decoder circuits. But now we can use one chip to do all the timing counting and
decoding functions. Not only that I have also added a light flashing routine for
the first few seconds when the dice is turned on. Try doing all that with one
chip – other than a microcontroller.
The best way to describe the action of a program is with a flowchart. The
flowchart for the dice is shown in Figure 18.2.

B5
A1
18
LED3
LED6
0v
9
8
LED5
B3
B2
7 × 470R
0v
LED4
0v
LED1
0v
12
11
0v
0v
LED2
B0
B1
6
7
10
B4
LED0
5
0v
0.1µ
5v
0v
14
0v
V+
16F818
B6
A2
1
1K
SW1
0v
5v
Figure 18.1 Circuit diagram for the electronic dice
Table 18.1 Truth table for the electronic dice
Throw B7 B6 B5 B4 B3 B2 B1 B0
1 0 0 0 0 0 0 1 0
2 0 0 1 0 1 0 0 0
3 0 0 1 0 1 0 1 0
4 0 1 1 0 1 1 0 0
5 0 1 1 0 1 1 1 0
6 0 1 1 1 1 1 0 1
258 Projects

Y
N
Y
Y
N
Y
N
Y
N
Y
N
Y
Y
Y
N
N
Is
COUNT=3
Is switch
Pressed?
Start TMR0
Is
TMR0=6?
Is switch
Released?
Is
COUNT=1?
Is
COUNT=2
Is
COUNT=6
Is
COUNT=4
Is
COUNT=5
Display 3
Buzz 3 times
Display 1
Buzz 1 time
Display 2
Buzz 2 times
Display 4
Buzz 4 times
Display 6
Buzz 6
Display 5
Buzz 5
Figure 18.2 Flowchart for the dice
Projects 259

Program listing for the dice
The full program listing for the dice is given below in ;DICE.ASM.
;DICE.ASM
PC EQU 2
COUNTA EQU 21H
;*********************************************************
;*********************************************************
;Configuration Bits
;function on B2,
;**********************************************************
260 Projects

SUBLW .3 ;TIME-3
BTFSS STATUS,
NOP
;0.3 second delay.
DELAY MOVLW .3
MOVWF COUNT
LOOPC CALL DELAYP1
DECFSZ COUNT
GOTO LOOPC
RETLW 0
;1 second delay.
DELAY1 MOVLW .10
MOVWF COUNT
LOOPA CALL DELAYP1
DECFSZ COUNT
GOTO LOOPA
RETLW 0
;**********************************************************
MOVWF TRISA
MOVWF ADCON1
MOVLW B’00000000’
MOVLW B’00000000’
Projects 261

;**********************************************************
CALL DELAY1
CALL DELAY1
CLRF PORTB ;Turn off LEDs and buzzer.
MOVLW .5
MOVWF COUNTA
SEC1 MOVLW 60H ;Light flashing routine.
MOVWF PORTB
CALL DELAY
MOVLW 13H
MOVWF PORTB
CALL DELAY
MOVLW 0CH
MOVWF PORTB
CALL DELAY
MOVLW 13H
MOVWF PORTB
CALL DELAY
DECFSZ COUNTA
GOTO SEC1
CALL DELAY1
BSF PORTA,1 ;Turn buzzer on
CALL DELAY1
BCF PORTA,1 ;Turn buzzer off
BEGIN BTFSC PORTA,2 ;Is switch pressed?
GOTO BEGIN ;NO
CALL DELAYP1 ;YES
CLRF PORTB ;Switch off LEDs
LOOP1 CLRF TMR0 ;Start Timer
LOOP2 MOVF TMR0,W ;Put time into W.
SUBLW 6 ;Is TMR0 ¼ 6?
BTFSC STATUS,
ZEROBIT ;Skip if TMR0 is not 6.
GOTO LOOP1 ;TMR0 is 6, so reset timer.
BTFSS PORTA,2 ;skip if button released?
GOTO LOOP2 ;No, Carry on timing
262 Projects

MOVF TMR0,W ;yes, put the TMR0 into W.
ADDWF PC ;Jump the value of W.
GOTO NUM1 ;TMR0¼0
GOTO NUM2 ;TMR0¼1
GOTO NUM3 ;TMR0¼2
GOTO NUM4 ;TMR0¼3
GOTO NUM5 ;TMR0¼4
GOTO NUM6 ;TMR0¼5
NUM1 MOVLW B’00000010’ ;Turn LED on
MOVWF PORTB
BSF PORTA,1 ;turn on buzzer for 1/4 sec.
CALL DELAY
BCF PORTA,1 ;Turn buzzer off.
GOTO BEGIN ;BEGIN AGAIN.
NUM2 MOVLW B’00101000’ ;TURN ON 2 LEDS.
MOVWF PORTB
CALL DELAY
BCF PORTA,1 ;turn off buzzer for 1/4 sec.
CALL DELAY
CALL DELAY
GOTO BEGIN
NUM3 MOVLW B’00101010’
MOVWF PORTB
CALL DELAY
CALL DELAY
CALL DELAY
CALL DELAY
CALL DELAY
BCF PORTA,1 ;Turn off buzzer.
GOTO BEGIN
MOVWF PORTB
Projects 263

CALL DELAY
CALL DELAY
CALL DELAY
CALL DELAY
CALL DELAY
CALL DELAY
CALL DELAY
GOTO BEGIN
MOVWF PORTB
CALL DELAY
CALL DELAY
CALL DELAY
CALL DELAY
CALL DELAY
CALL DELAY
CALL DELAY
CALL DELAY
CALL DELAY
BCF PORTA,1 ;turn off buzzer.
GOTO BEGIN
MOVWF PORTB
CALL DELAY
264 Projects

CALL DELAY
CALL DELAY
CALL DELAY
CALL DELAY
CALL DELAY
CALL DELAY
CALL DELAY
CALL DELAY
CALL DELAY
CALL DELAY
GOTO BEGIN
END
Modifications to the dice project
Can you think of any modifications you can make to this program? Perhaps
you could add a roll routine so that a few numbers are shown before the dice
finally comes to rest on the number.
The initial display routine could also be customized.
You could throw a 7.
Dice using 12C508
The dice circuit used 8 outputs and 1 input a total of 9 I/O.
But LEDs 0 and 6, 1 and 5, 2 and 4 work in pairs, i.e. they are on and off
together. If these LEDs were paralleled up, then we only need 6 I/O, e.g.:
Input from Switch
Output to Buzzer
Projects 265

Output to LEDs 0 and 6
Output to LED 3
This project can then be undertaken using the 6 I/O of the 12C508.
Project 2 Reaction timer
There are many question and answer games on the market that would benefit
from a reaction timer which indicates the first player of a team to press. This
project has the facility for up to 6 players.
The circuit diagram for this project illustrated in Figure 18.3 uses 6 inputs and
7 outputs.
0v
SW0
6
B0
B6
B7
A0
A1
V+
0v
14
5v
0v
0.1µ
16F818
5
12
13
17
18
6 × 680R
0v
SW1
0v
SW2
0v
SW3
7
8
9
B2
B3
B5
0v
0v
0v
0v
LED0
LED1
LED2
LED3
0v
SW4
10
SW5
11
0v
B1
B4
A2
A3
1
2
0v
0v
LED4
LED5
5v
A4
3
Figure 18.3 The reaction timer circuit
266 Projects

Reaction timer operation
If B0 is the first to press B6 output LED lights
If B1 is the first to press B7 output LED lights
If B2 is the first to press A0 output LED lights
The Buzzer is connected to A4.
The buzzer sounds for 4 seconds after a button is pressed. During this time no
further presses are acknowledged. After the 4 seconds the buzzer stops and the
LED is extinguished and the program resets.
The unit uses 13 I/O but not all 6 button/LED combinations need be used. The
program will not need altering.
Just one point in case you were wondering: B0–B5 have been used as inputs
instead of PORTA because PORTB has internal pull-up resistors on the inputs.
The switches do not need their own – no point in using 5 resistors if you don’t
have to.
The reaction timer program
;REACTION.ASM
;*********************************************************
Projects 267

;*********************************************************
;Configuration Bits
;function on B2,
;*****************************************************
SUBLW .3 ;TIME-3
BTFSS STATUS,
NOP
;4 second delay.
DELAY4 MOVLW .40
MOVWF COUNT
LOOPC CALL DELAYP1
DECFSZ COUNT
GOTO LOOPC
RETLW 0
;1 second delay.
DELAY1 MOVLW .10
MOVWF COUNT
LOOPA CALL DELAYP1
DECFSZ COUNT
GOTO LOOPA
RETLW 0
268 Projects

ON0 BSF PORTB,6 ;Turn on LED0
BSF PORTA,4 ;Turn on buzzer
BCF PORTB,6 ;Turn off LED0
BCF PORTA,4 ;Turn off buzzer
GOTO SCAN
ON1 BSF PORTB,7 ;Turn on LED1
BCF PORTB,7 ;Turn off LED1
GOTO SCAN
ON2 BSF PORTA,0 ;Turn on LED2
BCF PORTA,0 ;Turn off LED2
GOTO SCAN
GOTO SCAN
GOTO SCAN
GOTO SCAN
;**********************************************************
Projects 269

MOVLW B’0000000’ ;8 bits of PORTA are O/P
MOVWF TRISA
MOVWF ADCON1
MOVLW B’00111111’
MOVWF TRISB ;PORTB is mixed I/O
MOVLW B’00000000’
CLRF COUNT
;**********************************************************
MOVLW 0FFH
MOVWF PORTA ;Turn on PORTA outputs
MOVWF PORTB ;Turn on PORTB outputs
CLRF PORTA ;Turn off PORTA outputs
CLRF PORTB ;Turn off PORTB outputs
SCAN BTFSS PORTB,0 ;Has B0 been pressed
GOTO ON0 ;Yes
BTFSS PORTB,1 ;Has B1 been pressed
270 Projects

GOTO ON1 ;Yes
GOTO ON2 ;Yes
GOTO ON3 ;Yes
GOTO ON4 ;Yes
GOTO ON5 ;Yes
GOTO SCAN
END
How does it work?
The program starts by turning all the LEDs and the buzzer on for 1 second
to check they are all working.
The program then tests each input in turn starting with B0, if it is set i.e. not
pressed the program skips and checks the next input. When the last input B5
is checked and it is not pressed then the program skips the next instruction
and goes back to SCAN again.
If one of the inputs is pressed the program branches to the relevant subroutine
to turn on the appropriate LED and buzzer for 4 seconds before returning
to scan the switches again.
Reaction timer development
One way of making this program more interesting and to develop your
programming skills – when a button is pressed have the outputs jump around
B6, A0, A3, A1, A2 then B7 before landing on the correct output.
You could also have a flashing light routine at the start of the program to
check they are working, you could also pulse the buzzer. The buzzer could be
made to beep a number of times to give an audible indication of who was first
to press. Another modification you could make is – think of one yourself,
I’m not doing all the work.
Projects 271

Project 3 Burglar alarm
Operation
The circuit for the Burglar Alarm is shown in Figure 18.4 using the 16F818.
It uses two inputs, SW0 and SW1 which are both normally closed. They can
represent Door contacts, Passive Infra red sensor outputs, window contacts
or tilt switches.
SW0 has a delay on it but SW1 is immediately active.
Both switches can have additional switches wired in series with them to provide
extra security cover. If SW1 is a window contact in a caravan it could have
a tilt switch wired in series with it, so if the caravan was moved the siren
would sound immediately.
SW0 and SW1 are connected to PORTB so pull-ups are not required.
A buzzer is used to indicate entry and exit delays on the alarm and a siren is
connected to the micro via an IRF511 (Power MOSFET).
0v
SW0
6
B0
16F818
0v
SW1
7
B1
V+
0v
14
5v
0v
0.1µ
5
V+
0v
Buzzer
Siren
IRF511
B2
B3
8
9
Figure 18.4 Burglar alarm circuit
272 Projects

How does it work?
Consider the flow chart in Figure 18.5.
With reference to the flow chart:
When the alarm is switched on a 30 second exit delay is activated and the
buzzer sounds for this time.
Switches 0 and 1 are continually checked until one of them is open.
If SW0 is opened a 30 second entry delay is activated and the buzzer sounds for
this time, the siren will then sound for 5 minutes.
If SW1 is opened the siren will sound immediately for 5 minutes.
N
N
Y
Y
Y
Y
Switch on siren
Sound Buzzer for Entry Delay
Is SW1
Closed?
Is SW0
Closed?
Is SW1
Open?
Is SW0
Open?
Sound Buzzer for Exit Delay
N
N
Figure 18.5 Burglar alarm flowchart
Projects 273

The switches are then checked until they are both closed when the alarm resets
back to checking switches 0 and 1 until one of them opens again.
Switching off the power would disable the alarm.
Burglar alarm project code
The code for the Burglar Alarm is shown below in ALARM.ASM
;ALARM.ASM
;EQUATES SECTION
COUNTA EQU 21H
;*********************************************************
;*********************************************************
;Configuration Bits
;Background Debugger Mode disabled,
;CCP function on B2,
;**********************************************************
274 Projects

SUBLW .3 ;TIME-3
NOP
;0.5 second delay.
DELAYP5 MOVLW .5
MOVWF COUNT
LOOPC CALL DELAYP1
DECFSZ COUNT
GOTO LOOPC
RETLW 0
;1 second delay.
DELAY1 MOVLW .10
MOVWF COUNT
LOOPA CALL DELAYP1
DECFSZ COUNT
GOTO LOOPA
RETLW 0
;0.25 second delay
DELAYP25 MOVLW .3
MOVWF COUNT
LOOPD CALL DELAYP1
DECFSZ COUNT
GOTO LOOPD
RETLW 0
;5 second delay
DELAY5 MOVLW .50
MOVWF COUNT
LOOPE CALL DELAYP1
DECFSZ COUNT
GOTO LOOPE
RETLW 0
Projects 275

BUZZER MOVLW .5
MOVWF COUNTA ;5 2 SECONDS
BUZZ1 BSF PORTB,2
CALL DELAY1
BCF PORTB,2
CALL DELAY1
DECFSZ COUNTA
GOTO BUZZ1
MOVLW .10
MOVWF COUNTA ;10 1 SECOND
BUZZ2 BSF PORTB,2
CALL DELAYP5
BCF PORTB,2
CALL DELAYP5
DECFSZ COUNTA
GOTO BUZZ2
MOVLW .20
MOVWF COUNTA
BUZZ3 BSF PORTB,2 ;20 0.5 SECONDS
CALL DELAYP25
BCF PORTB,2
CALL DELAYP25
DECFSZ COUNTA
GOTO BUZZ3
RETLW 0
;**********************************************************
MOVWF TRISA
MOVWF ADCON1
MOVLW B’00000011’
MOVWF TRISB ;PORTB is MIXED I/O
MOVLW B’00000000’
276 Projects

CLRF COUNT
;**********************************************************
CALL BUZZER ;Exit delay
CHK_ON BTFSC PORTB,0 ;Check for alarm
GOTO ENTRY
BTFSC PORTB,1
GOTO SIREN
GOTO CHK_ON
ENTRY CALL BUZZER ;Entry delay
SIREN BSF PORTB,3 ;5 minute siren
MOVLW .60
MOVWF COUNTA
WAIT5 CALL DELAY5
DECFSZ COUNTA
GOTO WAIT5
BCF PORTB,3 ;Turn off Siren
CHK_OFF BTFSC PORTB,0 ;Check switches closed
GOTO CHK_OFF
BTFSC PORTB,1
GOTO CHK_OFF
GOTO CHK_ON
END
The Burglar Alarm uses 2 inputs and 2 outputs a total of 4 I/O.
We can therefore program the Alarm with a 12C508 chip.
Burglar alarm using the 12C508
The circuit diagram for the Alarm with the 12C508 is shown in Figure 18.6.
Note in the circuit of Figure 18.6, showing the alarm using the 12C508, that no
external oscillator circuit is required and that pull ups are not required on pins
GPIO,0 or GPIO,1 (or GPIO,3). N.B. GPIO,3 is an input only pin.
Projects 277

The flowchart of course is the same. The code is shown below as
ALARM_12.ASM using the header for the 12C508 from Chapter 15.
WARNING: The 12C508 only has a two level deep stack which means when
you do a CALL you can only do one more CALL from that subroutine
otherwise the program will get lost.
Program code for 12C508 burglar alarm
;ALARM_12.ASM FOR 12C508
OSCCAL EQU 5 ;Oscillator calibration.
COUNTB EQU 09H
;**********************************************************
0v
SW0
7
GP0
12C508
0v
SW1
6
GP1
V+
0v
1
5v
0v
0.1µ
8
V+
0v
Buzzer
Siren
IRF511
GP2
GP4
5
3
Figure 18.6 Burglar alarm using 12C508
278 Projects

;**********************************************************
;Configuration Bits
__CONFIG H’0FEA’ ;selects Internal RC oscillator, WDT off,
;**********************************************************
;1 second delay
DELAY1 MOVLW .100 ;100 1/100 SEC.
MOVWF COUNT
TIMEA CLRF TMR0 ;Start TMR0
GOTO LOOPB
DECFSZ COUNT
GOTO TIMEA
RETLW 0
;1/2 second delay
DELAYP5 MOVLW .50 ;50 1/100 SEC.
MOVWF COUNT
TIMEB CLRF TMR0 ;Start TMR0
BTFSS STATUS,ZEROBIT ;CHECK TIME-W¼0
GOTO LOOPC
DECFSZ COUNT
GOTO TIMEB
RETLW 0
;1/4 second delay
DELAYP25MOVLW .25 ;25 1/100 SEC.
MOVWF COUNT
TIMEC CLRF TMR0 ;Start TMR0
LOOPD MOVF TMR0,W ;Read TMR0 IN W
Projects 279

GOTO LOOPD
DECFSZ COUNT
GOTO TIMEC
RETLW 0
;2 second delay
DELAY2 MOVLW .200 ;200 1/100 SEC.
MOVWF COUNT
TIMED CLRF TMR0 ;Start TMR0
LOOPE MOVF TMR0,W ;Read TMR0 IN W
GOTO LOOPE
DECFSZ COUNT
GOTO TIMED
RETLW 0
BUZZER MOVLW .5
MOVWF COUNTB ;5 2 Seconds
BUZZ1 BSF GPIO,2
CALL DELAY1
BCF GPIO,2
CALL DELAY1
DECFSZ COUNTB
GOTO BUZZ1
MOVLW .10
MOVWF COUNTB ;10 1 Second
BUZZ2 BSF GPIO,2
CALL DELAYP5
BCF GPIO,2
CALL DELAYP5
DECFSZ COUNTB
GOTO BUZZ2
MOVLW .20
MOVWF COUNTB
BUZZ3 BSF GPIO,2 ;20 0.5 Seconds
CALL DELAYP25
BCF GPIO,2
CALL DELAYP25
280 Projects

DECFSZ COUNTB
GOTO BUZZ3
RETLW 0
;**********************************************************
START MOVWF OSCCAL
MOVLW B’00101011’ ;GPIO bits 2 and 4 are O/Ps.
TRIS GPIO
MOVLW B’00000111’
MOVLW .39
MOVWF TIME
;**********************************************************
CALL BUZZER ;Exit delay
CHK_ON BTFSC GPIO,0 ;Check for alarm
GOTO ENTRY
BTFSC GPIO,1
GOTO SIREN
GOTO CHK_ON
ENTRY CALL BUZZER ;Entry delay
SIREN BSF GPIO,4 ;5 minute siren
MOVLW .150
MOVWF COUNTB
WAIT5 CALL DELAY2 ;150 2 seconds
DECFSZ COUNTB
GOTO WAIT5
BCF GPIO,4 ;Turn siren off
CHK_OFF BTFSC GPIO,0 ;Check switches closed
GOTO CHK_OFF
BTFSC GPIO,1
GOTO CHK_OFF
GOTO CHK_ON
END
Projects 281

Fault finding
What if it all goes wrong!
The block diagram of the microcontroller in Figure 18.7 shows 3 sections:
Inputs, the microcontroller and outputs.
The microcontroller makes the output respond to changes in the inputs under
program control.
All microcontroller circuits will have outputs and most will have inputs.
Check the supply voltage
Check that the correct voltages are going to the pins. 5v on Vdd, pin 14 and
MCLR, pin 4 and 0v on Vss, pin 5, on the 16F84.
Checking inputs
If the inputs are not providing the correct signals to the micro then the outputs
will not respond correctly.
Before checking inputs or outputs it is best to remove the microcontroller
from the circuit – with the power switched off. You have inserted the micro
in an IC holder so that it can be removed easily! This is essential for
development work.
In order to check the inputs and outputs to the microcontroller let us consider
a circuit we have looked at before in Chapter 5, the Switch Scanning Circuit,
shown below in Figure 18.8.
The four switches sw0, sw1, sw2 and sw3 turned on LED0, LED1, LED2 and
LED3 respectively.
To test the inputs monitor the voltage on the input pins to the micro-
controller, pins 1, 2, 17 and 18. They should go high and low as you throw
the switches.
OUTPUTS
MICROCONTROLLER
INPUTS
Figure 18.7 Block diagram of the microcontroller circuit
282 Projects

Checking outputs
The microcontroller will output 5v to turn on the outputs.
To make sure the outputs are connected correctly, apply 5v to each output pin
in turn to make sure the corresponding LED lights.
When 5v is applied to pin 6, the B0 output then LED0 should light, etc. If it
doesn’t the resistor value could be incorrect or the LED faulty or in the wrong
way round.
Check the oscillator
Check the oscillator is operating by monitoring the signal on CLKOUT,
pin 15, with an oscilloscope or counter. Correct selection of the oscillator
5v
0v
1K
SW0
17
A0
68p
68p
0v
32kHz
1
15
B0
B1
B2
B3
V+
MCLR
0v
14
4
5v
0v
0.1µ
16F84
5
6
7
8
9
4 x 680R
5v
0v
1K
SW1
5v
0v
1K
SW2
5v
0v
1K
SW3
18
1
2
A1
A2
A3
0v
0v
0v
0v
LED0
LED1
LED2
LED3
Figure 18.8 The switch scanning circuit
Projects 283

capacitor values are important – use 68pF with the 16C54 and 16F84 when
using a 32kHz crystal.
Has the micro been programmed for the correct oscillator: R-C, LP, XT or
HS. Most programs in this book use the LP configuration for the 32kHz
Oscillator.
If everything is OK so far then the fault is with the microcontroller chip or the
program.
Checking the microcontroller
If the program is not running it could be that you have a faulty
microcontroller. You could of course try another, but how do you know if
that is a good one or not. The best course of action is to load a program you
know works, into the micro. Such as FLASHER.ASM from Chapter 2. This
flashes an LED on and off for one second, it doesn’t use any inputs and only
1 output B0.
Checking the code
If there are no hardware faults then the problem is in your code.
I find a useful aid is first of all turn an LED on for 1 second and then turn it
off. When this works you know that the microcontroller is ok, and that your
timing has been set correctly and the oscillator and power supply are
functioning correctly. With the switch scanning circuit you could turn all
4 LEDs on for 1 second anyway to serve as an LED check.
To check your code, break it up into sections. Look at were the program stops
running to identify the problem area.
If possible turn on LEDs on the outputs to indicate where you are in the
program. If you are supposed to turn LED3 on when you go into a certain
section of code and LED3 doesn’t turn on, then of course you have not gone
into that section you are stuck somewhere else.
These instructions can be removed later when the program is working.
Using a simulator
By using a simulator such as the one contained in MPLAB you can single
step through the program and check it out a line at a time. To use the
simulator from MPLAB select – Debugger, Select Tool, MPLAB SIM as
284 Projects

Common faults
Here are just a few daft things my students (or I!) have done:
Not switched the power on.
Put the chip in upside down.
Programmed the wrong program into the micro.
Corrected faults in the code but forgot to assemble it again, thus blowing the
previous incorrect HEX file again.
Programmed incorrect fuses, i.e. Watchdog Timer and Oscillator.
Development kits
There are a number of development kits on the market (and you can make your
own). They have a socket for your micro, inputs and outputs that you can
connect to your micro. They are ideal for program development. Once verified
using the kit if the system does not work then your circuit is at fault. I have
developed such a kit shown in Figure 18.10. Details of it can be found on the
SL Electrotech website at:
http://www.slelectrotech.com
Figure 18.9 Selecting MPLAB SIM
Projects 285

Figure 18.10 PIC microcontroller development kit
286 Projects

19
Instruction set, files and
registers
Microcontrollers work essentially by manipulating data in memory locations.
Some of these memory locations are special registers others are user files. In a
control application data may be read from an input port, manipulated and
passed to an output port.
To use the microcontroller you need to know how to move and manipulate this
data in the memory. There are 35 instructions in the PIC 16F84 to enable you
to do this. Using the Microcontroller is then about using these instructions in a
program. Like any vocabulary you do not use all the words all of the time,
some you never use others only now and again. The PIC Instruction Set is like
this – you can probably manage quite well with say 15 instructions.
Most of these instructions involve the use of the WORKING REGISTER or
Wreg. The W register is at the heart of the PIC Microcontroller. To move data
from File A to File B you have to move it from File A to W and then from
W to File B, rather like a telephone system routes one caller to another via
the exchange. The W reg also does the arithmetic and logical manipulating
on the data.
The PIC microcontroller instruction set
To communicate with the PIC microcontroller you have to learn how to
program it using its instruction set. The 16F84 chip has a 1k 14 bit word
EEPROM program memory, 68 8bit general purpose registers and a 35 word
instruction set made up of three groups of instructions, bit, byte and literal and
control operations.
The instructions can be sub-divided into 3 types:
Bit Instructions, which act on 1 bit in a file.
Byte Instructions, which act on all 8 bits in a file.
Literal and Control Operations, which modify files with variables or control
the movement of data from one file to another.

Bit instructions
The bit instructions act on a particular bit in a file, so the instruction would be
followed by the data which specifies the file number and bit number.
I.e. BSF 6,3 This code is not too informative so we would use something like
BSF PORTB,BUZZER where PORTB is file 6 and the buzzer is connected to
bit 3 of the output port. In the equates section we would see PORTB EQU 6
and BUZZER EQU 3.
BCF Bit Clear in File.
BSF Bit Set in File.
BTFSC Bit Test in File Skip if Clear.
BTFSS Bit Test in File Skip if Set.
Byte instructions
Byte instructions work on all 8 bits in the file. So a byte instruction would be
followed by the appropriate file number.
I.e. DECF 0CH. This statement is not too informative so we would again
indicate the name of the file such as DECF COUNT. Of course we would
need to declare in the equates section that COUNT was file 0CH, by COUNT
EQU 0CH.
ADDWF ADD W and F.
ANDWF AND W and F.
CLRF CLeaR File.
CLRW CLeaR Working register.
COMF COMplement File.
DECF DECrement File.
DECFSZ DECrement File Skip if Zero.
INCF INCrement File.
INCFSZ INCrement File Skip if Zero.
IORWF Inclusive-OR W and F.
MOVF MOVe F to W.
MOVWF MOVe W to F.
NOP No OPeration.
RLF Rotate File one place Left.
RRF Rotate File one place Right.
SUBWF SUBtract W from F.
SWAPF SWAp halves of F.
XORWF eXclusive-OR W and F.
288 Instruction set, files and registers

Literal and control operations
Literal and control operations manipulate data and perform program
branching (jumps).
ADDLW ADD Literal with W.
ANDLW AND Literal with W.
CALL CALL subroutine.
CLRWDT CLeaR watchdog Timer.
GOTO GOTO address.
IORLW Inclusive-OR Literal with W.
MOVLW MOVe Literal to W.
RETFIE RETurn From IntErrupt.
RETLW RETurn place Literal in W.
RETURN RETURN from subroutine.
SLEEP Go into standby mode.
SUBLW SUBtract Literal from W.
XORLW eXclusive-OR Literal and W.
These instructions operate mainly on two 8 bit registers – the Working register
or W register and a File F which can be one of the 15 special registers or one of
the 68 general purpose file registers which form the user memory (RAM) of the
16F84.
The memory map of the 16F84 is shown in Figure 6.1.
The PIC Microcontrollers are 8 bit devices – this means that the maximum
number that can be stored in any one memory location is 255. Some PICs like
the 17C43 have 454 bytes of data memory. So to address memory locations
greater than 255 the idea of pages or Banks has been introduced. Bank0 holds
address locations up to 255, while Bank1 can hold a further 255 and Bank2
a further 255 etc. So you need to know what Bank a particular register or file
is in.
Banks are not used in the 16C54.
Registers
Registers are made up of 8 bits as shown in Figure 19.1.
Bit 0 is the Least Significant Bit (LSB) and Bit 7 is the Most Significant Bit
(MSB).
1 0 1 1 0 0 1 0
MSB ------------------------------------------------------------------------------------------- LSB
Figure 19.1 Register layout
Instruction set, files and registers 289

Register 00 indirect data addressing register
See File Select Register, Register 04.
Register 01 TMR0, TIMER 0/counter register
This register can be written to or read like any other register. It is used for
counting or timing events. The contents of the register can be incremented
(add 1) by the application of an external pulse applied to the TOCKI pin
i.e. counting cars into a car park or by the internal instruction cycle clock
which runs at ¼ of the crystal frequency to time events.
Register 02 PCL, program counter
The Program Counter automatically increments to execute program instruc-
tions. An application of the use of the Program Counter is illustrated in the
section on the Look Up Table, in Chapter 8.
Register 03, status register
The Status Register contains the result of the arithmetic or logical
operations of the program. The 8 bits of the Status Register are shown in
Figure 19.2.
Bit 0, C, Carry Bit. This is (set to a 1) if there is a carry from an addition or
subtraction instruction.
E.g. if one 8 bit number is added to another;
IRP RP1 RP0 TO PD Z DC C
Figure 19.2 Status Register
+
No carry to this column, C = 0
0 0 1 0 1 0 0 1
1 0 1 1 0 0 1 1
1 1 0 1 1 1 0 0
E.g.
+
1
Carry to this column, C = 1
1 0 1 1 0 0 1 1
1 0 1 1 0 1 0 1
0 1 1 0 1 0 0 0

If the result of a subtraction is þve or zero then the carry bit is set.
If the result of a subtraction is ve then the carry bit is clear.
Bit 2, Z, Zero Bit. This is set if the result of an arithmetic or logic operation
is zero. i.e. countdown to zero.
An important use of this bit is checking if a variable in memory is equal to a
fixed value. I.e. does file CARS contain 150.
MOVLW .150 ;Put 150 in W
SUBWF CARS,W ;Subtract W from CARS, i.e. CARS-150
BTFSS STATUS,ZEROBIT ;Zerobit set if CARS ¼ 150
Bits 6 and 5, RP1 and RP0, are the bank select bits to address banks 0,1,2
and 3 to select the different registers and user files.
00 would select bank0, 01 selects bank1, 10 selects bank2 and 11 selects
bank3.
Register 04 FSR file select register
The file select register is used in conjunction with the Indirect Data Addressing
Register, Register 00. They are used in indirect addressing to read or write
data not from a specific file, but to or from a file indicated by the data in the
file select register.
Register 05 PORT A and register 06 PORT B
Ports are the pin connections that allow the microcontroller to communicate
with its surroundings. Port A is a 5 bit port on the 16F84, only the 5 LSB’s
are used. Port A bit0 can also be programmed to be a clock input (T0CKI).
Port B is an 8 bit port. To set up a port the instruction TRIS is used. Tris
is an abbreviation for tristate, three states which can be a high impedance
input, a high (5v) output or a low (0v) output.
Register 8FH oscillator control register (16F818)
The oscillator control register is used to select the clock frequency when using
the internal oscillator.

bit 6–4 IRCF2:IRFC0: Internal Oscillator Frequency Select Bits.
111 ¼ 8 MHz (8MHz source drives clock directly)
110 ¼ 4 MHz
101 ¼ 2 MHz
100 ¼ 1 MHz
011 ¼ 500 kHz
010 ¼ 250 kHz
001 ¼ 125 kHz
000 ¼ 31.25 kHz (INTRC source drives clock directly)
bit2 IOFS:INTOSC Frequency Stable Bit.
W Register
The W register holds the result of an operation or an internal data transfer. It is
like a telephone exchange – data comes into the W register and is transferred
out to another file.
Option Register
This register is used to prescale the Real Time Clock/Counter. TMR0 clock
runs at ¼ of the crystal frequency but can be divided down by the prescaler for
longer time measurements.
Stack
Stack is the name given to the memory location that keeps track of the
program address when a Call instruction is made. There is an eight level
stack in the 16F84, which means that the program can jump to a subroutine
and from there jump to another subroutine, making 8 jumps in total and
the stack will be able to return it back to the program. The 16C54 has a two
level stack.
Instruction set summary
ADDLW Adds a number (literal) to W.
E.g. ADDLW 7 will add 7 to W, the result is placed in W.
bit7 bit6 bit5 bit4 bit3 bit2 bit1 bit0
- IRCF2 IRCF1 IRCF0 - IOFS - -
Figure 19.3 Oscillator control register

ADDWF Adds the contents of W to F.
E.g. ADDWF 7 will add the contents of the W register and file 7
N.B. the result is placed in file 7.
E.g. ADDWF 7,W the result is placed in W.
Status affected C,DC,and Z.
ANDLW The contents of W are ANDed with an 8 bit number (literal).
The result is placed in W.
E.g. ANDLW 12H or ANDLW B’00010010’ or ANDLW .18
Status affected Z.
ANDWF The contents of W are ANDed with F.
E.g. ANDWF 12,W the contents of file 12 is ANDed to the
contents of W. N.B. The result is placed in W.
E.g. ANDWF 12 the result is placed in file 12.
Status affected Z.
BCF Clear the bit in file F.
E.g. BCF 6,4 bit 4 is cleared in file 6.
File 6 is port B this clears bit 4, i.e. bit 4 ¼ 0.
BSF Set bit in file F.
E.g. BSF 6,4 this sets bit 4 in File 6, i.e. bit 4 ¼ 1.
BTFSC Test bit in file skip if clear.
E.g. BTFSC 3,2 this tests bit 2 in file 3 if it is clear then the
next instruction is missed. File 3 is the status register bit 2 is the
zero bit so the program jumps if the result of an instruction was
zero.
BTFSS Test bit in file skip if set.
E.g. BTFSS 3,2 if bit 2 in file 3 is set then the next instruction is
skipped.
CALL This calls a subroutine in a program.
E.g. CALL WAIT1MIN This will call a routine (you have
written) to wait for 1 minute. May be to turn a lamp on for 1
minute, and then return back to the program.
CLRF This clears file F i.e. all 8 bits in file F are cleared.
E.g. CLRF 5.
Status affected Z.

CLRW This clears the W register.
Status affected Z.
CLRWT The watchdog timer is cleared. The watchdog is a safety device in
the microcontroller if the program crashes the watchdog timer
times out then restarts the program.
Status affected TO, PD.
COMF The 8 bits in file F are complemented i.e. inverted.
E.g. COMF 6.
Status affected Z.
DECF Subtract 1 from file F. Useful for counting down to zero.
E.g. DECF 12 will store the result in 12.
DECF 12,W will store the result in W leaving 12 unchanged.
Status affected Z.
DECFSZ The contents of F are decremented and the next instruction is
skipped if the result is zero.
E.g. DECFSZ 12 or DECFSZ COUNT
GOTO This is an unconditional jump to a specified location in the
program.
E.g. GOTO SIREN.
INCF Add 1 to F. This value could then be compared to another to see
if a total had been achieved.
E.g. INCF 14 or INCF COUNT
Status affected Z.
INCFSZ Add 1 to F if the result is zero then skip the next instruction.
E.g. INCFSZ 19 or INCFSZ COUNT
IORLW The contents of the W register are ORed with a literal.
E.g. IORLW 27.
i.e W = 1 0 0 1 1 0 1 1
L = 0 0 0 1 1 0 0 1
L+W = 1 0 0 1 1 0 1 1
This is a very useful way of determining if any bit in a file
is set i.e. by ORing a file with 00000000 if all the bits in the

file are zero the OR result is zero and the zero bit is set in
the status register.
Status affected Z.
IORWF The contents of the W register are ORed with the file F.
E.g. IORWF 7,W The result is stored in W.
E.g. IORWF 7 The result is stored in file 7.
Status affected Z.
MOVF The contents of the file F are moved into the W register, from
there the data can be moved to an output port.
E.g. MOVF 12,W File 12 is moved to W.
E.g. MOVF 12 File 12 is moved to file 12? Zero is affected.
Status affected Z.
MOVLW The 8 bit literal is moved directly into W.
E.g. MOVLW .127
Status affected Z.
MOVWF The contents of the W register are moved to F.
E.g. MOVWF 6 the data in the W register is placed on port B.
NOP No operation – may seem like a daft idea but it is very useful for
small delays. The NOP instruction delays for ¼ of the clock
speed.
OPTION The contents of W are loaded into the OPTION register. This
instruction is used to prescale i.e. set TMR0 timing rate as shown
in Figure 19.4.
RETFIE This instruction is used to return from an interrupt.
RETLW This instruction is used at the end of a subroutine to return to the
program following a CALL instruction. The literal value is
placed in the W register. This instruction can also be used with a
look up table.
E.g. RETLW 0
RETURN This instruction is used to return from a subroutine.

RLF The contents of the file F are rotated 1 place to the left through
the carry flag. Shifting a binary number to the left means that the
number has been multiplied by 2. This instruction is used when
multiplying binary numbers.
E.g. RLF 12,W The result is placed in W.
E.g. RLF 12 The result is placed in file 12.
The diagram below shows file 12 being rotated left.
7 6 5 4 3 2 1 0
RBPU T0CS T0SE PSA PS2 PS1 PS0
Prescaler Value TMR0 Rate WDT Rate
0 1:2 1:1
0 1:4 1:2
0 1:8 1:4
0 1:16 1:8
1 1:32 1:16
1 1:64 1:32
1 1:128 1:64
1
0
0
1
1
0
0
1
1
0
1
0
1
0
1
0
1 1:256 1:128
Prescaler Assignment bit 0 = TMR0
1 = WDT
TMR0 Signal Edge
0 = Increment on low – high Transition on T0CKI pin.
1 = Increment on high – low Transition on T0CKI pin.
TMR0 Signal Source 0 = instruction cycle clock on CLKOUT pin.
1 = transition on T0CKI pin.
PORTB Pull Up Enable Bit.
1= Pull ups are disabled.
0 = Pull ups are enabled.
Figure 19.4 Option register

0 0 0 1
1 1 0 0 1
Carry
Bit.
FILE 12 Before rotation.
0 1 0 1 1 0 0 1 0
Carry
Bit.
FILE 12 After rotation.
Status affected C.
RRF This is the same as RLF except the file is rotated one place to the
right.
SLEEP When executing this instruction the chip is put into sleep mode.
The power-down status bit (PD) is cleared, the time-out status
bit is set, the watchdog timer and its prescaler are cleared and the
oscillator driver is turned off. The watchdog timer still keeps
running from its own internal clock.
E.g. SLEEP
Status affected TO, PD.
SUBLW The contents of the W register are subtracted from a number.
E.g. SUBLW 14 executes 14-W the result is placed in W. The
carry bit and the zero bit in the status register are affected
N.B. If W 4 14 then C ¼ 0 the result is ve.
If W 5 14 then C ¼ 1 the result is þve or zero.
If W ¼ 14 then Z ¼ 1 the result is zero. This is
a very useful condition. To find out if something has
occurred 14 times subtract 14 from those occurrences
if the answer is zero – bingo.
Status affected C, DC, and Z.
SUBWF The contents of the W register are subtracted from the contents
of the file F.
E.g. SUBWF 14,W executes F-W the result is placed in W.
E.g. SUBWF 14 executes F-W the result is placed in F.
NB. If W 4 F then C ¼ 0 the result is ve.
If W 5 F then C ¼ 1 the result is þve or zero.
If W ¼ F then Z ¼ 1 the result is zero.
Status affected C, DC, and Z.
SWAPF The upper and lower nibbles (4 bits) of file F are swapped.
E.g. SWAPF 12,W The result is placed in W.
E.g. SWAPF 12 The result is placed in file 12.

File 12 before SWAPF
0 1 1 0 1 1 0 1
File 12 after SWAPF
1 1 0 1 0 1 1 0
TRIS Load the TRIS register.
The contents of the W register are loaded into the TRIS register.
This then configures an I/O port as input or output.
E.g. MOVLW B’00001111’
MOVWF TRISB
This sets the 4 LSB’s of port B as inputs and the 4 MSB’s as
outputs. N.B. 1 for an input, 0 for an output.
XORLW The contents of the W register are Exclusive Ored with the
literal. If the result is zero then the contents match.
i.e. If a number on the input port, indicating temperature, is the
same as the literal then the result is zero and the zero bit is set.
i.e. 0 0 ¼ 0, 0 1 ¼ 1, 1 0 ¼ 1, 1 1 ¼ 0.
E.g. XORLW 67
Status affected Z.
XORWF The contents of the W register are Exclusive Ored with the
contents of the file F. i.e. If a number on the input port,
indicating temperature, is the same as the W register then the
result is zero and the zero bit is set. N.B. you can not Exclusive
OR the input port directly with a file, you have to do this by
loading the file into the W register with an MOVF instruction.
E.g. XORWF 17,W The result is placed in W.
E.g. XORWF 17 The result is placed in 17.
Status affected Z.
Did you notice how vital the W register is in the operation of the
microcontroller?
Data cannot go directly from A to B, it goes from A to W and then from
W to B.

Appendix
A
Microcontroller
data
Product
Program
Memory
Bytes
(words)
EEPROM
Data
Memory
Bytes
RAM
Bytes
I/O
Pins
A/D
Channels
Timers
Max
Speed
MHz
Internal
Oscillator
MHz
12C508 768
(512)
- 25 6 - 1–8 bit 4 4
12C509 1536
(1024)
- 41 6 - 1–8 bit 4 4
12CE518 768
(512)
16 25 6 - 1–8 bit 4 4
12CE519 1536
(1024)
16 41 6 - 1–8 bit 4 4
12CE673 1792
(1024)
16 128 6 4 (8 bit) 1–8 bit 4 10
12CE674 3584
(2048)
16 128 6 4 (8 bit) 1–8 bit 4 10
12F629 1792
(1024)
128 64 6 - 1–8 bit
1–16 bit
20 4
12F675 1792
(1024)
128 64 6 4 (10 bit) 1–8 bit
1–16 bit
20 4

Product
Program Memory E2
Prom
Data
Memory
RAM
Bytes
8-Bit
ADC
Channels
I/O
Ports
Timers
MAX
Speed
MHz
Bytes Words
PIC16CXXX – 4-12 Interrupts, 200ns Instruction Execution, 35 Instructions, 4MHz Internal Oscillator, 4/5 Oscillator Selections
PIC1F83 896 512 14 64 36 - 13 1–8bit, 1-WDT 10
PIC16F84 1792 1024 14 64 68 - 13 1–8bit, 1-WDT 10
PIC16F872 3584 2048 14 64 128 5 (10 bit) 22 1–16bit, 2–8bit, 1-WDT 20
PIC16C923 7168 4096 14 - 176 - 52 1–16bit, 2–8bit, 1-WDT 8
PIC16C924 7168 4096 14 - 176 5 52 1–16bit, 2–8bit, 1-WDT 8
PIC17CXXX – 4-12 Interrupts, 200ns Instruction Execution, 35 Instructions, 4MHz Internal Oscillator, 4/5 Oscillator Selections
PIC17C42A 4096 4096 14 - 192 8 33 1–16bit, 2–8bit, 1-WDT 20
PIC17C43 8192 8192 14 - 368 5 22 1–16bit, 2–8bit, 1-WDT 20
PIC17C44 16384 8192 14 - 368 8 33 1–16bit, 2–8bit, 1-WDT 20
PIC17C752 16384 2048 14 - 256 6 (12 bit) 16 1–16bit, 2–8bit, 1-WDT 20
PIC18CXXX – 10 MIPS, 77 Instructions, C-compiler Eﬃcient Instruction Set, Table Operation, Switchable Oscillator Sources
300
Appendix
A

Appendix B
Electrical characteristics
Absolute maximum ratings: (16F818/9)
Absolute maximum ratings: (16F818/9)
Ambient temperature 558C to þ1258C
Storage temperature 658C to þ1508C
Voltage on any pin with respect to Vss
(except Vdd and MCLR) 0.6V to Vdd þ0.6V
Voltage on Vdd with respect to Vss 0 to þ7.5V
Voltage on MCLR with respect to Vss 0 to þ14V
Total power dissipation 1W
Max. current out of Vss pin 200mA
Max. current into Vdd pin (16C54) 50mA
Max. current into Vdd pin 200mA
Max. output current sunk by any I/O pin 25mA
Max. output current sourced by any I/O pin 25mA
Max. output current sourced by PORTA 100mA
Max. output current sourced by PORTB 100mA
Max. output current sunk by PORTA 100mA
Max. output current sunk by PORTB 100mA

DC Characteristics.
PIC12F629/675
Characteristic Symbol Min. Typ. Max. Units Conditions.
Supply Voltage Vdd
2.0
2.2
3.0
5.5
5.5
5.5
V
V
V
Fosc = DC to 4MHz
With A/D off
PIC12F675 withA/D on
Fosc = 4 to 10MHz
RAM dataretention voltage Vdr 1.5 V Device in Sleep Mode
Supply Current Idd 0.4
0.9
5.2
20
2
4
15
48
mA
mA
mA
µA
Fosc = 4MHz, Vdd = 2V
Fosc = 4MHz, Vdd = 5.5V
Fosc = 32KHz, Vdd = 2V,
WDT disabled.
Power down Current
(sleep mode)
Ipd 1
0.9
18 µA
µA
Vdd = 2.0V, A/Don
Vdd = 2.0V, WDT disabled
PIC16F818/9
SupplyVoltage Vdd 2.0 5.5 V HS, XT, RC and LP osc modes
Supply Current Idd 28
874
µA
µA
Fosc = 32KHz, Vdd = 5.0V
Power down Current (sleep) Ipd 0.5 µA Vdd = 5.0V
PIC16F84
Supply Voltage
PIC16F84-XT
PIC16F84-RC
PIC16F84-HS
PIC16F84-LP
Vdd
4.0
4.0
4.5
4.0
6.0
6.0
5.5
6.0
V
V
V
V
Supply Current
PIC16F84-XT
PIC16F84-RC
PIC16F84-HS
PIC16F84-LP
Idd
7.3
7.3
5
35
10
10
10
400
mA
mA
mA
µA
Fosc = 32KHz, Vdd = 3.0V,
WDT disabled.
Power down Current
(sleep mode)
Ipd 40
38
100
100
µA
µA
Vdd = 4.0V, WDT enabled
Vdd = 4.0V, WDT disabled
PIC16F87X
Supply Voltage Vdd 4.0
4.5
5.5
5.5
V
V
LP, XT, RC osc configuration
HS osc configuration
Supply Current Idd 1.6
7
20
4
15
35
mA
mA
µA
Fosc = 32KHz, Vdd = 3.0V,
WDT disabled.
Power down Current (sleep) Ipd 1.5 19 µA Vdd = 4.0V, WDT enabled
302 Appendix B

Appendix C
Decimal, binary and
hexadecimal numbers
Homosapiens are used to Decimal numbers, i.e. 0,1,2,3 . . . . . . 9. Electronic
machines or chips use Binary numbers 0 and 1, (OFF and ON).
Decimal numbers increase in tens, i.e. 267 means 7 ones, 6 tens and 2 hundreds.
100 10 1
2 6 7
Binary numbers increase in twos, i.e. 1010. The right hand 0 means no ones,
the next digit means 1 two, the next means no fours, the next 1 eight etc.
8 4 2 1
1 0 1 0
The binary number 1010 consists of 4 BInary digiTs it is called a 4 BIT
number. 1010 is equivalent to 10 in decimal numbers.
We can change decimal numbers to binary and binary numbers to decimal.
Digital systems, i.e. Computers are a little better than we are at this.
Consider the decimal number 89, to turn this into a binary number write the
binary scale:
128 64 32 16 8 4 2 1
To make 89 we need (0 128) þ (1 64) þ (0 32) þ (1 16) þ (1 8) þ
(0 4) þ (1 2) þ (1 1).
So 89 in decimal ¼ 01011001 in binary.
To convert a binary number to decimal add up the various multiples of 2,
i.e. 10011010 is:
128 64 32 16 8 4 2 1
1 0 0 1 1 0 1 0
¼ 128 þ 16 þ 8 þ 2 ¼ 154:
A long string of binary numbers is difficult to read, i.e. 11010101 to make this
shorter and therefore easier to put into a microcontroller Hexadecimal

numbers are used. Hexadecimal numbers increase in sixteen’s and are described
by sixteen digits. Table C.1 shows these 16 digits and their decimal and binary
equivalents.
Table C.1 4 BIT Decimal, binary and hexadecimal representation
Decimal Binary Hexadecimal
0 0000 0
1 0001 1
2 0010 2
3 0011 3
4 0100 4
5 0101 5
6 0110 6
7 0111 7
8 1000 8
9 1001 9
10 1010 A
11 1011 B
12 1100 C
13 1101 D
14 1110 E
15 1111 F
The PIC microcontrollers are 8 bit micros, they use 8 binary digits for number
representation like
10010101 this is
128 64 32 16 8 4 2 1
1 0 0 1 0 1 0 1
¼ 149
The largest decimal number that can be represented by an 8 bit number is:
11111111 which represents:-
128 64 32 16 8 4 2 1
1 1 1 1 1 1 1 1
¼ 255
But we can program our microcontroller to increase our number representa-
tion from 8 bits i.e. up to 255:
to 16 bits, numbers up to 65,535
to 24 bits, numbers up to 16,777,215
to 32 bits, numbers up to 4,294,967,295 etc.
304 Appendix C

As mentioned earlier hexadecimal numbers are a shorter way of writing binary
numbers. To do this divide the binary number into groups of 4 and write each
group of 4 as a hex number.
i.e. 10010110 as 1001 0110 in binary
¼ 9 6 in hex:
i.e. 11011010 as 1101 1010 in binary
¼ D A in hex:
Table C.2 shows some of the 255 numbers represented by 8 bits.
Table C.2 8 BIT Decimal, binary and hexadecimal representation
Decimal Binary Hexadecimal
0 00000000 00
1 00000001 01
2 00000010 02
3 0000011 03
4 00000100 04
5 00000101 05
8 00001000 08
15 00001111 0F
16 00010000 10
31 00011111 1F
32 00100000 20
50 00110010 32
63 00111111 3F
64 01000000 40
100 01100100 64
127 01111111 7F
128 10000000 80
150 10010110 96
200 11001000 C8
250 11111010 FA
251 11111011 FB
252 11111100 FC
253 11111101 FD
254 11111110 FE
255 11111111 FF
Appendix C 305

Appendix D
Useful contacts
Author
d.w.smith@mmu.ac.uk
A Microcontroller Design Company
S.L. Electrotech Limited.
%þ44(0) 782 566626 http://www.slelectrotech.com
Arizona Microchip, the company that manufacture the PICs. This Website
is a must.
http://www.MICROCHIP.COM
Places to buy your components
Farnell %þ44(0) 113 263 6311 http://www.Farnell.com
Rapid Electronics %þ44(0) 1206 751166
RS Components %þ44(0) 1536 444105 http://www.rs-components.com/rs
Maplin Electronics %þ44(0) 1702 554000 http://www.maplin.co.uk
A recommended Magazine
Everyday Practical Electronics
http://www.epemag.wimborne.co.uk

Index
ADCON0 register 169
ADCON1 register 169
ADDLW 292
ADDWF 119, 293
ADRES register 171
ADSEL register 227
ANDLW 293
ANDWF 293
Anti-bounce routine 107
Assembling code 19
Banks 85
BCF 19, 293
BSF 18, 293
BTFSC 65, 293
BTFSS 66, 293
Burglar Alarm 272
CALL 19, 293
Carry Bit 173
Clock 3
CLRF 68, 84, 110, 293
CLRW 294
CLRWT 294
CMCON register 228
COMF 294
Compiling 22
Configuration bits 83, 90
Counting 110
Data 16F818 50
Data 16F84 49
Data types 13
DECF 110, 294
DECFSZ 43, 294
DELAY 19
Development kits 285
Dice 257
EECON1 198
EECON2 198
EEPROM 198
Equates 82
Fault finding 282
GOTO 19, 85, 294
Greenhouse control 236
Header 12C508 51
Header 12F629 52
Header 12F675 53
Header 16C54 139
Header 16F627 55
Header 16F818 59, 88
Header 16F84 14, 57, 82
Header 16F872 61
I/O 12
INCF 132, 294
INCFSZ 132, 294
INTCON register 208
Internal Oscillator 91
Interrupt sources 208
Interrupts 207
IORLW 295
Keypad 93
LIST 83
Look up table 115
Memory 2
Memory map 12C508 217
Memory map 12F629/675 218
Memory map 16C54 142
Memory map 16F818 92

Memory map 16F84 87
MOVF 79, 189, 295
MOVLW 41, 295
MOVWF 41, 295
MPLAB 19–25
NOP 295
OPTION 295
Option Register 292
OSCCAL register 228
Oscillator calibration, OSCCAL 218
OSCON 90, 291
OTP device 139
Power supply 6
Prescaler 86
Program Counter 290
Pull ups 98, 219
Reaction Timer 266
Registers 289
RETFIE 211, 296
RETLW 68, 84, 296
RETURN 296
RLF 296
RRF 296
Scan routine 100
SLEEP 297
Stack 292
Status Register 91, 290
SUBLW 68, 85, 185, 297
Subroutine 83
SUBWF 79, 108, 297
SWAPF 297
Temperature measurement 174
Timing 12
TMR0 290
TRIS 298
TRISA 46, 86
TRISB 86
Voltage measurement 178
W Register 292
XORLW 298
XORWF 298
Zerobit 84
308 Index

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