UART project report by Tarun Khaneja ( 09034406598 )

1
A
Summer Training Report On
Embedded System Design using
8051 based microcontroller
EMSEEKER (DESIGNNOVA)
Submitted to: Submitted by:
Er. Nidhi Uppal Tarun Khaneja
A. P. & Head 09034406598
3rd
year
Under the guidance of
Er. Sumeet Pahwa
Director of EMSEEKER
Department of Electronics & Communication Engineering
Doon Valley Institute of Engineering & Technology
Karnal- Haryana
(Aug. 2012)

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Chapter-1
1.1 The UART: What it is and how it works
A UART (Universal Asynchronous Receiver/Transmitter) is the microchip with programming that
controls a computer's interface to its attached serial devices. Specifically, it provides the computer
with the RS-232C Data Terminal Equipment (DTE) interface so that it can "talk" to and exchange
data with modems and other serial devices. As part of this interface, the UART also:
Converts the bytes it receives from the computer along parallel circuits into a single serial bit stream
for outbound transmission
On inbound transmission, converts the serial bit stream into the bytes that the computer handles
Adds a parity bit (if it's been selected) on outbound transmissions and checks the parity of incoming
bytes (if selected) and discards the parity bit
Adds start and stop delineators on outbound and strips them from inbound transmissions
Handles interrupts from the keyboard and mouse (which are serial devices with special ports)
May handle other kinds of interrupt and device management that require coordinating the
computer's speed of operation with device speeds
Serial transmission is commonly used with modems and for non-networked communication between
computers, terminals and other devices.
The communications links across which computers—or parts of computers—talk to one another may
be either serial or parallel. A parallel link transmits several streams of data
(perhaps representing particular bits of a stream of bytes) along multiple channels (wires, printed
circuit tracks, optical fibres, etc.); a serial link transmits a single stream of data.
Figure 1.1
At first sight it would seem that a serial link must be inferior to a parallel one, because it can transmit
less data on each clock tick. However, it is often the case that serial links can be clocked considerably

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faster than parallel links, and achieve a higher data rate. A number of factors allow serial to be
clocked at a greater rate:
Clock skew between different channels is not an issue (for unlocked serial links)
A serial connection requires fewer interconnecting cables (e.g. wires/fibres) and hence occupies less
space. The extra space allows for better isolation of the channel from its surroundings
Crosstalk is less of an issue, because there are fewer conductors in proximity.
In many cases, serial is a better option because it is cheaper to implement. Many ICs have serial
interfaces, as opposed to parallel ones, so that they have fewer pins and are therefore cheaper.
Figure 1.2
In telecommunications and computer science, serial communications is the process of sending data
one bit at one time, sequentially, over a communications channel or computer bus. This is in contrast
to parallel communications, where all the bits of each symbol are sent together. Serial
communications is used for all long-haul communications and most computer networks, where the
cost of cable and synchronization difficulties makes parallel communications impractical. Serial
computer buses are becoming more common as improved technology enables them to transfer data at
higher speeds.
1.2 Serial versus parallel
The Serial Port is harder to interface than the Parallel Port. In most cases, any device you connect to
the serial port will need the serial transmission converted back to parallel so that it can be used. This
can be done using a UART. On the software side of things, there are many more registers that you
have to attend to than on a Standard Parallel Port. (SPP)

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So what are the advantages of using serial data transfer rather than parallel?
1. Serial Cables can be longer than Parallel cables. The serial port transmits a '1' as -3 to - 25 volts and a
'0' as +3 to +25 volts where as a parallel port transmits a '0' as 0v and a '1' as 5v. Therefore the serial
port can have a maximum swing of 50V compared to the parallel port which has a maximum swing of
5 Volts. Therefore cable loss is not going to be as much of a problem for serial cables than they are
for parallel.
2. You don't need as many wires than parallel transmission. If your device needs to be mounted a far
distance away from the computer then 3 core cable (Null Modem Configuration) is going to be a lot
cheaper that running 19 or 25 core cable. However you must take into account the cost of the
interfacing at each end.
3. Infra Red devices have proven quite popular recently. You may of seen many electronic diaries and
palmtop computers which have infra red capabilities build in. However could you imagine
transmitting 8 bits of data at the onetime across the room and being able to (from the devices point of
view) decipher which bits are which? Therefore serial transmission is used where one bit is sent at a
time. IrDA-1 (The first infra red specifications) was capable of 115.2k baud and was interfaced into a
UART. The pulse length however was cut down to 3/16th of a RS232 bit length to conserve power
considering these devices are mainly used on diaries, laptops and palmtops.
4. Microcontrollers have also proven to be quite popular recently. Many of these have in built SCI
(Serial Communications Interfaces) which can be used to talk to the outside world. Serial
Communication reduces the pin count of these MPU's. Only two pins are commonly used, Transmit
Data (TXD) and Receive Data (RXD) compared with at least 8 pins if you use a 8 bit Parallel method
(You may also require a Strobe).
There are two primary forms of serial transmission: Synchronous and Asynchronous. Depending on
the modes that are supported by the hardware, the name of the communication sub-system will
usually include a A if it supports Asynchronous communications, and a S if it supports Synchronous
communications. Both forms are described below.
Figure 1.3
1.3 Synchronous Serial Transmission
Synchronous serial transmission requires that the sender and receiver share a clock with one another,
or that the sender provide a strobe or other timing signal so that the receiver knows when to “read” the
next bit of the data. In most forms of serial Synchronous communication, if there is no data available
at a given instant to transmit, a fill character must be sent instead so that data is always being
transmitted. Synchronous communication is usually more efficient because only data bits are
transmitted between sender and receiver, and synchronous communication can be more costly if extra
wiring and circuits are required to share a clock signal between the sender and receiver. A form of

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Synchronous transmission is used with printers and fixed disk devices in that the data is sent on one
set of wires while a clock or strobe is sent on a different wire. Printers and fixed disk devices are not
normally serial devices because most fixed disk interface standards send an entire word of data for
each clock or strobe signal by using a separate wire for each bit of the word. In the PC industry, these
are known as Parallel devices. The standard serial communications hardware in the PC does not
support Synchronous operations. This mode is described here for comparison purposes only.
Figure 1.4 serial cable
1.4 Asynchronous Serial Transmission
Asynchronous serial communication describes an asynchronous transmission protocol in which a start
signal is sent prior to each byte, character or code word and a stop signal is sent after each code word.
The start signal serves to prepare the receiving mechanism for the reception and registration of a
symbol and the stop signal serves to bring the receiving mechanism to rest in preparation for the
reception of the next symbol. A common kind of start-stop transmission is ASCII over RS-232, for
example for use in teletypewriter operation.
In the diagram, a start bit is sent, followed by eight data bits, no parity bit and one stop bit, for a10-
bit character frame. The number of data and formatting bits, and the transmission speed, must be pre

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Agreed by the communicating parties. After the stop bit, the line may remain idle indefinitely, or
another character may immediately be started.
1.5 Bits, Baud and Symbols
Baud is a measurement of transmission speed in asynchronous communication. Because of advances
in modem communication technology, this term is frequently misused when describing the data rates
in newer devices. Traditionally, a Baud Rate represents the number of bits that are actually being sent
over the media, not the amount of data that is actually moved from one DTE device to the other. The
Baud count includes the overhead bits Start, Stop and Parity that are generated by the sending UART
and removed by the receiving UART. This means that seven-bit words of data actually take 10 bits to
be completely transmitted. Therefore, a modem capable of moving 300 bits per second from one place
to another can normally only move 307-bit words if Parity is used and one Start and Stop bit are
present. If 8-bit data words are used and Parity bits are also used, the data rate falls to 27.27 words per
second, because it now takes 11 bits to send the eight-bit words, and the modem still only sends 300
bits per second. The formula for converting bytes per second into a baud rate and vice versa was
simple until error- correcting modems came along. These modems receive the serial stream of bits
from the UART in the host computer (even when internal modems are used the data is still frequently
serialized) and converts the bits back into bytes. These bytes are then combined into packets and sent
over the phone line using a Synchronous transmission method. This means that the Stop, Start, and
Parity bits added by the UART in the DTE (the computer) were removed by the modem before
transmission by the sending modem. When these bytes are received by the remote modem, the remote
modem adds Start, Stop and Parity bits to the words, converts them to a serial format and then sends
them to the receiving UART in the remote computer, who then strips the Start, Stop and Parity bits.
The reason all these extra conversions are done is so that the two modems can perform error
correction, which means that the receiving modem is able to ask the sending modem to resend a block
of data that was not received with the correct checksum. This checking is handled by the modems, and
the DTE devices are usually unaware that the process is occurring. By striping the Start, Stop and
Parity bits, the additional bits of data that the two modems must share between themselves to
perform error-correction are mostly concealed from the effective transmission rate seen by the
sending and receiving DTE equipment. For example, if a modem sends ten 7-bit words to another
modem without including the Start, Stop and Parity bits, the sending modem will be able to add 30
bits of its own information that the receiving modem can use to do error-correction without impacting
the transmission speed of the real data. The use of the term Baud is further confused by modems that
perform compression. A single 8-bit word passed over the telephone line might represent a dozen
words that were transmitted to the sending modem. The receiving modem will expand the data back to
its original content and pass that data to the receiving DTE. Modern modems also include buffers that
allow the rate that bits move across the phone line (DCE to DCE) to be a different speed than the
speed that the bits move between the DTE and DCE on both ends of the conversation. Normally the
speed between the DTE and DCE is higher than the DCE to DCE speed because of the use of
compression by the modems. Because the number of bits needed to describe a byte varied during the
trip between the two machines plus the differing bits-per-seconds speeds that are used present on
the DTE-DCE and DCE-DCE links, the usage of the term Baud to describe the overall
communication speed causes problems and can misrepresent the true transmission speed. So Bits Per
Second (bps) is the correct term to use to describe the transmission rate seen at the DCE to DCE
interface and Baud or Bits Per Second are acceptable terms to use when a connection is made between
two systems with a wired connection, or if a modem is in use that is not performing error-
correction or compression. Modern high speed modems (2400, 9600, 14,400, and 19,200bps) in
reality still operate at or below 2400 baud, or more accurately, 2400 Symbols per second. High speed
modem are able to encode more bits of data into each Symbol using a technique called Constellation
Stuffing, which is why the effective bits per second rate of the modem is higher, but the modem
continues to operate within the limited audio bandwidth that the telephone system provides. Modems
operating at 28,800 and higher speeds have variable Symbol rates, but the technique is the same.

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1.6 Asynchronous Serial Reception
A multiplexed data communication pulse can only be in one of two states but there are many names
for the two states. When on, circuit closed, low voltage, current flowing, or a logical zero, the pulse is
said to be in the "space" condition. When off, circuit open, high voltage, current stopped, or a logical
one, the pulse is said to be in the "mark" condition. A character code begins with the data
communication circuit in the space condition. If the mark condition appears, a logical one is recorded
otherwise a logical zero.
Figure. 1.5 Asynchronous Code Format.
The least-significant bit is always transmitted first. If parity is present, the parity bit comes after the
data bits but before the stop bit(s).
The start bit is always a '0' (logic low), which is also called a space. Ironically, the logic low '0'
corresponds to a high voltage on the data wire. The start bit signals the receiving DTE that a character
code is coming. The next five to eight bits, depending on the code set employed, represent the
character. In the ASCII code set the eighth data bit may be a parity bit. The next one or two bits are
always in the mark (logic high, i.e., '1') condition and called the stop bit(s). They provide a "rest"
interval for the receiving DTE so that it may prepare for the next character which may be after the
stop bit(s). The rest interval was required by the old mechanical Teletypes which used a motor driven
camshaft to
decode each character. At the end of each character the motor needed time to strike the character bail
(print the character) and reset the camshaft.
There are six basic steps in receiving a serial character code into a parallel register. First, to keep track
of time, the receiver employs a clock which "ticks." When the line is in the space condition, the
receiver samples the line 16 times the data rate. In other words, a data interval is equal to 16 clock
ticks. In this way the receiver can determine the beginning of the start bit and "move over" to the
center of the bit time for data sampling. Second, when the line goes into the mark state, declare a
"looking for start bit" condition and wait one half the bit interval or eight clock ticks. Third, sample
the line again and if it has not remained in the mark condition, consider this to be a spurious voltage
change and go back to step one. Fourth, if the line was still in the mark state, then consider this a valid
start bit. Shift the start bit into an eight-bit shift register and wait one bit time or 16 clock ticks. Fifth,
after one bit time sample the line (the data should have been there for the last eight clock ticks, and
should remain for eight more clock ticks). Now shift the sample into the shift register. Sixth, continue
start|<-five to eight databits ->| stop bit(s)
0---- -- - - - - - - - - -
Space(logic low, high data-wire voltage)
| | || | | || | | | |
| S | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | S | S |
| | || | | || | | | |
1 - - - - - - - -- - - --------
Mark(logic high, low data-wirevoltage)

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steps four and five seven more times. After the eighth shift, the start bit will "migrate" into a flip-
flop indicating character received. Go to step one.
Before the transmitter and receiver UARTs will work, they must also agree on the same values of five
parameters. First, both sides must agree on the number of bits per character. Second, the speed or
Baud of the line must be the same on both sides. Third, both sides must agree to use or not use parity.
Fourth, if parity is used, both sides must agree on using odd or even parity. Fifth, the number of stop
bits must be agreed upon. Having said all this, most DTEs today employ eight data bits, no parity, and
one stop bit. Thus there is a rule-of-thumb that the number of characters per second is equal to the
Baud divided by 10 for a typical RS-232 or RS-423data line.
1.7 Other UART Functions
In addition to the basic job of converting data from parallel to serial for transmission and from serial
to parallel on reception, a UART will usually provide additional circuits for signals that can be used to
indicate the state of the transmission media, and to regulate the flow of data in the event that the
remote device is not prepared to accept more data. For example, when the device connected to the
UART is a modem, the modem may report the presence of a carrier on the phone line while the
computer may be able to instruct the modem to reset itself or to not take calls by raising or lowering
one more of these extra signals. The function of each of these additional signals is defined in the
EIA RS232-C standard.

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Chapter-2
The RS-232 Standard
2.1 The RS232-C and V.24 Standards
In most computer systems, the UART is connected to circuitry that generates signals that comply with
the EIA RS232-C specification. There is also a CCITT standard named V.24 that mirrors the
specifications included in RS232-C.
Figure 2.1 RS 232

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2.2 RS232-C Bit Assignments (Marks and Spaces)
In RS232-C, a value of 1 is called a Mark and a value of 0 is called a Space. When a communication
line is idle, the line is said to be “Marking”, or transmitting continuous 1 values.
The Start bit always has a value of 0 (a Space). The Stop Bit always has a value of 1 (a Mark). This
means that there will always be a Mark (1) to Space (0) transition on the line at the start of every
word, even when multiple word are transmitted back to back. This guarantees that sender and receiver
can resynchronize their clocks regardless of the content of the data bits that are being transmitted.
The idle time between Stop and Start bits does not have to be an exact multiple (including zero) of the
bit rate of the communication link, but most UARTs are designed this way for simplicity.
In RS232-C, the "Marking" signal (a 1) is represented by a voltage between -2 VDC and -12VDC,
and a "Spacing" signal (a 0) is represented by a voltage between 0 and +12 VDC. The transmitter is
supposed to send +12 VDC or -12 VDC, and the receiver is supposed to allow for some voltage loss
in long cables. Some transmitters in low power devices (like portable computers) sometimes use only
+5 VDC and -5 VDC, but these values are still acceptable to aRS232-C receiver, provided that the
cable lengths are short.
2.3 RS232-C Break Signal
RS232-C also specifies a signal called a Break, which is caused by sending continuous Spacing values
(no Start or Stop bits). When there is no electricity present on the data circuit, the line is considered to
be sending Break.The Break signal must be of a duration longer than the time it takes to send a
complete byte plus Start, Stop and Parity bits. Most UARTs can distinguish between a Framing Error
and a Break, but if the UART cannot do this, the Framing Error detection can be used to identify
Breaks.In the days of teleprinters, when numerous printers around the country were wired in series
(such as news services), any unit could cause a Breakby temporarily opening the entire circuit so that
no current flowed. This was used to allow a location with urgent news to interrupt some other location
that was currently sending information.In modern systems there are two types of Break signals. If the
Break is longer than 1.6 seconds, it is considered a "Modem Break", and some modems can be
programmed to terminate the conversation and go on-hook or enter the modems' command mode
when the modem detects this signal. If the Break is smaller than 1.6 seconds, it signifies a Data Break
and it is up to the remote computer to respond to this signal. Sometimes this form of Break is used as
an Attention or Interrupt signal and sometimes is accepted as a substitute for the ASCIICONTROL-
C character.Marks and Spaces are also equivalent to “Holes” and “No Holes” in paper tape systems.
Note: Breaks cannot be generated from paper tape or from any other byte value, since bytes are
always sent with Start and Stop bit. The UART is usually Spacing signal in response to a special
command from the host processor.
2.4 RS232-C DTE and DCE Devices
The RS232-C specification defines two types of equipment: the Data Terminal Equipment (DTE) and
the Data Carrier Equipment (DCE). Usually, the DTE device is the terminal (or computer), and the
DCE is a modem. Across the phone line at the other end of a conversation, the receiving modem is
also a DCE device and the computer that is connected to that modem is a DTE device. The DCE
device receives signals on the pins that the DTE device transmits on, and vice versa. When two

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devices that are both DTE or both DCE must be connected together without a modem or a similar
media translator between them, a NULL modem must be used. The NULL modem electrically re-
arranges the cabling so that the transmitter output is connected to the receiver input on the other
device, and vice versa. Similar translations are performed on all of the control signals so that each
device will see what it thinks are DCE (or DTE) signals from the other device. The number of signals
generated by the DTE and DCE devices is not symmetrical. The DTE device generates fewer signals
for the DCE device than the DTE device receives from the DCE.
Table 2.1
2.5 RS232-C Pin Assignments
The EIA RS232-C specification (and the ITU equivalent, V.24) calls for a twenty-five pin connector
(usually a DB25) and defines the purpose of most of the pins in that connector.
In the IBM Personal Computer and similar systems, a subset of RS232-C signals is provided via nine
pin connectors (DB9). The signals that are not included on the PC connector deal mainly with
synchronous operation, and this transmission mode is not supported by the UART that IBM selected
for use in the IBM PC.
Depending on the computer manufacturer, a DB25, a DB9, or both types of connector may be used
for RS232-C communications. (The IBM PC also uses a DB25 connector for the parallel printer
interface which causes some confusion.)

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Chapter-3
8051 Microcontroller
The most commonly used microcontroller is 8051 families’ AT89C51 microcontroller which is
produced by Atmel. It is widely used in most of the application for having an advantage of simple
programming and low cost.
3.1 AT89C51
AT89C51 is an 8-bit, 40 pin microcontroller that belongs to Atmel's 8051 family. ATMEL
89C51 has 4KB of Flash programmable and erasable read only memory (PEROM) and 128 bytes of
RAM. It can be erased and program to a maximum of 1000 times.
In 40 pin AT89C51, there are four ports designated as P1, P2, P3 and P0. All these ports are 8-bit bi-
directional ports, i.e., they can be used as both input and output ports. Except P0 which needs external
pull-ups, rest of the ports have internal pull-ups. When 1s are written to these port pins, they are
pulled high by the internal pull-ups and can be used as inputs. These ports are also bit addressable and
so their bits can also be accessed individually.
3.1.1 Salient Features of AT89C51-
 4K Bytes of In-System Reprogrammable Flash Memory
 Fully Static Operation: 0 Hz to 24 MHz
 Three-level Program Memory Lock
 28 x 8-bit Internal RAM
 32 Programmable I/O Lines
 Two 16-bit Timer/Counters
 Six Interrupt Sources
 Programmable Serial Channel
 Low-power Idle and Power-down Modes
 40-pin DIP

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3.1.2 Pin Diagram of AT89C51
Figure 3.1
5.1.3 Pin Description of AT89C51
Pins 1-8 (Port 1) - Each of these pins can be configured as an input or an output.
Pin 9 (Reset) - A logic one on this pin disables the microcontroller and clears the contents of most
registers. In other words, the positive voltage on this pin resets the microcontroller. By applying logic
zero to this pin, the program starts execution from the beginning.
Pins 10- 17 (Port 3) -Similar to port 1, each of these pins can serve as general input or output.
Besides, all of them have alternative functions:

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Pin 10 (RXD) - Serial asynchronous communication input or Serial synchronous communication
output.
Pin 11(TXD) - Serial asynchronous communication output or Serial synchronous communication
clock output.
Pin 12 (INT 0) - Interrupt 0 input.
Pin 13(INT 1) - Interrupt 1 input.
Pin 14(T0) - Counter 0 clock input.
Pin 15(T1) - Counter 1 clock input.
Pin 16(WR) -Write to external (additional) RAM.
Pin 17 (RD) -Read from external RAM.
Pin 18 and 19(X1, X2) - Internal oscillator input and output. A quartz crystal which specifies
operating frequency is usually connected to these pins. Instead of it, miniature ceramics resonators
can also be used for frequency stability. Later versions of microcontrollers operate at a frequency of 0
Hz up to over 50 Hz.
Pin 20 (GND) - Ground.
Pin 21-28 (Port 2) - If there is no intention to use external memory then these port pins are
configured as general inputs/outputs. In case external memory is used, the higher address byte, i.e.
addresses A8-A15 will appear on this port. Even though memory with capacity of 64Kb is not used,
which means that not all eight port bits are used for its addressing, the rest of them are not available as
inputs/outputs.
Pin 29 (PSEN) - If external ROM is used for storing program then a logic zero (0) appears on it every
time the microcontroller reads a byte from memory.
Pin 30 (ALE) - Prior to reading from external memory, the microcontroller puts the lower address
byte (A0-A7) on P0 and activates the ALE output. After receiving signal from the ALE pin, the
external register memorizes the state of P0 and uses it as a memory chip address. Immediately after
that, the ALU pin is returned its previous logic state and P0 is now used as a Data Bus. As seen, port
data multiplexing is performed by means of only one additional (and cheap) integrated circuit. In
other words, this port is used for both data and address transmission.
Pin 31 (EA) - By applying logic zero to this pin, P2 and P3 are used for data and address transmission
with no regard to whether there is internal memory or not. It means that even there is a program
written to the microcontroller, it will not be executed. Instead, the program written to external ROM
will be executed. By applying logic one to the EA pin, the microcontroller will use both memories,
first internal then external (if exists).
Pin 32-39 (Port 0) - Similar to P2, if external memory is not used, these pins can be used as general
inputs/outputs. Otherwise, P0 is configured as address output (A0-A7) when the ALE pin is driven
high (1) or as data output (Data Bus) when the ALE pin is driven low (0).
Pin 40 (Vcc) - +5V power supply.

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Architecture of 8051 Microcontroller
6.1 Block Diagram of 8051 Microcontroller
Figure 3.2
Address bus-For a device (memory or I/O) to be recognized by the CPU, it must beassigned an
address. The address assigned to a given device must be unique. The CPU puts the address on the
address bus, and the decoding circuitry finds the device.
Data bus-The CPU either gets data from the deviceor sends data to it.
Control bus-Provides read or write signals to the device to indicate if the CPU is asking for
information or sending it information .
3.2 Memory and Registers
The 8051 microcontroller has a total of 256 bytes of RAM in which 128 is visible or useraccessible
and extra 128 is for special function registers.
The user accessible RAM is used for temporary data storage. The user accessible RAM is from the
address range 00 to 7Fh.
From the user accessible RAM, 32 bytes of RAM is used for registers and rest for Stack operations.
The 32 Bytes of RAM is divided into four register Banks i.e. Bank0, Bank 1, Bank 2, Bank3. Each of
these banks have 8 Registers i.e. R0 to R7 each.

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RAM locations from 0 to 7 are set aside for bank 0 of R0 – R7 where R0 is RAM location 0, Rl is
RAM location 1, and R2 is location 2, and so on, until memory location 7, which belongs to R7 of
bank 0. The second bank of registers R0 – R7 starts at RAM location 08 and goes to location 0FH.
The third bank of R0 – R7 starts at memory location 10H and goes to location 17H. Finally, RAM
locations 18H to 1FH are set aside for the fourth bank of R0 – R7.
Figure 3.3
Generally for normal operations, Register bank Bank0 is set by default. But we can switch to other
banks by using PSW Commands.
Figure 3.4
3.2.1 SFRs (Special Function Register) - These Registers are in extra 128 bytes of the memory.
This part of memory is not user accessible and these registers are used for special purposes. These
registers range from 80h to FFh. There are a total of only 21 SFRs in this range and all other addresses
from 80h to FFh are invalid and there use can cause errors and not valuable results.
Some of the SFRs are TCON, SBUF, ACC, B, SCON, TMOD SP, P0, PSW, TL0, and TL1. These all
the registers have some specific function that has to be performed after they are programmed.
(i) 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

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TH1 – Timer 1 High bytes = 80H
TL1 – Timer 1 Low order byte = 86H
SBUF – Serial data buffer = 99H
PCON – Power control – 87H.
3.2.2 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.
3.3 Flags and PSW (Program Status Word) Register in 8051
 The program status word (PSW)register, also referred to as the flag register, is an 8 bit
register.
 Only 6 bits are used These four are CY (carry), AC (auxiliary carry), P(parity), and OV
(overflow) They are called conditional flags, meaning that they indicate some conditions that
resulted after an instruction was execute.
 The PSW3 and PSW4 are designed as RS0 and RS1, and are used to change the bank.
 The two unused bits are user-definable.
PSW 7 PSW 6 PSW 5 PSW 4 PSW 3 PSW 2 PSW 1 PSW 0
Figure 3.5
CY- PSW.7- Carry flag.
AC- PSW.6- Auxiliary Carry flag.
F0 (-----) - PSW.5- Available to the user for general purpose
RS1 -PSW.4 - Register Bank selector bit 1.
RS0- PSW.3 -Register Bank selector bit 0.
OV -PSW.2 -Overflow flag.
F0 (-----) - PSW.1- User definable bit.
P- PSW.0 -Parity flag. Set/cleared by hardware each.
3.4 Instructions that affects Flags
C A F0 RS1 RS0 OV --------- P
Instructions CY OV AC
ADD X X X
ADDC X X X
SUBB X X X
MUL 0 X
DIV 0 X
DA X
RPC X
PLC X
SETB C 1
CLR C 0
CPL C X

18
Table 3.1
3.5 I/O PROGRAMMING
 The four 8-bit I/O ports P0, P1, P2 and P3 each use 8 pins.
 All the ports upon RESET are configured as input, ready to be used as input ports.
When the first 0 is written to a port, it becomes an output.
To reconfigure it as an input, a 1 must be sent to the port.
To use any of these ports as an input port, it must be programmed.
3.5.1 Port 0
 It can be used for input or output; each pin must be connected externally to a 10K ohm pull-
up resistor.
 This is due to the fact that P0 is an open drain, unlike P1, P2, and P3.
 Open drain is a term used for MOS chips in the same way that open collector is used for TTL
chips.
Figure 3.6
 In order to make port 0 an input, the port must be programmed by writing 1 to all the bits.
 Port 0 is also designated as AD0-AD7, allowing it to be used for both address and data.
3.5.2 Port 1
 Port 1 can be used as input or output.
 In contrast to port 0, this port does not need any pull-up resistors since it already has pull-up
resistors internally.
ANL C, bit X
ANL C, /bit X
ORL C,/bit X
ORL C, bit X
MOV C,bit X
CJNE X

19
 Upon reset, port 1 is configured as an input port.
 To make port 1 an input port, it must be programmed as such by writing 1 to all its bits.
3.5.3 Port 2
 Just like P1, port 2 does not need any pull-up resistors since it already has pull-up resistors
internally.
 Upon reset, port 2 is configured as an input portP0.4.
 Port 2 is also designated as A8 – A15, indicating its dual function. Port 0 provides the lower 8
bits via A0 – A7.
3.5.3 Port 3
 Port 3 does not need any pull-up resistors.
 Port 3 is configured as an input port upon reset; this is not the way it is most commonly used.
 Port 3 has the additional function of providing some extremely important signals
Serial Communication
External Interrupts
Timers
Read/ Write Signals
Table 3.2
3.6 TIMERS AND COUNTERS
3.6.1 Timers
The 8051 comes equipped with two timers, both of which may be controlled, set, read, and configured
individually. The 8051 timers have three general functions:
1) Keeping time and/or calculating the amount of time between events,
2) Counting the events themselves,
3) Generating baud rates for the serial port.
Both Timer 0 and Timer 1 are 16 bits wide.
P3 Function Pin
P3.0 Rxd 10
P3.1 Txd 11
P3.2
Int0
12
P3.3
Int1
13
P3.4 T0 14
P3.5 T1 15
P3.6
Wr
16
P3.7
Rd
17

20
Since 8051 has an 8-bit architecture, each 16-bits timer is accessed as two separate registers of low
byte and high byte.
One timer is TIMER0 and the other is TIMER1. The two timers share two SFRs (TMOD and TCON)
which control the timers, and each timer also has two SFRs dedicated to itself (TH0/TL0 and
TH1/TL1).
The upper higher bits are TH0 and TH1 and the lower bits are TL0 AND TL1
The TMOD and TCON are two control registers for the two timers.
Figure 3.7
(i) TMOD Register
It is used to set the various timer operation mode.
– TMOD is an 8-bit register where the lower 4 bits are set aside for timer 0 and the upper 4 bits are set
aside for timer 1.
MSB
LSB
Gate C/T M0 M1 GATE C/T M0 MI
Timer 1 Timer 0
Figure 3.8
GATE: To start and stop the timer
 GATE=1 _HW control: is enabled only while INTx pin is ‘1’and TRx control pin (in TCON)
is set.
 GATE=0 _SW control (used frequently)

21
 C/T: Timer or counter selection
 C/T = 0 _Timer (input from internal system clock) the crystal (1/12) is used to trigger the
timer.
 C/T = 1 _Counter (input from Tx input pin)
M1 and M0: Mode selection for timer and counter
Mode M1 M0
0 0 0 13-bit timer/counter mode
1 0 1 16-bit timer/counter mode
2 1 0 8-bit auto reload timer/counter mode
3 1 1 split timer/counter mode
(ii) TCON Register
MSB
LSB
TF1 TF0 TR1 TR0 IE1 IE0 IT1 IT0
TIMER 1 TIMER 0 TIMER1 TIMER0
Figure 3.9
TF1: Timer 1 overflows flag
 TF1=1: Timer/counter 1 overflows.
 TF1=0: processor vectors to the interrupt services.
TR1: Timer 1 run control bit
 TR1=1: turn Timer 1 ON
 TR1=0: turn Timer 1 OFF
IE1: External interrupt 1 edge flag
 IE1=1: external interrupt is detected.
 IE1=0: when interrupt is processed.
IT1: Interrupt 1 type control bit
 IT1=1: falling edge.
 IT1=0: low level triggered external interrupt.
Gate=0, SETB TR1 _Run Timer 1
SETB TR0 _Run Timer 0
Gate=0, CLR TR1 _OFF Timer 1
CLR TR0 _OFF Timer 0
Timer Mode 0
 Mode 0: 13-bit Timer/counter mode
 0000 ~ 1FFFH
Timer Mode 2
Mode 2: 8-bit auto reload Timer/counter mode (00 ~ FFH).
In auto reload, TH is loaded with the initial count and a copy of it is given to TL.
This reloading leaves TH unchanged still holding a copy of original values.
This mode has many applications, including setting the baud rate in serial communication.
Mode 2 Programming
 8 bit - 00 ~FFH

22
 TH copy to TL
 Start SETB TR0, or TR1
 TL increased FFH (OV monitoring)
 TH reloads to TL.
3.6.2 Counters
Counter is used to count input pulses.
C/T=0: As Time, using 8051’s crystal as the source ofthe frequency.
C/T=1: As counter, a pulse outside of the 8051 that increments the TH and TL register.
When the C/T=1, the counter counts up as pulses are fed from Pins P3.4 (for counter 0) or P3.5 (for
counter 1).
3.7Serial Port Communication in 8051 microcontroller
The 8051 microcontroller transmits data serially as well as parallel communication is also done. For
serial communication, the microcontroller comes with serial communication pin 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.
The two registers used for controlling the communication are SCON and SBUF.
Serial Communication Parallel Communication
Figure 3.10
Serial data communication uses two methods
 Synchronous method transfers a block of data at a time
 Asynchronous method transfers a single byte at a time
It is possible to write software to use either of these methods, but the programs can be tedious and
long
There are special IC chips made by many manufacturers for serial communications
 UART (universal asynchronous Receiver transmitter)
 USART (universal synchronous-asynchronous Receiver-transmitter)
The rate of data transfer in serial data communication is stated in bps (bits per second).
Another widely used terminology for bps is baud rate.

23
3.7.1 SCON
Figure 3.11
RI(Receive Interrupt Flag)-Set by hardware on receiving. Must be cleared by software
TI (Transmit Interrupt Flag) -Set by hardware on transmitted, must clear by hardware
RB8 (Receive bit 8)
 Mode 2,3 : copy of bit 8
 Mode 1 & SM2 clear : copy of stop bit
TB8 Transmit bit 8- The 9th data bit of mode 2, 3. Set or clear by software
REN Receive Enable- Set by software to enable reception, if is cleared reception will be blocked.
SM2 Serial Mode (bit 2) -Use in mode 2, 3 for multiprocessor communications.
SM1 & SM0 Serial Mode (bit 6 & 7)
Operating modes
 Mode 0- 8-bit shift register, f/12
1Mbit with 12 MHz Oscillator Frequency
 Mode 1- 8-bit UART, variable baud rate
 Mode 2 -9-bit UART, f/64 or f/32
187.5K and 375K with 12MHz Oscillator Frequency
 Mode 3- 9-bit UART, variable baud rate.
3.7.2 SBUF
These are two separate data buffers for transmit and receive.
The register SBUF is used to hold both the transmitter and receiver serial port data.
To transmit the data, load SBUF register with data.
 MOV SBUF, source
 When transmission is complete the TI bit will be set in the SCONregister.
When a data frame is received the RI bit in SCON is set high.
The received data may then be loaded from SBUF
 MOV destination, SBUF
 Data reception is double buffered.
3.7.3 SMOD
 Addition bit to double baud speed.

24
3.8 INTERUPTS
An interrupt is an external or internal event that interrupts the microcontroller to inform it that a
device needs its service.
 The advantage of interrupts is that the microcontroller can serve many devices.
 Each device can get the attention of the microcontroller based on the assigned priority.
 The microcontroller can also ignore (mask) a device request for service.
3.8.1 Hardware and Software interrupt
The interrupts in a controller can be either hardware or software. If the interrupts are generated by the
controller’s inbuilt devices, like timer interrupts; or by the interfaced devices, they are called the
hardware interrupts. If the interrupts are generated by a piece of code, they are termed as software
interrupts.
The 8051 controller has six hardware interrupts of which five are available to the programmer.
1. RESET Interrupt - This is also known as Power on Reset (POR). When the RESET interrupt is
received, the controller restarts executing code from 0000H location. This is an interrupt which is not
available to or, better to say, need not be available to the programmer.
2. Timer interrupts - Each Timer is associated with a Timer interrupt. A timer interrupt notifies the
microcontroller that the corresponding Timer has finished counting. Therefore these are two interrupts
for the timers.
3. External interrupts - There are two external interrupts EX0 and EX1 to serve external devices.
Both these interrupts are active low. In AT89C51, P3.2 (INT0) and P3.3 (INT1) pins are available for
external interrupts 0 and 1 respectively. An external interrupt notifies the microcontroller that an
external device needs its service.
4. Serial interrupt - This interrupt is used for serial communication. When enabled, it notifies the
controller whether a byte has been received or transmitted.
Figure 3.12
The interrupts must be enabled by software in order for the microcontroller to respond to them.

25
Interrupt Enable Register- There is a register called IE (interrupt enable) that is responsible for
enabling(unmasking) and disabling (masking) theinterrupts.
EA -------- ET2 ES ET1 EX1 ET0 EX0
Figure 3.13
 To enable any of the interrupts, first the EA bit must be set to 1. After that the bits
corresponding to the desired interrupts are enabled.
 ET0, ET1 and ET2 bits are used to enable the Timer Interrupts 0, 1 and 2, respectively. In
AT89C51, there are only two timers, so ET2 is not used.
 EX0 and EX1 are used to enable the external interrupts 0 and 1. ES is used for serial interrupt.
EA bit acts as a lock bit. If any of the interrupt bits are enabled but EA bit is not set, the interrupt will
not function. By default all the interrupts are in disabled mode.

26
Chapter-4
INTERFACING DEVICES WITH 8051 MICROCONTROLLER
4.1 LED INTERFACING
Interfacing an LED with 8051 is easy. The I/O pins are used as output pins. When any of the bit is set
to 1, the LED glows if LED n side is connected to ground and p side with bit. And if p side is
connected to power and n side to bit, then on bit low, the LED glows.
Figure 4.1
4.2 SEVEN SEGMENT INTERFACING
A seven segment consists of eight LEDs which are aligned in a manner so as to display digits from 0
to 9 when proper combination of LED is switched on. Seven segment uses seven LED’s to display
digits from 0 to 9 and the eighth LED is used for the dot. A typical seven segment looks like as shown
in the figure below.

28
4.3 LCD INTERFACING
A 16x2 LCD means it can display 16 characters per line and there are 2 such lines. In this LCD each
character is displayed in 5x7 pixel matrix. This LCD has two registers.
1. Command/Instruction Register - stores the command instructions given to the LCD. A command
is an instruction given to LCD to do a predefined task like initializing, clearing the screen, setting the
cursor position, controlling display etc.
2.Data Register - stores the data to be displayed on the LCD. The data is the ASCII value of the
character to be displayed on the LCD.
Code-
Figure 4.4

30
4.4 INTERFACING MOTOR WITH 8051
Interfacing DC motor to 8051 forms an essential part in designing embedded robotic projects. A well
designed 8051-DC motor system has essentially two parts. Firstly an 8051 with the required software
to control the motor and secondly a suitable driver circuit.
L293D-L293 is a dedicated quadruple Half H Bridge motor driver IC available in 16 pin package. The
L293 is designed to provide bidirectional drive currents of up to 1 A at voltages
From 4.5 V to 36 V.
Figure 4.7

31
Chapter-5
Components required
5.1 MAX232
The MAX232 is an IC, first created in 1987 by Maxim Integrated Products, that converts signals from
an RS-232 serial port to signals suitable for use in TTL compatible digital logic circuits. The
MAX232 is a dual driver/receiver and typically converts the RX, TX, CTS and RTS signals.
The drivers provide RS-232 voltage level outputs (approx. ± 7.5 V) from a single + 5 V supply via on-
chip charge pumps and external capacitors. This makes it useful for implementing RS-232 in devices
that otherwise do not need any voltages outside the 0 V to + 5 V range, as power supply design does
not need to be made more complicated just for driving the RS-232 in this case.
The receivers reduce RS-232 inputs (which may be as high as ± 25 V), to standard 5 V TTL levels.
These receivers have a typical threshold of 1.3 V, and a typical hysteresis of 0.5 V.
Figure 5.1
It is helpful to understand what occurs to the voltage levels. When a MAX232 IC receives a TTL level
to convert, it changes a TTL logic 0 to between +3 and +15 V, and changes TTL logic 1 to between -3
to -15 V, and vice versa for converting from RS232 to TTL. This can be confusing when you realize
that the RS232 data transmission voltages at a certain logic state are opposite from the RS232 control
line voltages at the same logic state.

32
The MAX232(A) has two receivers (converts from RS-232 to TTL voltage levels), and two drivers
(converts from TTL logic to RS-232 voltage levels). This means only two of the RS-232 signals can
be converted in each direction. Typically, a pair of a driver/receiver of the MAX232 is used for TX
and RX signals, and the second one for CTS and RTS signals.
There are not enough drivers/receivers in the MAX232 to also connect the DTR, DSR, and DCD
signals. Usually these signals can be omitted when e.g. communicating with a PC's serial interface. If
the DTE really requires these signals, either a second MAX232 is needed, or some other IC from the
MAX232 family can be used. Also, it is possible to directly wire DTR (DB9 pin #4) to DSR (DB9 pin
#6) without going through any circuitry. This gives automatic (brain dead) DSR acknowledgment of
an incoming DTR signal.
5.2 7805
The 78xx (sometimes L78xx, LM78xx, MC78xx...) is a family of self-contained fixed linear voltage
regulator integrated circuits. The 78xx family is commonly used in electronic circuits requiring a
regulated power supply due to their ease-of-use and low cost. For ICs within the family, the xx is
replaced with two digits, indicating the output voltage (for example, the 7805 has a 5 volt output,
while the 7812 produces 12 volts). The 78xx line are positive voltage regulators: they produce a
voltage that is positive relative to a common ground.
Figure 5.2
78xx ICs have three terminals and are commonly found in the TO220 form factor, although smaller
surface-mount and larger TO3 packages are available. These devices support an input voltage
anywhere from a couple of volts over the intended output voltage, up to a maximum of 35 to 40 volts
depending on the make, and typically provide 1 or 1.5 amperes of current

33
Advantages
 78xx series ICs do not require additional components to provide a constant, regulated source
of power, making them easy to use, as well as economical and efficient uses of space. Other
voltage regulators may require additional components to set the output voltage level, or to
assist in the regulation process. Some other designs (such as a switched-mode power supply)
may need substantial engineering expertise to implement.
 78xx series ICs have built-in protection against a circuit drawing too much power. They have
protection against overheating and short-circuits, making them quite robust in most
applications. In some cases, the current-limiting features of the 78xx devices can provide
protection not only for the 78xx itself, but also for other parts of the circuit.
5.3 ULN2003
The ULN2002A, ULN2003A, ULN2003AI, ULN2004A, ULQ2003A, and ULQ2004A are high-
voltage high-current. Darlington transistor arrays. Each consists of seven npn Darlington pairs that
feature high-voltage outputs with common-cathode clamp diodes for switching inductive loads. The
collector-current rating of a single Darlington pair is 500 mA. The Darlington pairs can be paralleled
for higher current capability. Applications include relay drivers, hammer drivers, lamp drivers,
display drivers (LED and gas discharge), line drivers, and logic buffers.
For 100-V (otherwise interchangeable) versions of the ULN2003A and ULN2004A, see the
SN75468 andSN75469, respectively. The ULN2002A is designed specifically for use with 14-V to
25-V PMOS devices. Each input of this device has a Zener diode and resistor in series to control the
input current to a safe limit. The ULN2003A and ULQ2003A have a 2.7-kΩ series base resistor for
each Darlington pair for operation directly with TTL or 5-V CMOS devices.
Figure 5.3

34
5.4 Preset
A preset is a three legged electronic component which can be made to offer varying resistance in a
circuit. The resistance is varied by adjusting the rotary control over it. The adjustment can be done by
using a small screw driver or a similar tool. The resistance does not vary linearly but rather varies in
exponential or logarithmic manner. Such variable resistors are commonly used for adjusting
sensitivity along with a sensor.
The variable resistance is obtained across the single terminal at front and one of the two other
terminals. The two legs at back offer fixed resistance which is divided by the front leg. So whenever
only the back terminals are used, a preset acts as a fixed resistor. Presets are specified by their fixed
value resistance.
Figure 5.4
5.5 LCD (Liquid Crystal Display)
LCD-LCD (Liquid Crystal Display) screen is an electronic display module and find a wide range of
applications. A 16x2 LCD display is very basic module and is very commonly used in various devices
and circuits. These modules are preferred over seven segments and other multi segment LEDs. The
reasons being: LCDs are economical; easily programmable; have no limitation of displaying special &
even custom characters (unlike in seven segments), animations and so on.
A 16x2 LCD means it can display 16 characters per line and there are 2 such lines. In this LCD each
character is displayed in 5x7 pixel matrix. This LCD has two registers, namely, Command and Data.
The command register stores the command instructions given to the LCD. A command is an
instruction given to LCD to do a predefined task like initializing it, clearing its screen, setting the
cursor position, controlling display etc. The data register stores the data to be displayed on the LCD.
The data is the ASCII value of the character to be displayed on the LCD.
Figure 5.5

UART project report by Tarun Khaneja ( 09034406598 )

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