Microcontroller 8051 features and applications

MICROCONTROLLER-8051
Features & Applications

What is a Microcontroller ?
• A smaller computer used for a particular task
• On-chip RAM, ROM, I/O ports...
• Example ： Motorola’s 6811, Intel’s 8051, Zilog’s
Z8 and PIC 16X
RAM ROM
I/O
Port
Timer
Serial
COM
Port
CPU
A single chip
Microcontroller

• Personal information products: Cell phone(basic
model), pager, watch, pocket recorder, calculator
• Laptop components: mouse, keyboard, modem, fax
card, sound card, battery charger
• Home appliances: door lock, alarm clock, thermostat,
air conditioner, TV remote, VCR, small refrigerator,
exercise equipment, washer/dryer, microwave oven
• Industrial equipment: Temperature/pressure
controllers, Counters, timers, RPM Controllers
• Toys: video games, cars, dolls, etc.

How is it different from a Microprocessor ??
• General-purpose microprocessor CPU for
Computers
• No RAM, ROM, I/O on CPU chip itself
Example ： Intel’s x86, Motorola’s 680x0
CPU
General-
Purpose
Micro-
processor
RAM ROM I/O
Port
Timer
Serial
COM
Port
Data Bus
Address Bus

Microprocessor vs. Microcontroller

Microcontroller Architectures
CPU
Program
+ Data
Address Bus
Data Bus
Memory
Von Neumann
Architecture
CPU
Program
Address Bus
Data Bus
Harvard
Architecture
Memory
Data
Address Bus
Fetch Bus
0
0
0
2n

• 4K bytes ROM
• 128 bytes RAM
• Four 8-bit I/O ports
• Two 16-bit timers
• Serial interface
• 64K external code memory space
• 64K data memory space
Important Features of 8051

40 –lead PDIP(Plastic Dual In-line Package)
44 –lead PLCC(Plastic leaded chip carrier)
44 –lead TQFP(Thin Quad Flat Package)

Microcontroller 8051 features and applications

PC:
program counter (which points the next instruction to be executed)
Oscillator
• It is used for providing the clock to MC8051 which decides the speed or
baud rate of MC.
• We use crystal which frequency vary from 4MHz to 30 MHz, normally
we use 11.0592 MHz frequency.
Interrupts
• Interrupts are defined as requests because they can be refused
(masked) if they are not used, that is when an interrupt is acknowledged.
A special set of events or routines are followed to handle the interrupts.
These special routines are known as interrupt handler or interrupt service
routines (ISR). These are located at a special location in memory.
• INT0 and INT1 are the pins for external interrupts.

4KB ROM(read only memory)
• is available for program storage.
• Permanent data storage.
• This is nonvolatile memory; the data saved in this memory does not
disappear
after power failure.
• We can interface up to 64KB ROM memory externally if the application is
large. These sizes are specified different by their companies.

128 Byte- RAM (Random Access memory) for Data Storage
• memory is volatile memory.
•During execution for storing the data the RAM is used.
•RAM consists of the 4 register banks(4*8 byte), stack for temporary data
storage and bit addressable storage-16Bytes.
•It also consists of some special function register (SFR) which are used
for some
specific purpose like timer, input output ports etc.
•This is above the stack.

RAM memory space allocation in the 8051
7FH
30H
2FH
20H
1FH
17H
10H
0FH
07H
08H
18H
00H
Register Bank 0
(
Stack
)
Register Bank
1
Register Bank 2
Register Bank 3
Bit-Addressable RAM
Scratch pad RAM

Serial Port
• There are two pins available for serial communication TXD and RXD.
• Normally TXD is used for transmitting serial data which is in SBUF register,
RXD is used for receiving the serial data.
• SCON register is used for controlling the operation

Pin Description of the 8051
• The 8051 is a 40 pin
device, but out of these
40 pins, 32 are used for
I/O.
• 24 of these are dual
purpose, i.e. they can
operate as I/O or a control
line or as part of address
or date bus.

Input Output Ports
• four input output ports P0, P1, P2, P3.
• Each port is 8 bit wide and has special function register P0, P1, P2,
P3 which are bit addressable means each bit can be set or reset by the
Bit instructions (SETB for high, CLR for low) independently.
• The data at any port which is transmitting or receiving is in these
registers.
• The port 0 can perform dual works. It is also used as Lower order
address bus (A0 to A7) multiplexed with 8 bit data bus P0.0 to P0.7 is
AD0 to AD7 respectively the address bus and data bus is demultiplex
by the ALE signal and latch which is further discussed in details.
• Port 2 can be used as I/O port as well as higher order address bus A8
to A15.

• Port 3 also have dual functions it can be worked as I/O as well as each pin of P3
has specific function.
P3.0 – RXD – {Serial I / P for Asynchronous communication & Serial O / P for
synchronous communication.}
P3.1 – TXD – Serial data transmit.
P3.2 – INT0 – External Interrupt 0.
P3.3 – INT1 – External Interrupt 1.
P3.4 – T0 – Clock input for counter 0.
P3.5 – T1 – Clock input for counter 1.
P3.6 – WR – Signal for writing to external memory.
P3.7 – RD – Signal for reading from external memory.
When external memory is interfaced with 8051 then P0 and P2 can’t be worked
as I/O port they works as address bus and data bus, otherwise they can be
accessed as I/O ports.

Timers and Counters:
Timer - can give the delay of particular time between some events.
Ex: ON & OFF the lights after every 2 sec. This delay can be provided
through some assembly program.
The 2 HW pins are used – T0 & T1(2 – 16 bit timers)
-used for time delay generation (Timer mode).
-used for counting some external events/pulses (counter mode).
How much times a number is repeated in the given table is calculated by the
counter.
• With these timers, we can generate delay between 0000H to FFFFH.
• If we want to load T0 with 16 bit data then we can load separate lower 8 bit
in TL0 and higher 8 bit in TH0.
• In the same way for T1.
• TMOD, TCON registers are used for controlling timer operation.

Description of each pin :
• VCC → 5V supply
• VSS → GND
• XTAL2/XTALI are for oscillator input
• Port 0 – 32 to 39 – AD0/AD7 and P0.0 to P0.7
• Port 1 – 1 to 8 – P1.0 to P1.7
• Port 2 – 21 to 28 – P2.0 to P2.7 and A 8 to A15
• Port 3 – 10 to 17 – P3.0 to P3.7
• P 3.0 – RXD – Serial data input – SBUF
• P 3.1 – TXD – Serial data output – SBUF
• P 3.2 – INT0 – External interrupt 0 – TCON 0.1
• P 3.3 – INT1 – External interrupt 1 – TCON 0.3
• P 3.4 – T0 – External timer 0 input – TMOD
• P 3.5 – T1 – External timer 1 input – TMOD
• P 3.6 –WR – External memory write cycle – Active LOW
• P 3.7 – RD – External memory read cycle – Active LOW
• RST – for Restarting 8051
• ALE – Address latch enable
1 – Address on AD 0 to AD 7
0 – Data on AD 0 to AD 7
• PSEN – Program store enable

ALU — Arithmetic Logical Unit:
This unit is used for the arithmetic calculations.
A-Accumulator:
This register is used for arithmetic operations. This is also bit addressable and 8
bit register.
B-Register
This register is used in only two instructions MUL AB and DIV AB. This is also
bit addressable and 8 bit register.
ROM Memory Map in 8051
→ 4KB, 8KB, 16KB, 32KB, 64KB on chip ROM is available.
→ Max ROM space is 64 KB because 16 bit address line is available in
8051.
→ Starting address for ROM is 0000H (because PC which points the ROM
is 16 bit wide).

PC-Program Counter
• Points to the address of next instruction to be executed from ROM
• It is 16 bit register means the 8051 can access program address from
0000H to FFFFH. A total of 64KB of code. 16 bit register means.

8051 Flag Bits and PSW Register:
→ Used to indicate the Arithmetic condition of ACC.
→ Flag register in 8051 is called as program status word (PSW). This special
function register PSW is also bit addressable and 8 bit wide means each bit
can be set or reset independently.
There are four flags in 8051
• P → Parity flag → PSW 0.0
1 – odd number of 1 in ACC
0 – even number of 1 in ACC
• OV(PSW 0.2) → overflow flag → this is used to detect error in signed
arithmetic operation. This is similar to carry flag but difference is only that
carry flag is used for unsigned operation.

for selecting Bank 1, we use following commands
SETB PSW0.3 (means RS0=1)
CLR PSW0.4 (means RS1=0)
Initially by default always Bank 0 is selected.
• F0 → user definable bit
• AC → Auxiliary carry flag → when carry is generated from D3 to D4, it
is set to 1, it is used in BCD arithmetic.
Since carry is generated from D3 to D4, so AC is set.
• CY → carry flag → Affected after 8 bit addition and subtraction. It is used
to detect error in unsigned arithmetic opr. We can also use it as single bit
storage.
SETB C → for cy = 1
CLR C → for cy = 0

Structure of RAM or 8051 Register Bank and Stack
→ 128 byte RAM is available in 8051
→ 128 byte = 2^7 B
Address range of RAM is 00H to 7FH.
→ In MC8051, 128 byte visible or user accessible RAM. Extra 128B RAM which is
not user accessible. 80H to FFH used for storage of SFR (special function register)

→ Four Register Banks
→ There are four register banks, in each register bank there are eight 8 bit register available
from R0 to R7
→ By default Bank 0 is selected.
For selecting banks we use RS0 and RS1 bit of PSW.
→ R0 to R7 registers are byte addressable means.
If we want to set the bit 3 of R0 then we can’t use SETB R0.3
We use MOV R0, #08H;
For changing single bit we can modify all the other bits of R0.
→ Locations 20H to 2FH is bit addressable RAM means each bit from 00H to
FFH in this we can set or reset CF rather than changing whole byte.
→ Locations 30H to 7FH is used as scratch pad means we can use this space for
data reading and writing or for data storage.

Stack in 8051
→ RAM locations from 08H to 1FH can be used as stack. Stack is used to store the
data temporarily.
Stack is last in first out (LIFO)
→ Stack pointer (SP) →
• 8bit register
• It indicate current RAM address available for stack or it points the top of stack.
• Initially by default at 07H because first location of stack is 08H.
• After each PUSH instruction the SP is incremented by one while in MC after PUSH
instruction SP is decremented.
• After each POP instruction the SP is decremented.
Example:
MOV R6,#25H;
MOV R1,#12H;
MOV R4,#OF3H;
PUSH 06H;
PUSH 01H;
POP 04H;

→ if we want to use more than 24byte (08H to 1FH) of stack. We can change
SP to point RAM address 30H to 7FH by MOV SP, #XX(Any value from 30 to 7FH)
Conflicting of Register Banks and Stack
→ We know locations from 08H to 1FH is used as stack and it is also used as
register bank.
→ If in the program, we use the Register Bank 1 to 3 and also use the stack
then conflicts exist and error can be possible.
For removing this situation we use the stack from location 30H to 7FH by
shifting SP to 2FH.
MOV SP,#2FH;

DPTR → Data Pointer in 8051
→ 16 bit register, it is divided into two parts DPH and DPL.
→ DPH for Higher order 8 bits, DPL for lower order 8 bits.
→ DPTR, DPH, DPL these all are SFRs in 8051.
Special Function Register
→ RAM scratch pad, there is extra 128 byte RAM which is used to store the SFRs
→ Following figure shows special function bit address, all access to the four
I/O ports CPU register, interrupt control register, timer/counter, UART,
power control are performed through registers between 80H and FFH.

Byte Addressable SFR with byte address
SP – Stack printer – 81H
DPTR – Data pointer 2 bytes
DPL – Low byte – 82H
DPH – High byte – 83H
TMOD – Timer mode control – 89H
TH0 – Timer 0 Higher order bytes – 8CH
TL0 – Timer 0 Low order bytes – 8AH
TH1 – Timer 1 High bytes = 80H
TL1 – Timer 1 Low order byte = 86H
SBUF – Serial data buffer = 99H
PCON – Power control – 87H.

An Assembly language instruction
consists of four fields:
[label:] Mnemonic [operands] [;comment]

MOV destination, source ;copy source to dest.
The instruction tells the CPU to move (in reality, COPY) the source operand to
the destination operand

ADD A, source ;ADD the source operand ;to the accumulator
“ADD R4, A” and “ADD R2, #12H” are invalid since A must be the destination of
any arithmetic operation
ORG (origin)
􀂾 The ORG directive is used to indicate the beginning of the address
􀂾 The number that comes after ORG can be either in hex and decimal
􀂃 If the number is not followed by H, it is decimal and the assembler will
convert it to hex
END
􀂾 This indicates to the assembler the end of the source (asm) file
􀂾 The END directive is the last line of an 8051 program
􀂃 Mean that in the code anything after the END directive is ignored by the
assembler

EQU (equate)
􀂾 This is used to define a constant without occupying a memory location
􀂾 The EQU directive does not set aside storage for a data item but associates a
constant value with a data label
􀂃 When the label appears in the program, its constant value will be substituted for
the label
􀂾 Assume that there is a constant used in many different places in the program,
and the programmer wants to change its value throughout
􀂃 By the use of EQU, one can change it once and the assembler will change all of
its occurrences

Structure of Assembly Language

Steps:
• Type your program in editor of software widely used editors are notepad.
• Save this file with file name . asm } depending on assembler or file name . src
notepad produce ASCII file.
• “ASM” source file contains program code assembler converts the instruction
into machine code and produces “obj” file and “lst file” (object file and
list file).
→ Linker program takes one or more object files and produces an absolute
object file with extension “abs”.
→ “abs” file is used by 8051 trainer that have a monitor program.
→ “abs” file is fed into a program code called “OH” (object to Hex converter).
Which create a file with extension “Hex” that is ready to burn into ROM
→ lst file contains all opcode and address as well as error that assembler
detected.

Examine the list file and how the code is placed in ROM

The DB directive is the most widely used data directive in the assembler
􀂾 It is used to define the 8-bit data
􀂾 When DB is used to define data, the numbers can be in decimal,
binary, hex, ASCII formats
DATA1: DB 28
DATA2: DB 00110101B
DATA3: DB 39H

Repeating a sequence of instructions a certain number of times is
called a loop
Loop action is performed by
DJNZ reg, Label
􀂃 The register is decremented
􀂃 If it is not zero, it jumps to the target address referred to by the label
􀂃 Prior to the start of loop the register is loaded with the counter for the
number of repetitions
􀂃 Counter can be R0 – R7 or RAM location
;This program adds value 3 to the ACC ten times
MOV A,#0 ;A=0, clear ACC
MOV R2,#10 ;load counter R2=10
AGAIN: ADD A,#03 ;add 03 to ACC
DJNZ R2,AGAIN ;repeat until R2=0,10 times
MOV R5,A ;save A in R5

􀂉 Jump only if a certain condition is met
JNC label ;jump if no carry, CY=0
􀂾 If CY = 0, the CPU starts to fetch and execute instruction from the
address of the label
􀂾 If CY = 1, it will not jump but will execute the next instruction below
JNC

The unconditional jump is a jump in which control is transferred
unconditionally to the target location
LJMP (long jump)
􀂾 3-byte instruction
􀂃 First byte is the opcode
􀂃 Second and third bytes represent the 16-bit target address
– Any memory location from 0000 to FFFFH
SJMP (short jump)
􀂃 Second byte is the relative target address
– 00 to FFH (forward +127 and backward -128 bytes from the current
PC)

Call instruction is used to call subroutine
􀂾 Subroutines are often used to perform tasks that need to be performed
frequently
􀂾 This makes a program more structured in addition to saving memory
space
LCALL (long call)
􀂃 Second and third bytes are used for address of target subroutine
– Subroutine is located anywhere within 64K byte address space
ACALL (absolute call)
􀂃 11 bits are used for address within 2K-byte range

􀂉 When a subroutine is called, control is transferred to that subroutine,
the processor
􀂾 Saves on the stack the the address of the instruction immediately
below the LCALL
􀂾 Begins to fetch instructions form the new location
􀂉 After finishing execution of the subroutine
􀂾 The instruction RET transfers control back to the caller
􀂃 Every subroutine needs RET as the last instruction

CPL A ;complements the register A
This is called 1’s complement
Note:
To get the 2’s complement, all we have to do is to to add 1 to the 1’s
complement

CJNE destination, source, rel. addr.
􀂉 The actions of comparing and jumping are combined into a single
instruction called CJNE (compare and jump if not equal)
􀂾 The CJNE instruction compares two operands, and jumps if they are
not equal
􀂾 The destination operand can be in the accumulator or in one of the
Rn registers

ADDRESSING MODES
The CPU can access data in various ways, which are called addressing modes
􀂾 Immediate
􀂾 Register
􀂾 Direct (Accessing Memory)
􀂾 Register indirect (Accessing Memory)
􀂾 Indexed (Accessing Memory)
Examples of immediate Addressing mode
MOV A, #25H ;load 25H into A
MOV R4, #62 ;load 62 into R4
MOV P1, #55H(We can also use immediate addressing mode to send data to
8051 ports)
Examples of register addressing mode
MOV A,R0 ;copy contents of R0 into A
MOV R2,A ;copy contents of A into R2
-> The source and destination registers must match in size
-> The movement of data between Rn registers is not allowed

Examples of Direct Addressing Mode
MOV A,4 ;is same as
MOV A,R4 ;which means copy R4 into A
Only direct addressing mode is allowed for pushing or popping
the stack
Register Indirect Addressing Mode
􀂉 A register is used as a pointer to the data
􀂾 Only register R0 and R1 are used for this purpose
􀂾 R2 – R7 cannot be used to hold the address of an operand
located in RAM
􀂉 When R0 and R1 hold the addresses of RAM locations, they must
be preceded by the “@” sign
􀂾 Whether accessing externally connected RAM or on-chip ROM,
we need 16-bit pointer
􀂾 In such case, the DPTR register is used

Indexed Addressing Mode and On-chip ROM Access
􀂉 Indexed addressing mode is widely used in accessing data
elements of look-up table entries located in the program ROM
􀂉 The instruction used for this purpose is MOVC A,@A+DPTR
􀂾 Use instruction MOVC, “C” means code
􀂾 The contents of A are added to the 16-bit register DPTR to form the
16-bit address of the needed data

Example
In this program, assume that the word “USA” is burned into ROM locations starting
at 200H. And that the program is burned into ROM locations starting at 0. Analyze
how the program works and state where “USA” is stored after this program is run.
Solution:
ORG 0000H ;burn into ROM starting at 0
MOV DPTR,#200H ;DPTR=200H look-up table addr
CLR A ;clear A(A=0)
MOVC A,@A+DPTR ;get the char from code space
MOV R0,A ;save it in R0
INC DPTR ;DPTR=201 point to next char
CLR A ;clear A(A=0)
MOVC A,@A+DPTR ;get the next char
INC DPTR ;DPTR=202 point to next char
CLR A ;clear A(A=0)
MOVC A,@A+DPTR ;get the next char
Here: SJMP HERE ;stay here
;Data is burned into code space starting at 200H
ORG 200H
MYDATA:DB “USA”
END ;end of program

WHY PROGRAM 8051 IN C
􀂉 Compilers produce hex files that is downloaded to ROM of microcontroller
􀂾 The size of hex file is the main concern
􀂃 Microcontrollers have limited on-chip ROM
􀂃 Code space for 8051 is limited to 64K bytes
􀂉 C programming is less time consuming, but has larger hex file size
􀂉 The reasons for writing programs in C
􀂾 It is easier and less time consuming to write in C than Assembly
􀂾 C is easier to modify and update
􀂾 You can use code available in function libraries
􀂾 C code is portable to other microcontroller with little of no modification
DATA TYPES
􀂉 A good understanding of C data types for 8051 can help programmers to
create smaller hex files
􀂾 Unsigned char
􀂾 Signed char
􀂾 Unsigned int
􀂾 Signed int
􀂾 Sbit (single bit)
􀂾 Bit and sfr

􀂉 There are two ways to create a time delay in 8051 C
􀂾 Using the 8051 timer
􀂾 Using a simple for loop be mindful of three factors that can affect the
accuracy of the delay
􀂃 The 8051 design
– The number of machine cycle
– The number of clock periods per machine cycle
􀂃 The crystal frequency connected to the X1 – X2 input pins
􀂃 Compiler choice
– C compiler converts the C statements and functions to
Assembly language instructions
– Different compilers produce different code

CPU executing an instruction takes a certain number of clock cycles
􀂾 These are referred as to as machine cycles
􀂉 The length of machine cycle depends on the frequency of the crystal
oscillator connected to 8051
􀂉 In original 8051, one machine cycle lasts 12 oscillator periods
if an instruction takes one machine cycle to execute, it will take 12 pulses of the
crystal to execute. Since we know the crystal is pulsing 11,059,000 times per
second and that one machine cycle is 12 pulses, we can calculate how many
instruction cycles the 8051 can execute per second:
11,059,000 / 12 = 921,583
This means that the 8051 can execute 921,583 single-cycle instructions per second.

􀂉 Logical operators
􀂾 AND (&&), OR (||), and NOT (!)
􀂉 Bit-wise operators
􀂾 AND (&), OR (|), EX-OR (^), Inverter (~), Shift Right (>>), and Shift Left (<<)
􀂃 These operators are widely used in software engineering for embedded
systems and control

The 8051 C compiler allocates RAM locations
􀂾 Bank 0 – addresses 0 – 7
􀂾 Individual variables – addresses 08 and beyond
􀂾 Array elements – addresses right after variables
􀂃 Array elements need contiguous RAM locations and that limits the size of the array
due to the fact that we have only 128 bytes of RAM for everything
􀂾 Stack – addresses right after array elements

􀂉 The 8051 has an on-chip oscillator but requires an external clock to run it
􀂾 A quartz crystal oscillator is connected to inputs XTAL1 (pin19) and XTAL2
(pin18)
􀂃 The quartz crystal oscillator also needs two capacitors of 30 pF value
􀂉 If you use a frequency source other than a crystal oscillator, such as a TTL
oscillator
􀂾 It will be connected to XTAL1
􀂾 XTAL2 is left unconnected

INTERFACING LCD TO 8051
􀂉 LCD is finding widespread use replacing LEDs
􀂾 The declining prices of LCD
􀂾 The ability to display numbers, characters, and graphics
􀂾 Incorporation of a refreshing controller into the LCD, thereby relieving the CPU
of the task of refreshing the LCD
􀂾 Ease of programming for characters and graphics

Above is the quite simple schematic. The LCD
panel's Enable and Register Select is connected to the Control Port. The
Control Port is an open collector / open drain output. While most Parallel
Ports have internal pull-up resistors, there are a few which don't. Therefore
by incorporating the two 10K external pull up resistors, the circuit is more
portable for a wider range of computers, some of which may have no
internal pull up resistors.
We make no effort to place the Data bus into reverse direction. Therefore
we hard wire the R/W line of the LCD panel, into write mode. This will cause
no bus conflicts on the data lines. As a result we cannot read back the
LCD's internal Busy Flag which tells us if the LCD has accepted and
finished processing the last instruction. This problem is overcome by
inserting known delays into our program.
The 10k Potentiometer controls the contrast of the LCD panel. Nothing
fancy here. As with all the examples, I've left the power supply out. You can
use a bench power supply set to 5v or use a onboard +5 regulator.
Remember a few de-coupling capacitors, especially if you have trouble with
the circuit working properly.

 The 2 line x 16 character LCD modules are available from a wide
range of manufacturers and should all be compatible with the
HD44780. The one I used to test this circuit was a Power tip PC-
1602F and an old Philips LTN211F-10 which was extracted from a
Poker Machine! The diagram to the right, shows the pin numbers for
these devices. When viewed from the front, the left pin is pin 14 and
the right pin is pin 1.
Logic status on control lines:
• E - 0 Access to LCD disabled
- 1 Access to LCD enabled
• R/W - 0 Writing data to LCD
- 1 Reading data from LCD
• RS - 0 Instructions
•1 Character

Microcontroller 8051 features and applications

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