S6 EEE MPLC FULL NOTE.pdf subject in electrical and electronics engineering

 A PROGRAMMABLE LOGIC CONTROLLER (PLC)
is an industrial computer control system that
continuously monitors the state of input
devices and makes decisions based upon a
custom program to control the state of
output devices.
Their primary goal of PLC is
 To eliminate the high costs associated with
inflexible, relay-controlled systems
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 Prior to PLCs, many control tasks were performed by contactors,
control relays and other electromechanical devices. This is often
referred to as hard-wired control.
 Circuit diagrams had to be designed, electrical components
specified and installed, and wiring lists created. If an error was
made, the wires had to be reconnected correctly. A change in
function or system expansion required extensive component
changes and rewiring.
 Requires periodic maintenance and testing.
 Relay operation can be affected due to ageing of the components
and dust, pollution resulting in spurious trips
 Operation speed for an electromagnetic relays is limited by the
mechanical inertia of the component
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 Smaller physical size than hard-wire
solutions.
 Easier and faster to make changes.
 PLCs have integrated diagnostics and override
functions.
 Diagnostics are centrally available.
 Applications can be immediately
documented.
 Applications can be duplicated faster and less
expensively.
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 1. SMALL
 it covers units with up to 128 I/O’s and memories
up to 2 Kbytes.
 2. MEDIUM
 They have up to 2048 I/O’s and memories up to
32 Kbytes.
 3. LARGE
 They have up to 8192 I/O’s and memories up to
750 Kbytes.
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• Less wiring.
• Wiring between devices and relay contacts are done in
the PLC program.
• Easier and faster to make changes.
• Trouble shooting aids make programming easier and
reduce downtime.
• Reliable components make these likely to operate for
years before failure.
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 Manufacturing / Machining
 Food / Beverage
 Metals
 Power
 Mining
 Petrochemical / Chemical
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Working of PLC
Prepared by Mrs. Alka Kalra

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Major Components of a Common PLC
PROCESSOR
POWER
SUPPLY
I M
N O
P D
U U
T L
E
O M
U O
T D
P U
U L
T E
PROGRAMMING
DEVICE
From
SENSORS
Pushbuttons,
contacts,
limit switches,
etc.
To
OUTPUT
Solenoids,
contactors,
alarms
etc.
Prepared by Alka Kalra

 Read all field input devices via the input
interfaces, execute the user program stored in
application memory, then, based on whatever
control scheme has been programmed by the
user, turn the field output devices on or off, or
perform whatever control is necessary for the
process application.
 This process of sequentially reading the inputs,
executing the program in memory, and updating
the outputs is known as scanning.

PHASE 1 – Input Status scan
 A PLC scan cycle begins with the CPU reading the status of its inputs.
PHASE 2– Logic Solve/Program Execution
 The application program is executed using the status of the inputs
PHASE 3– Logic Solve/Program Execution
 Once the program is executed, the CPU performs diagnostics and
communication tasks
PHASE 4 - Output Status Scan
 An output status scan is then performed, whereby the stored
output values are sent to actuators and other field output
devices. The cycle ends by updating the outputs.

 As soon as Phase 4 are completed, the entire cycle
begins again with Phase 1 input scan.
 The time it takes to implement a scan cycle is called
SCAN TIME.
◦ The scan time composed of the program scan time, which is
the time required for solving the control program, and the I/O
update time, or time required to read inputs and update
outputs.
 The program scan time generally depends on the
amount of memory taken by the control program and
type of instructions used in the program. The time to
make a single scan can vary from 1 ms to 100 ms

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While the PLC is running, the scanning process includes the
following four phases, which are repeated continuously as
individual cycles of operation:
PHASE 2
Program
Execution
PHASE 3
Diagnostics/
Comm
PHASE 4
Output
Scan
PHASE 1
Read Inputs
Scan

 The input sources convert the real-time
analog electric signals to suitable digital
electric signals and these signals are applied
to the PLC through the connector rails.
 These input signals are stored in the PLC
external image memory in locations known as
bits. This is done by the CPU
 The control logic or the program instructions
are written onto the programming device
through symbols or through mnemonics and
stored in the user memory.

 The CPU fetches these instructions from the user memory and
executes the input signals by manipulating, computing, processing
them to control the output devices.
 The execution results are then stored in the external image
memory which controls the output drives.
 The CPU also keeps a check on the output signals and keeps
updating the contents of the input image memory according to the
changes in the output memory.
 The CPU also performs internal programming functions like setting
and resetting of the timer, checking the user memory.

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ANSWER : d) All of the above
4. Programmable Logic Controller:
4.1. Introduction:
A programmable logic controller (PLC) is a specialized Programmable device which is used
to control machines and processes.
It uses a programmable memory to store instructions and execute specific functions that
include on/off control, timing, counting, sequencing, arithmetic, and data handling.
Advantages
 Increased Reliability
 More Flexibility
 Lower Cost
 Communications Capability
 Faster Response Time
 Easier to Troubleshoot
4.2 PLC Block diagram

 The basic elements of a PLC include input
modules or points, a Central Processing Unit
(CPU), output modules or points, and
a programming device.
 The type of input modules or points used by
a PLC depend upon the types of input devices
used. Some input modules or points respond
to digital inputs, also called discrete inputs,
which are either on or off. Other modules or
inputs respond to analog signals.
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 PLCs are capable of monitoring the inputs
continuously from sensors and producing the
output decisions to operate the actuators
based on the program. Every PLC system
needs at least these three modules:
 CPU Module
 Power Supply Module
 One or more I/O Module

 The primary function of a PLC’s input circuitry
is to convert the signals provided by these
various switches and sensors into logic
signals that can be used by the CPU.
 The CPU evaluates the status of inputs,
outputs, and other variables as it executes a
stored program.
 The CPU then sends signals to update the
status of outputs.
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 CPU module consists of a central processor
and its memory. The Processor is responsible
for doing all the necessary computations and
data processing by accepting the inputs and
producing appropriate outputs.

 The main function of the microprocessor is to
analyze data coming from field sensors through
input modules, make decisions based on the
user’s defined control program and return signal
back through output modules to the field devices.
 Field sensors(Input): switches, flow, level,
pressure, temp. transmitters, etc.
 Field output devices: motors, valves, solenoids,
lamps, or audible devices.

 These PLCs use retentive memory to save
user programs and data when the power
supply breaks or fails and to resume the
execution of a user program ones the power
is restored.
 Thus, these PLCs do not need any use of a
keyboard or monitor for re programming the
processor each time.
 The retentive memory can be implemented
with the use of long-life batteries, EEPROM
modules and flash memory methods.

 VOLATILE.
 A volatile memory is one that loses its stored
information when power is removed.
 Even momentary losses of power will erase any
information stored or programmed on a volatile
memory chip.
 Common Type of Volatile Memory
 RAM. Random Access Memory(Read/Write)

 Read/write indicates that the information stored in
the memory can be retrieved or read, while write
indicates that the user can program or write
information into the memory.
 The words random access refer to the ability of
any location (address) in the memory to be
accessed or used. Ram memory is used for both
the user memory (ladder diagrams) and storage
memory in many PLC’s.

 The CMOS-RAM (Complimentary Metal Oxide
Semiconductor) is probably one of the most
popular. CMOS-RAM is popular because it has a
very low current drain when not being accessed
(15microamps), and the information stored in
memory can be retained by as little as 2Vdc.
 RAM memory must have battery backup to retain
or protect the stored program.

 NON-VOLATILE
 Has the ability to retain stored information when
power is removed, accidentally or intentionally.
These memories do not require battery back-up.
 Common Type of Non-Volatile Memory
 ROM, Read Only Memory
 Read only indicates that the information stored in
memory can be read only and cannot be
changed. Information in ROM is placed there by
the manufacturer for the internal use and
operation of the PLC.

 PROM, Programmable Read Only Memory
 Allows initial and/or additional information to be
written into the chip.
 PROM may be written into only once after being
received from the PLC manufacturer;
programming is accomplish by pulses of current.
 The current melts the fusible links in the device,
preventing it from being reprogrammed. This type
of memory is used to prevent unauthorized
program changes.

 EPROM, Erasable Programmable Read Only Memory
 Ideally suited when program storage is to be semi-
permanent or additional security is needed to prevent
unauthorized program changes.
 The EPROM chip has a quartz window over a silicon
material that contains the electronic integrated circuits.
This window normally is covered by an opaque
material, but when the opaque material is removed
and the circuitry exposed to ultra violet light, the
memory content can be erased.
 The EPROM chip is also referred to as UVPROM.

Electrically Erasable Programmable Read Only
Memory
 Also referred to as E2PROM, is a chip that can be
programmed using a standard programming
device and can be erased by the proper signal
being applied to the erase pin.
 EEPROM is used primarily as a non-volatile
backup for the normal RAM memory. If the
program in RAM is lost or erased, a copy of the
program stored on an EEPROM chip can be down
loaded into the RAM.

 Processor module includes both ROM and RAM
memories.
◦ ROM (Program Memory/System Memory) contains the
operating system, driver and application programs,
◦ RAM (Data Memory/Application Memory)stores user-
written programs and working data.
 The program information or the control logic is stored
in the user memory or the program memory from
where the CPU fetches the program instructions.
 The input and output signals and the timer and counter
signals are stored in the input and output external
image memory respectively.

 In some modular PLCs bus or rack is provided
in the backplane of the circuit into which all
the modules like CPU and other I/O modules
are plugged to the corresponding slots.
 This bus enables the communication between
CPU and I/O modules to send or receive the
data.
 This communication is established by
addressing the I/O modules according to the
location from CPU module along the bus.

 The I/O interface section of a PLC connects it to
external field devices.
 The input and out modules of the
programmable logic controller are used to
connect the sensors and actuators to the
system to sense the various parameters such
as temperature, pressure and flow, etc.
 These I/O modules are of two types: digital
or analog.

• The main purpose of the I/O interface is to
condition the various signals received from or sent
to the external input and output devices.
• Input modules converts signals from discrete or
analog input devices to logic levels acceptable to
PLC’s processor.
• Output modules converts signal from the
processor to levels capable of driving the
connected discrete or analog output devices.

OPTO-
ISOLATOR
IS NEEDED TO:
 Prevent voltage
transients from damaging
the processor.
Helps reduce the effects
of electrical noise
Current
Limiting
Resistor
FROM INPUT
DEVICE
USE TO DROP
THE VOLTAGE
TO LOGIC LEVEL
Buffer, Filter,
hysteresis
Circuits TO
PROCESSOR

Pilot Duty Outputs
 Outputs of this type typically are used to drive high-current electromagnetic
loads such as solenoids, relays, valves, and motor starters.
These loads are highly inductive and exhibit a large current.
General - Purpose Outputs
 These are usually low- voltage and low-current and are used to drive indicating
lights and other non-inductive loads.
 Noise suppression may or may not be included on this types of modules.
Discrete Inputs
 Circuits of this type are used to sense the status of limit switches, push buttons,
and other discrete sensors.
 Noise suppression is of great importance in preventing false indication of inputs
turning on or off because of noise.

 Circuits of this type sense or drive analog signals.
 Analog inputs come from devices, such as
thermocouples, strain gages, or pressure sensors,
that provide a signal voltage or current that is
derived from the process variable.
 Standard Analog Input signals: 4-20mA; 0-10V
 Analog outputs can be used to drive devices such
as voltmeters, X-Y recorders, servomotor drives,
and valves through the use of transducers.
 Standard Analog Output signals: 4-20mA; 0-5V; 0-
10V

 Circuits of this type are used to interface PLCs to
very specific types of circuits such as
servomotors, stepping motors PID (proportional
plus integral plus derivative) loops, high-speed
pulse counting, resolver and decoder inputs,
multiplexed displays, and keyboards.
 This module allows for limited access to timer and
counter presets and other PLC variables without
requiring a program loader.

PLC
INPUTS
OUTPUTS
MOTOR
LAMP
CONTACTOR
PUSHBUTTONS

 The programming device is used to enter or
change the PLC’s program or to monitor or
change stored values.
 Once entered, the program and associated
variables are stored in the CPU.
 In addition to these basic elements, a PLC
system may also incorporate an operator
interface device of some sort to simplify
monitoring of the machine or process.
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 It is used to enter the desired program that will
determine the sequence of operation and control
of process equipment or driven machine.
 Also known as:
 Industrial Terminal ( Allen Bradley )
 Program Development Terminal ( General
Electric )
 Programming Panel ( Gould Modicon )
 Programmer ( Square D )
 Program Loader ( Idec-Izumi )
 Programming Console ( Keyence / Omron )

 Various types of programming devices are used to enter,
modify and troubleshoot a PLC program.
 Hand held unit with LED / LCD display
 Personal Computer (PC)
 Desktop type with a CRT display /Desktop Console


 In the handheld programming device method, a
proprietary device is connected to PLC through
a connecting cable.
 This device consists of a set of keys that allows
to enter, edit and dump the code into the PLC.
 These handheld devices consist of small display
to make the instruction that has been
programmed visible.
 These are compact and easy to use devices, but
these handheld devices have limited capabilities.

 PC is used for programming the PLC in conjunction with the
software given by the manufacturer.
 By using this PC we can run the program in either online or offline
mode, and can also edit, monitor, diagnose and troubleshoot the
program of the PLC.
 The way of transferring the program to the PLC is shown in the
above figure wherein the PC consists of program code
corresponding to control application which is transferred to the
PLC CPU via programming cable.
 . A major advantage of using a computer is that the program can
be stored on the hard disk or a CD and copies can be easily made.

Changing resident PLC programs -
uploading/downloading from a supervisory
controller (Laptop or desktop computer).
• Forcing I/O points and memory elements from a
remote terminal.
• Linking a PLC into a control hierarchy containing
several sizes of PLC and computer.
• Monitoring data and alarms, etc. via printers or
Operator Interface Units (OIUs).

 A PLC power supply is the workhorse of
the PLC system. It converts your line voltage, 120 or
240 volts AC, to a lower DC voltage, commonly 24 volts
DC. This DC voltage is then sent into the rack
to power the rest of the PLC components.
 The output 5V DC drives the computer circuitry, and in
some PLCs 24DC on the bus rack drives few sensors
and actuators.

 Line voltage is stepped down with a
transformer, rectified to convert it to DC,
filtered with capacitors, and protected during
this process. All of this is packed into that
small looking power supply.
 This DC voltage is used to power the rest of
the PLC and components.
 The common current ratings for PLC’s are
anywhere from 2 to 10 amps for smaller
systems and up to 50 amps for larger, more
powerful controllers.

Number of logical inputs and outputs
This specifies the number of I/O devices that can be
connected to the controller.
There should be sufficient I/O ports to meet present
requirements with enough spares to provide for
moderate future expansion.

 MEMORY CAPACITY
The amount of memory required for a particular
application is related to the length of the program
and the complexity of the control system.
 Simple applications having just a few relays do not
require significant amount of memory.
 Program length tend to expand after the system
have been used for a while.
 It is advantageous to a acquire a controller that
has more memory than is presently needed.

OUTPUT-PORT POWER RATINGS
Each output port should be capable of supplying sufficient
voltage and current to drive the output peripheral connected
to it.
Scan Time
This is the speed at which the controller executes the relay
ladder logic program. This variable is usually specified as the
scan time per 1000 logic nodes and typically ranges from 1 to
200 milliseconds.

 The difference between a PLC and relay logic
is that a PLC is a programmable device where
as relay logic is a network of hardwired
electrical devices.
 Both a PLC and relay logic can perform logical
computation, but a PLC does it using a
microprocessor and relay logic does it using
electric circuits
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Communications Port
 RS-232, RS 422 / RS 485,LAN
Software
 1. Allen-Bradley – Rockwell Software RSLogix500
 2. Modicon - Modsoft
 3. Omron - Syswin
 4. GE-Fanuc Series 6 – LogicMaster6
 5. Square D- PowerLogic
 6. Texas Instruments – Simatic
 6. Telemecanique – Modicon TSX Micro

A Detailed Design Process
1. Understand the process
2. Hardware/software selection
3. Develop ladder logic
4. Determine scan times and memory requirements
PLC Status Indicators
1. Power On
2. Run Mode
3. Programming Mode
4. Fault

1. Look at the process
2. PLC status lights
 HALT - something has stopped the CPU
 RUN - the PLC thinks it is OK (and probably is)
 ERROR - a physical problem has occurred with the
PLC
3. Indicator lights on I/O cards and sensors
4. Consult the manuals, or use software if
available.
5. Use programming terminal / laptop.

 Problem Statement :
 Providing lubricant for the gear box before the lathe spindle starts to run
which aims to ensure that the oil pump motor starts first and the main motor
starts subsequently.
 Number of PLC Inputs Required
 X0 – START pushbutton to Start Oil Pump Motor
 X1 – START pushbutton to Stop Main Motor
 X2 – STOP pushbutton to Stop Oil Pump Motor
 X3 – STOP pushbutton to Stop Main Motor
 Number of PLC Outputs Required

Y0 – Oil Pump Motor
 Y1 – Main Motor

There are only 5 languages that are considered to
be standard languages for use on PLCs,
according to IEC section 61131–3.
 Ladder Diagram (LD)
 A sequential function chart
 Function Block Diagram
 Instruction List
 Structured Text
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 Ladder Diagram is the oldest PLC language.
This graphical programming language was
modeled from relay logic to allow engineers
and electricians to transition smoothly into
programming PLCs.
 Within Ladder, rungs and rails represent the
real world electrical connections. Specifically,
the vertical “rails” represent the supply power
of the device while the rungs that are
connected to the rails are equal to the
amount of control circuits.
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4
1. The vertical lines of the diagram represent the power rails between which circuits are
connected. The power flow is taken to be from the left-hand vertical across a rung.
2. Each rung on the ladder defines one operation in the control process.
3. A ladder diagram is read from left to right and from top to bottom, showing the
scanning motion employed by the PLC.
The top rung is read from left to right. Then the second rung down is read from left to
right and so on.
4. Each rung must start with an input or inputs and must end with at least one output. The
term input is used for a control action, such as closing the contacts of a switch, used as an
input to the PLC. The term output is used for a device connected to the output of a PLC,
e.g., a motor.
5. Electrical devices are shown in their normal condition. Thus a switch, which is
normally open until some object closes it, is shown as open on the ladder diagram. A
switch that is normally closed is shown closed.

5
6. A particular device can appear in more than one rung of a ladder. For example, we
might have a relay that switches on one or more devices. The same letters and/or numbers
are used to label the device in each situation.
7. The inputs and outputs are all identified by their addresses, the notation used depending
on the PLC manufacturer. This is the address of the input or output in the memory of the
PLC.
8.The instructions used are the relay equivalent of normally open (NO) and normally
closed (NC) contacts and coils.
9.Contact symbolism is a simple way of expressing the control logic in terms of symbols
that are used on relay control schematics.
10.A rung is the contact symbolism required to control an output. Some PLCs allow a
rung to have multiple outputs.
11.A complete ladder logic program thus consists of several rungs. each of which controls
an output.
12.Because the PLC uses ladder logic diagrams. the conversion from any existing relay
logic to programmed logic is simple.
13. Each rung is a combination of input conditions (symbols) connected from left to right,
with the symbol that represents the output at the far right.
14.The symbols that represent the inputs are connected in series, parallel. or some
combination of the two to obtain the desired logic.
15.Because the PLC uses ladder logic diagrams. the conversion from any existing relay
logic to programmed logic is simple.
16. Each rung is a combination of input conditions (symbols) connected from left to right,
with the symbol that represents the output at the far right.
17.The symbols that represent the inputs are connected in series, parallel. or some
combination of the two to obtain the desired logic.
RUNG
 A type of line diagram that uses the input and output symbols used by PLC ladder
logic.
 Line diagrams are converted to programming diagrams before being entered into
a PLC

 This program is a typical application of the conditional
control circuit.
Y0 = ON when Oil Pump START button is pressed.
Therefore, the oil pump will start to provide lubricant
for the gear box of main motor(Y1)
 Under the precondition of the operating state of the
Oil pump, the main motor (Y1) will be ON when the Main
motor START button is pressed.
 During the operation of main motor (Y1), oil pump
(Y0) needs to provide lubricant continuously.
 The oil pump will be stopped when Oil pump STOP
button X2 is activated, and the main motor will be stopped
when Main motor STOP button X3 is activated.

 Problem Statement: Detecting the standing
bottles on the conveyor and pushing falling
bottles in tray.


Number of PLC Inputs Required
X0 – Proximity Sensor to sense bottom of the Bottle i.e. X0 = ON
when the detected input signal from the bottle-bottom is
sheltered.
X1 – Proximity Sensor to sense upper part of the Bottle i.e. X1 = ON
when the detected input signal from the bottle-neck is sheltered.
Number of PLC Outputs Required
Y0 – To operate Pushing Cylinder/Rod

 A sequential function chart is a graphical programming language
that mimics a flow chart. You use steps and transitions to get
output.
 Steps are functions within the program and house events that are
activated based on state and other specified conditions.
 Transitions are instructions based on true/false values that move
you from one step to another.
 Branches are used to initiate multiple steps at a time. The
branches act like threads where functions can run concurrently.
 All of these steps, transitions, and branches are housed in a
series of scripts that execute in a procedural manner. The visual
nature of the language allows users to monitor processes that
both heavily use conditional logic and run parallel instructions.
PLCs that are prone to suffering from bottlenecks can be more
intuitively maintained and troubleshooted using the chart to
follow the logic of the program.
15

 This is the PLC’s equivalent to assembly
language. This gives you immediate access to the
machine itself, which allows you to write code
that is compressed and fast. The code is
represented in the manner that the language’s
name suggests: in a list of commands.
 Structured Text is a high level language designed
to program PLCs. This is essentially the C++ of
the PLC world. Any PLC that requires complex
data handling will most likely use ST.
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 Block based programming languages are a type of
graphical language that minimizes code into blocks,
which allows for a simple way to create executable
commands.
 FBD in particular describes a function between inputs
and outputs that are connected by connection lines.
The logic of the inputs and outputs are stored in
blocks. The blocks are programmed onto sheets and
the PLC scans these sheets in order or by specified
connections between blocks, much like procedural
languages.
 The I/O focus mirrors that of ladder logic. Yet, the
code that the blocks contain allow engineers to
develop more complex batch control tasks among
other repeatable tasks.
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 If the bottle on the conveyor belt is upstanding, the input signal
from monitoring photocell at both bottle-bottom and bottle-
neck will be detected. In this case, X0 = ON, and X1 = ON. The
normally open (NO) contact X0 will be activated as well as the
normally closed (NC) contact X1. Y0 remains OFF and pneumatic
pushing pole will not perform any action.
 If the bottle from the conveyor belt is down, only the input
signal from monitoring photocell at the bottle-bottom will be
detected. In this case, X0 = ON, X1 = OFF. The state of output
YO will be ON because the NO contact X0 activates and the NC
contact X1 remains OFF. The pneumatic pushing pole will push
the fallen bottle out of the conveyor belt.

 Structured Text is a high level language
designed to program PLCs. This is essentially
the C++ of the PLC world. Any PLC that
requires complex data handling will most
likely use ST.
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6
4.2 PLC Instructions
RELAY-TYPE INSTRUCTIONS
 The ladder diagram language is basically a
symbolic set of instructions used to create the controller program.
 The three fundamental symbols that are used to translate relay control logic to contact
symbolic logic are EXAMINE IF CLOSED,EXAMINE IF OPEN, and OUTPUT
ENERGIZE
Normally open
 Analogous to the normally open relay contact. For this instruction.
 we ask the processor to EXAMINE IF (the contact is) CLOSED.
 The status bit will be either1(ON) or 0 (OFF)
 The status bit is examined for an ON condition.
 If the status bit is I (ON). Then the instruction is TRUE.

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 If the status bit is 0 (OFF). Then the instruction is FALSE.
Normally closed
 instruction, we ask the processor to EXAMINE IF (the contact is) OPEN.
 The status bit will be either 1 (ON) or 0 (OFF).
 The status bit is examined for an OFF condition.
 If the status bit is 0 (OFF), then the instruction is TRUE.
 It the status bit is 1 (ON), then the Instruction is FALSE.

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Relay coil (OTE)
 The processor ( makes it is Instruction true (analogous to energizing a coil) when
there is a path 01 true XIC and XIO instructions in the rung.
 If any left to right path of input conditions is TRUE. the output is energized (turned
ON).
 The status bit 01 the addressed OUTPUT Energize instruction is set to 1 (ON) when
the rung is TRUE.

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Parallel input Branching
 Input branching by formation of parallel branches can be used in your application pro-
gram to allow more than one combination of input conditions,
 If at least one of these parallel branches forms a true logic path, the rung logic is
enabled.
 If none of the parallel branches forms a true logic path. rung logic is not enabled and
the output instruction logic will not be TRUE.
 In the example shown, either A and B or C provides a true logical path.
Parallel input Branching with Output branching
 branches can be established at both input and output portions of a rung. With output
branching.
 you can Program parallel outputs on a rung to allow a true logic path to control
multiple outputs.
 When there is a true logic path. all parallel outputs become TRUE.
 either A or B provides a true logical path to all three output instructions: C, D, and E.
Parallel output branching with input

11
 Additional input logic instructions (conditions) can be programmed in the output
branches to enhance condition contra) of the outputs.
 When there is a true logic path, including extra input conditions on an output branch.
That branch becomes TRUE.
 either A and D or Band D provide a true logic path to E.
Nested
 Input and output branches can be nested to avoid redundant instructions and to speed
up processor scan time.
 Input and output branches can be nested to avoid redundant instructions and to speed
up processor scan time.
4.3. Implementation of logic gates using ladder
AND Logic
It is a basic gate. Whenever the all the input condition must true then output becomes
true
Figure (a)Truth table of the AND logic (b) block diagram of AND logic

12
Logic equation
Ladder Logic diagram for AND Logic
OR LOGIC
It is basic gate. Whenever any one of the input conditions is true the output becomes true

13
Figure (a)Truth table of the OR logic (b) block diagram of OR logic
Logic equation
Y= A+B
Ladder logic diagram of the OR logic
 Two limit switches connected in parallel and used to control a solenoid valve.
NOT logic
It is one of the basic logic gate output always the complement of input
Truth Table of NOT logic
Ladder logic diagram of
NOT logic

14
NAND LOGIC
It is an universal gate. Whenever the all the input must true condition the output becomes
false or
If any one of the inputs is false condtion then output becomes true.
Truth Table of the NAND Logic
Logic Equation of the NAND Logic
Ladder Logic diagram for NAND logic

15
NOR Logic
It is an universal gate. Whenever all the input conditions are false then output becomes true
TRUTH TABLE of the NOR logic
LOGIC equation
Ladder logic diagram for NOR logic
Exclusive OR
 The output of this circuit is ON only when pushbutton A or B is pressed, but not both.
Case 1When A= 0 and B= 0:
Let us analyze main rung. When I1 = 0, the normally open instruction is false and,
normally closed instruction is true, but since normally open instruction is false, there is no
logical continuity and output cannot be energized. Similar analysis can be done in parallel

16
rung, normally closed instruction will be true and normally open instruction will be false
and output is not energized.
Case 2When A= 0 and B = 1:
In main rung, normally open instruction will be false and, normally closed instruction will
be true, but since there is no logical continuity this rung logic cannot energize the output.
But, in parallel rung, normally closed instruction will be true, as well as normally open
instruction will also be true, hence there is logical continuity, and output is energized.
Case 3 When I1 = 1 and I2 = 0:
This case is similar to case 2, only the role of inputs are interchanged i.e. here main rung is
true and energizes the output and parallel rung is false.
Case 4
 When both inputs are true, the main ladder rung as well as the parallel ladder rung
goes false. In main rung, normally open instruction is true but normally closed
instruction is false. Hence, there is no logical continuity. In parallel ladder rung,
normally closed instruction is false and normally open instruction is true, and here
also there is no logical continuity. Hence, the output is not energized.

17
Latch circuit
 There are often situations where it is necessary to hold an output energized, even
when the input ceases.
 A simple example of such a situation is a motor, which is started by pressing a push
button switch.
 Though the switch contacts do not remain closed, the motor is required to continue
running until a stop push button switch is pressed.
 The term latch circuit is used for the circuit used to carry out such an operation.
 It is a self-maintaining circuit in that, after being energized, it maintains that state
until another input is received.
4.4.Simple Programs
 A lighting control system is to be developed. The system will be controlled by four switches,
SWITCH1, SWITCH2, SWITCH3, and SWITCH4.
These switches will control the lighting in a room based on the following criteria:
1. Any of three of the switches SWITCH1, SWITCH2, and SWITCH3, if turned ON can turn
the lighting on, but all three switches must be OFF before the lighting will turn OFF.
2. The fourth switch SWITCH4 is a Master Control Switch. If this switch is in the ON
position, the lights will be OFF and none of the other three switches have any control.
Design the wiring diagram for the controller connections, assign the inputs and outputs and
develop the ladder diagram which will accomplish the task.
Number of PLC Inputs Required
 INPUT IN1 = SWITCH1
 INPUT IN4 = SWITCH4 (Master Control Switch)

18
Number of PLC Outputs Required
 OUTPUT OUT1 = Lights control relay coil CR1
 Any of three of the switches SWITCH1, SWITCH2, and SWITCH3, if turned ON can turn
the lighting on
 The fourth switch SWITCH4 is a Master Control Switch. If this switch is in the ON position,
the lights will be OFF
Boolean logic
CR1=(IN1 OR IN2 OR IN3) AND IN4
5. POST MCQ
1. Function Block Diagram (FBD) is a type of____________.
a) PLC Language

17
1) Computer with SIMATIC software.
2) Siemens S7-1200 PLC.
3) LEDs.
4) Switches.
5) Connecting wires
THEORY
TIMERS
On-Delay Timer The On-Delay Timer (TON) instruction counts time when the
enabling input is ON. When the current value (Txxx) is greater than or equal to the
present time (PT), the timer bit is ON. The On-Delay timer current value is cleared
when the enabling input is OFF. This timer continues counting after the Preset is
reached, and it stops counting at the maximum value of 32767.
The timer shown in figure will delay for 1000*1ms= 1000ms = 1 second then it
will be activated ON. Here when you activate the input I0.0, the timer T37 starts
delaying time, until it reaches 1000ms, then T37 bit turns ON, hence Q0.0 will be
ON. As a result Q0.0 will be activated after 1 second activating I0.0.
Ladder logic diagram for on-delay timer. the
timing diagram

20
the count-up input CU makes the transition from off to on. When the current
value (Cxxx) is greater than or equal to the Preset Value (PV), the counter bit
(Cxxx) turns on. The counter is reset when the Reset (R) input turns on, or
when the Reset instruction is executed. The counter stops counting when it
reaches the maximum value (32,767).
Counter ranges: Cxxx=C0 through C255
Count Down Counter
The Count Down (CTD) instruction counts down from the current value of
that counter each time the countdown input CD makes the transition from off to
on. When the current value Cxxx is equal to zero, the counter bit (Cxxx) turns
on. The counter resets the counter bit (Cxxx) and loads the current value with
the preset value (PV) when the load input (LD) turns on. The Down Counter
stops counting when it reaches zero, and the counter bit Cxxx turns on.
Counter ranges: Cxxx=C0 through C255

19
(Ladder logic diagram for off-delay timer. The timing diagram)
COUNTER
Count Up Counter
The Count Up (CTU) instruction counts up from the current value each time

18
Off-Delay Timer
The Off-Delay Timer (TOF) is used to delay turning an output OFF for a
fixed period of time after the input turns OFF. When the enabling input turns
ON, the timer bit turns ON immediately, and the current value is set to 0.
When the input turns OFF, the timer counts until the elapsed time reaches the
preset time. When the preset is reached, the timer bit turns OFF and the current
value stops counting. If the input is OFF for a time shorter than the preset
value, the timer bit remains ON. The TOF instruction must see an ON to OFF
transition to begin counting.

21
PROCEDURE:
1. Turn the PC ON.
2. Turn the PLC ON.
3. Start V4.0 STEP 7 Micro WIN SP5.
4. Select View>>Ladder.
5. Form Bit Logic double, click the normally open contact.
6. Click on the red question marks, and rename the normally open contactor with
a physical input name, I0.0- I0.7 or I1.0-I1.5, let us say I0.0.
7. Now from the Bit Logic, double click the output, and name it with a physical
output name, Q0.0-Q0.7 or Q1.0-Q1.1, let us say Q0.0, the result rung will be
as shown in the next figure.
8. Select PLC>>Compile All; make sure that the "Total Errors" is zero.
9. Select PLC>>STOP to place the PLC in the stop mode.
10. To download the ladder diagram to the PLC, click download (shown to the
right), the window for downloading appears then click to download.
11. Now place the PLC in the run mode, by selecting PLC>>RUN.
12. Now the PLC is ready for use performing the function of the ladder diagram
that you have drawn.
Conclusion:
This is all about different timer and counter using PLC ladder programming.
As well as PLC programming rules and programming instructions are also
important for writing the program.
…………………………………………………………………………………

 PLC Ladder Practice Problem:
 The production line may be powered off accidentally or
turned off for noon break. The program is to control the
counter to retain the counted number and resume counting
after the power is turned ON again. When the daily production
reaches 500, the target completed indicator will be ON to
remind the operator for keeping a record. Press the Clear
button to clear the history records. The counter will start
counting from 0 again.

Latched 16 bit UP counter
 X0 – Product Detecting Sensor. X1 – Production Counter RESET/Clear
Y0 – Production Counter Target Completed.
 Number of PLC Counter Required:
C120 – 16 Bit Latched Counter. (Max Count =32,768)

 The latching counter is demanded for the
situation of retaining data when power-off.
 When a product is completed, C120 will count
for one time. When the number reaches
500, target completed indicator Y0 will be
ON.
 For different series of PLC, the setup range of
16-bit latching counter is different.

 Enabling the indicator to be ON immediately
when switch pressed and OFF after a 5 sec
delay by the switch.


X1 – Start Switch.

Y1 – Output Indicator
 Number of PLC Timer Required

T0 – 5 second Timer, 100 ms Time Base. (See K50 Preset Value for Timer)
When X1 = ON, TMR instruction will be executed. Timer T1 will be ON and
start counting for 3 sec. When T1 reaches its set value, the NO (Normally
Open) contact T1 will be activated and indicator YI will be ON.
When X1 = OFF, TMR instruction will not be executed. Timer T1 will be OFF
and so will NO contact T1. Therefore, the indicator Y1 will be OFF.

S6 EEE MPLC FULL NOTE.pdf subject in electrical and electronics engineering

 A practical application for a comparative function is
something called alternating motor control, where the run-
times of two redundant electric motors are monitored, with
the PLC determining which motor to turn on next based on
which motor has run the least

 In this program, two retentive on-delay timers keep track of each
electric motor’s total run time, storing the run time values in two
registers in the PLC’s memory:
 Motor A runtime and Motor B runtime. These two integer values
are input to the “greater than” instruction box for comparison.
 If motor A has run longer than motor B, motor B will be the one
enabled to start up next time the “start” switch is pressed.
 If motor A has run less time or the same amount of time as
motor B (the scenario shown by the blue-highlighted status
indications), motor A will be the one enabled to start.
 The two series-connected virtual contacts OUT motor A and OUT
motor B ensure the comparison between motor run times is not
made until both motors are stopped.
 If the comparison were continually made, a situation might arise
where both motors would start if someone happened to press
the Start pushbutton with one motor is already running.

27
Execute Ladder diagrams with model applications (i) Stair case lighting (ii)
Traffic light controller
AIM: Execute Ladder diagrams with model applications Stair case lighting.
Equipment:
 I0.0 & I0.1: Input toggle switches.
 Q0.0: Output signaling device.
THEORY;
This ladder logic of stair-case wiring works on the X-OR logic i.e.Y is (A.B^) +
(A^. B)
Suppose initially only I0.0 is pressed and I0.1 remains open physically as shown
then the upper branch of rung 1 goes TRUE and Q0.0 gets energized.
Now, if I0.0 is also pressed then it will make its interlock N-C contact I0.1 to go
open and as a result Q0.0 de-energizes .
Again, if switch I0.0 is pressed it will also make its interlock contact I0.0 in
parallel branch to go again N-C, which in turn makes the parallel branch logic true
and Q0.0 again energizes but if the I0.1 input is also pressed again the logic goes
FALSE and Q0.0 again de-energizes. This is the basic concept of an X-OR logic
gate.
.

28
PROCEDURE:
 For this application, we used S7-1200 PLC and TIA portal software for
programming.
 In above program, we have added two NO contacts of SW 1 (I0.1) and SW 2
(I0.2) in series and NC contacts of SW1 (I0.1) and SW2 (I0.2) in parallel of
this series SW1 & SW2 NO Contacts.
 If the status of the bottom switch (SW1) and status of the top switch (SW2)
are same then lamp will be ON. And if either status of the bottom or top
switch is different from other then lamp (Q0.0) will be OFF.
 When lamp (Q0.0) is OFF then user can ON the lamp by changing status of
any switch. Also user can turn OFF the lamp by changing the status of one of
the two switches.
List of inputs/outputs
Digital Inputs
 SW1 : I0.1
 SW2 : I0.2
Digital Outputs
 Lamp : Q0.0
PLC Ladder diagram for two ways switch logic

29
Runtime Test Cases
CONCLUSION:
The above PLC Logic provided for basic idea about application of PLC for Stair
case lighting Two Way Switch Logic.
……………………………………………………………………………………….

 They are so many ways to write a program for
traffic light control ex: sequencer output
method but in this normal input, outputs and
timers are used.
 Timers are used to give time delay for output
to turn ON and OFF.
 Reset coil is used at the end to run the
program continuously.
 Comparator blocks are used to reduce the
number of timers used.

22
Execute the Ladder Diagrams with model applications (i) DOL starter
(ii)Star- Delta starter
AIM: Execute the Ladder Diagrams with model applications (i) DOL starter
(ii)Star- Delta starter
THEORY
Star Delta Motor Starter
The following figure shows the winding connections in star and delta configuration
one by one.
It can be seen that in star connection, one end of all three windings are shorted to
make star point while other end of each winding is connected to power supply.
In delta configuration, the windings are connected such that to make a close loop.
The connection of each winding is shown in above figure. In actual motor the three
phase connections are provided in the following order as shown

23
So in order to make winding connection in star and delta style in practical motor,
the connection is shown above.
Main contractor is used to supply power to the windings. It must be turned on all
the time. Initially the star contactor is closed while delta contactor is open It makes
the motor windings in star configuration.
The contactors are controlled by using PLC. The following section of PLC
tutorial will explain the ladder programming for star delta motor starter.
PROCEDURE
PLC program for star delta motor starter:

24
PLC Ladder Logic
Rung 1 Main contactor :
The main contactor depends upon the normally open input start push button (I1),
normally closed stop button (I2) and normally closed overload relay.
It means that Main contactor will only be energized if start button is pressed, while
stop is not pressed and overload relay is not activated. A normally open input
named (Q1) is added in parallel to the start button I1.
By doing so, a push button is created which means that once motor is started, it
will be kept started even if start button is released

25
Rung 2 Star contactor:
Star contactor depends upon main contactor, normally close contacts of timer (T1),
and normally close contacts of output delta contactor (Q3).
So star contactor will only be energized if main contactor is ON, time output is not
activated and delta contactor is not energized.
Timer T1:
Timer T1 measures the time after which the winding connection of star delta starter
is to be changed. It will start counting time after main contractor is energized.
Rung 3 Delta contactor:
Delta contactor will be energized when main contactor (Q1) is energized, timer T1
is activated and star contactor (Q3) is de-energized.
Also see Programming of push button and other requirements for simple motor
starter is explained in PLC Tutorial: Motor starter
DOL Starter Ladder Diagram
The ladder diagram of the dol starter.dol (Direct-On-Line) starter is used to start
the motor by applying full line voltage to the motor. Dol starter is used for the
motors having rating up to the 05HP. Figure shows the control diagram of the dol
starter.

 One method of starting electric motors is
using direct on line (DOL) or across the line
starter. In this method full line voltage is
applied to the motor terminals. This is
simplest type of motor starter.

 DOL motor starter contains fuse and over load
relay (OLR) for protection purpose. The starter
can be contain momentary contact or maintained
contact push buttons. The example considered
here is momentary contact push buttons. For
starting purpose normally open (NO) push button
is preferred whereas normally closed (NC) push
button is used to stop the motor.
The excessive supply voltage drop causing high
inrush current is the criteria to limit the use of
DOL starter. Conveyor motors, water pumps are
the applications where DOL starters are used.

 Listing of Input and Output devices:
Inputs: PB1- To start the motor
PB2- To stop the motor
Output: M1- Motor
1. When Start push button (PB1) is pressed, Motor (M1) has
to start.
2. If Start pushbutton (PB1) is released and Stop
pushbutton (PB2) is not pressed, Motor (M1) should remain
on.
3. When Stop push button (PB2 is pressed, Motor (M1) has
tol stop.
4. If stop push button is released and start is not pressed
(released) motor shouldl remain off.

Micro Controller Series
(MCS)-51 Over View

 The microprocessor is the core of computer
systems.
 Nowadays many communication, digital
entertainment, portable devices, are
controlled by them.
 A designer should know what types of
components he needs, ways to reduce
production costs and product reliable.
 Hardware :Interface to the real world
 Software :order how to deal with inputs

 CPU: Central Processing Unit
 I/O: Input /Output
 Bus: Address bus & Data bus
 Memory: RAM & ROM
 Timer
 Interrupt
 Serial Port
 Parallel Port

CPU
General-
Purpose
Micro-
processor
RAM ROM I/O
Port
Timer
Serial
COM
Port
Data Bus
Address Bus
 CPU for Computers
 No RAM, ROM, I/O on CPU chip itself
 Example：Intel’s 8085,8086, Motorola’s 680x0
Many chips on mother’s board
General-purpose microprocessor

Microprocessor
 CPU is stand-alone, RAM,
ROM, I/O, timer are separate
 Designer can decide on the
amount of ROM, RAM and
I/O ports.
 expansive
 versatility
 general-purpose
Microcontroller
• CPU, RAM, ROM, I/O and
timer are all on a single chip
• fix amount of on-chip ROM,
RAM, I/O ports
• for applications in which cost,
power and space are critical
• single-purpose

 4K bytes internal ROM
 128 bytes internal RAM
 Four 8-bit I/O ports (P0 - P3).
 Two 16-bit timers/counters
 One serial interface
RAM
I/O
Port
Timer
Serial
COM
Port
Microcontroller
CPU
A single chip
ROM

CPU
Interrupt
Control
OSC Bus
Control
4k
ROM
Timer 1
Timer 2
Serial
128 bytes
RAM
4 I/O Ports
TXD RXD
External Interrupts
P0 P2 P1 P3
Addr/Data

 only 1 On chip oscillator (external crystal)
 6 interrupt sources (2 external , 3 internal, Reset)
 64K external code (program) memory(only
read)PSEN
 64K external data memory(can be read and write) by
RD,WR
 Code memory is selectable by EA (internal or
external)
 We may have External memory as data and code

89XX ROM RAM Timer Int
Source
IO pin Other
8951 4k 128 2 6 32 -
8952 8k 256 3 8 32 -
8953 12k 256 3 9 32 WD
8955 20k 256 3 8 32 WD
898252 8k 256 3 9 32 ISP
891051 1k 64 1 3 16 AC
892051 2k 128 2 6 16 AC
WD: Watch Dog Timer
AC: Analog Comparator
ISP: In System Programable

 meeting the computing needs of the task
efficiently and cost effectively
◦ speed, the amount of ROM and RAM, the number of I/O
ports and timers, size, packaging, power consumption
◦ easy to upgrade
◦ cost per unit
 availability of software development tools
◦ assemblers, debuggers, C compilers, emulator,
simulator, technical support
 wide availability and reliable sources of the
microcontrollers

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
40
39
38
37
36
35
34
33
32
31
30
29
28
27
26
25
24
23
22
21
P1.0
P1.1
P1.2
P1.3
P1.4
P1.5
P1.6
P1.7
RST
(RXD)P3.0
(TXD)P3.1
(T0)P3.4
(T1)P3.5
XTAL2
XTAL1
GND
(INT0)P3.2
(INT1)P3.3
(RD)P3.7
(WR)P3.6
Vcc
P0.0(AD0)
P0.1(AD1)
P0.2(AD2)
P0.3(AD3)
P0.4(AD4)
P0.5(AD5)
P0.6(AD6)
P0.7(AD7)
EA/VPP
ALE/PROG
PSEN
P2.7(A15)
P2.6(A14)
P2.5(A13)
P2.4(A12)
P2.3(A11)
P2.2(A10)
P2.1(A9)
P2.0(A8)
8051
(8031)
(8751)
(8951)

 One of the most useful features of the 8051 is that it
contains four I/O ports (P0 - P3)
 Port 0 （pins 32-39）：P0（P0.0～P0.7）
◦ 8-bit R/W - General Purpose I/O
◦ Or acts as a multiplexed low byte address and data bus for external
memory design
 Port 1 （pins 1-8）：P1（P1.0～P1.7）
◦ Only 8-bit R/W - General Purpose I/O
 Port 2 （pins 21-28）：P2（P2.0～P2.7）
◦ 8-bit R/W - General Purpose I/O
◦ Or high byte of the address bus for external memory design
 Port 3 （pins 10-17）：P3（P3.0～P3.7）
◦ General Purpose I/O
◦ if not using any of the internal peripherals (timers) or external
interrupts.
 Each port can be used as input or output (bi-direction)

 PSEN (out): Program Store Enable, the read signal
for external program memory (active low).
 ALE (out): Address Latch Enable, to latch address
outputs at Port0 and Port2
 EA (in): External Access Enable, active low to access
external program memory locations 0 to 4K
 RXD,TXD: UART pins for serial I/O on Port 3
 XTAL1 & XTAL2: Crystal inputs for internal
oscillator.
 Vcc（pin 40）：
◦ Vcc provides supply voltage to the chip.
◦ The voltage source is +5V.
 GND（pin 20）：ground

 RST（pin 9: Reset
◦ input pin and active high（normally low）.
 The high pulse must be high at least 2
machine cycles.
◦ Power-on reset
 Upon applying a high pulse to RST, the
microcontroller will reset and all values
in registers will be lost.
 Reset values of some 8051 registers

 8 bit CPU with registers A and B
 16 bit PC and DPTR(data pointer).
 8 bit program status word(PSW)
 8 bit Stack Pointer
 32 I/O pins arranged as 4 8 bit ports:P0 to P3
 Two 16 bit timer/counters:T0 and T1
 Full duplex serial data receiver/transmitter
 Control registers : TCON,TMOD,SCON,PCON,IP and IE
 Two external and Three internal interrupt sources.
 Oscillator and Clock Circuits.
 4K Internal ROM
 128bytes Internal RAM
- 4 register banks each having 8 registers
16 bytes, which may be addressed at the bit level.
80 bytes of general purpose data memory
Wednesday, November 1,
2023

07
06
05
04
03
02
01
00
R7
R6
R5
R4
R3
R2
R1
R0
0F
08
17
10
1F
18
Bank 3
Bank 2
Bank 1
Bank 0
Four Register Banks
Each bank has R0-R7
Selectable by psw.2,3

 Four banks of 8 byte-sized registers, R0 to R7
 Addresses are :
18 - 1F for bank 3
10 - 17 for bank 2
08 - 0F for bank 1
00 - 07 for bank 0 (default)
 Active bank selected by bits [ RS1, RS0 ] in PSW.
 Permits fast “context switching” in interrupt service
routines (ISR).

A
B
R0
R1
R3
R4
R2
R5
R7
R6
DPH DPL
PC
DPTR
PC
Some 8051 16-bit Register
Some 8-bit
Registers of the
8051

 It is used as a general register to accumulate the
results of a large number of instructions.
 It can hold an 8-bit (1-byte) value and is the most
versatile register the 8051 has due to the shear
number of instructions that make use of the
accumulator.
 More than half of the 8051’s 255 instructions
manipulate or use the accumulator in some way.

 The "R" registers are a set of eight registers that are named
R0, R1, etc. up to and including R7.
 These registers are used as auxiliary registers in many
operations
 The "R" registers are also used to temporarily store values.
 For example, let’s say you want to add the values in R1
and R2 together and then subtract the values of R3 and R4.
One way to do this would be:
MOV A,R3 ;Move the value of R3 into the accumulator
ADD A,R4 ;Add the value of R4
MOV R5,A ;Store the resulting value temporarily in R5
MOV A,R1 ;Move the value of R1 into the accumulator
ADD A,R2 ;Add the value of R2
SUBB A,R5 ;Subtract the value of R5 (which now contains R3 +
R4)

 The "B" register is very similar to the Accumulator in
the sense that it may hold an 8-bit (1-byte) value.
 The "B" register is only used by two 8051
instructions: MUL AB and DIV AB. Thus, if you
want to quickly and easily multiply or divide A by
another number, you may store the other number in
"B" and make use of these two instructions.

 The Data Pointer (DPTR) is the 8051’s only user-
accessable 16-bit (2-byte) register. The
Accumulator, "R" registers, and "B" register are
all 1-byte values.
 DPTR, as the name suggests, is used to point to
data. It is used by a number of commands which
allow the 8051 to access external memory.
 DPTR is most often used to point to data in
external memory.

 The Program Counter (PC) is a 2-byte
address which tells the 8051 where the
next instruction to execute is found in
memory.
 When the 8051 is initialized PC always
starts at 0000h and is incremented each
time an instruction is executed.
 It is important to note that PC isn’t always
incremented by one. Since some
instructions require 2 or 3 bytes the PC will
be incremented by 2 or 3 in these cases.

 The Stack Pointer, like all registers except DPTR and
PC, may hold an 8-bit (1-byte) value. The Stack
Pointer is used to indicate where the next value to be
removed from the stack should be taken from.
 When a value is push onto the stack, the 8051 first
increments the value of SP and then stores the value
at the resulting memory location.

0000
DPTR
0007
SP
0000
PSW
0000
B
0000
ACC
0000
PC
Reset Value
Register
RAM are all zero


2023
 Port 0（pins 32-39）
 When connecting an 8051 to an external memory, the 8051 uses
ports to send addresses and read instructions.
◦ 16-bit address：P0 provides both address A0-A7, P2 provides
address A8-A15.
◦ Also, P0 provides data lines D0-D7.
 When P0 is used for address/data multiplexing, it is connected to
the 74LS373 to latch the address.
 Port 1（pins 1-8）
 Port 1 is denoted by P1.
◦ P1.0 ~ P1.7
◦ P1 as an output port (i.e., write CPU data to the external pin)
◦ P1 as an input port (i.e., read pin data into CPU bus)

2023
 Although port 3 is configured as an output port upon reset,
this is not the way it is most commonly used.
 Port 3 has the additional function of providing signals.
◦ Serial communications signal：RxD, TxD
◦ External interrupt：/INT0, /INT1
◦ Timer/counter：T0, T1
◦ External memory accesses ：/WR, /RD

 The 8051 microcontroller's memory is
divided into Program Memory and Data
Memory.
 Program Memory (ROM) is used for
permanent saving program being executed,
 Data Memory (RAM) is used for temporarily
storing and keeping intermediate results and
variables.

 Program Memory (ROM) is used for permanent
saving program (CODE) being executed. The
memory is read only.
 Depending on the settings made in compiler,
program memory may also used to store
a constant variables. The 8051 executes
programs stored in program memory only.
 Code memory type specifier is used to refer to
program memory.
 8051 memory organization allows external
program memory to be added.
 How does the microcontroller handle external
memory depends on the pin EA logical state.

 Up to 256 bytes of internal data memory is
available
 Locations available to the user occupy
addressing space from 0 to 7Fh, i.e. first 128
registers and this part of RAM is divided in
several blocks.
 The first 128 bytes of internal data memory
are both directly and indirectly addressable.
 The upper 128 bytes of data memory (from
0x80 to 0xFF) can be addressed only
indirectly.

2023
 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
Memory Organization

 Memory block in the range of 20h to 2Fh is
bit-addressable, which means that each bit
being there has its own address from 0 to
7Fh.
 Since there are 16 such registers, this block
contains in total of 128 bits with separate
addresses ( Bit 0 of byte 20h has the bit
address 0, and bit 7 of byte 2Fh has the bit
address 7Fh).

 Access to external memory is slower than access
to internal data memory.
 There may be up to 64K Bytes of external data
memory.
 Several 8051 devices provide on-chip XRAM
space that is accessed with the same instructions
as the traditional external data space.
 This XRAM space is typically enabled via proper
setting of SFR register and overlaps the external
memory space.
 Setting of that register must be manually done in
code, before any access to external memory or
XRAM space is made.

Figure 2-8
Accessing
external
code
memory

Figure
2-11
Interface
to 1K
RAM

 SFRs which are also bit addressable
A, B, IP, IE, TCON, SCON, PSW, P0, P1, P2,
P3
 Other SFRs
TMOD, THO, TLO, TH1, TL1, SBUF, PCON,
SP, DPTR

DATA registers
CONTROL registers
Timers
Serial ports
Interrupt system
Analog to Digital converter
Digital to Analog converter
Etc.
Addresses 80h – FFh
Direct Addressing used to
access SPRs

Name Function Name Function
A Accumulator SBUF Serial Port data
buffer
B Arithmetic SP Stack Pointer
DPH Addressing Ext
Memory
TMOD Timer/Counter
mode cntrl
DPL Addressing Ext
Memory
TCON Timer/Counter
cntrl
IE Interrupt enable TL0 Timer0 lower byte
IP Interrupt Priority TH0 Timer0 higher
byte
P0 I/O Port Latch TL1 Timer1 lower byte
P1 I/O Port Latch TH1 Timer1 higher
byte
P2 I/O Port Latch
P3 I/O Port Latch
PCON Power Control

Gate : When set, timer only runs while INT(0,1)
is high.
C/T : Counter/Timer select bit.
M1 : Mode bit 1.
M0 : Mode bit 0.

TF1: Timer 1 overflow flag.
TR1: Timer 1 run control bit.
TF0: Timer 0 overflag.
TR0: Timer 0 run control bit.
IE1: External interrupt 1 edge flag.
IT1: External interrupt 1 type flag.
IE0: External interrupt 0 edge flag.
IT0: External interrupt 0 type flag.

Instruction Set and
Addressing Modes of 8051

Sunday, August 23, 2020
Immediate
Register
Direct
Register Indirect
Indexed
The way in which the instruction is specified.

Immediate Data is specified in the instruction itself
Egs:
MOV A,#65H
MOV A,#’A’
MOV R6,#65H
MOV DPTR,#2343H
MOV P1,#65H

MOV Rn, A ;n=0,..,7
ADD A, Rn
MOV DPL, R6
MOV DPTR, A
MOV Rm, Rn

Although the entire of 128 bytes of RAM can be
accessed using direct addressing mode, it is most often
used to access RAM loc. 30 – 7FH.
MOV R0, 40H
MOV 56H, A
MOV A, 4 ; ≡ MOV A, R4
MOV 6, 2 ; copy R2 to R6
; MOV R6,R2 is invalid !

 In this mode, register is used as a pointer to the data.
MOV A,@Ri
; move content of RAM loc. Where address is held by Ri into
A
( i=0 or 1 )
MOV @R1,B
In other word, the content of register R0 or R1 is sources or
target in MOV, ADD and SUBB insructions.
⌦ jump

This mode is widely used in accessing data elements
of look-up table entries located in the program (code)
space ROM at the 8051
MOVC A,@A+DPTR
A= content of address A +DPTR from ROM
Note:
Because the data elements are stored in the program
(code ) space ROM of the 8051, it uses the instruction
MOVC instead of MOV. The “C” means code.

Data transfer instructions
Addressing modes
Data processing (arithmetic and logic)
Program flow instructions

MOV dest, source dest 🡨 source
Stack instructions
PUSH byte ;increment stack pointer,
;move byte on stack
POP byte ;move from stack to byte,
;decrement stack pointer
Exchange instructions
XCH a, byte ;exchange accumulator and byte
XCHD a, byte ;exchange low nibbles of
;accumulator and byte

Immediate Mode – specify data by its value
mov A, #0 ;put 0 in the accumulator
;A = 00000000
mov R4, #11h ;put 11hex in the R4 register
;R4 = 00010001
mov B, #11 ;put 11 decimal in b register
;B = 00001011
mov DPTR,#7521h ;put 7521 hex in DPTR
;DPTR = 0111010100100001

Immediate Mode – continue
MOV DPTR,#7521h
MOV DPL,#21H
MOV DPH, #75
COUNT EGU 30
~
~
mov R4, #COUNT
MOV DPTR,#MYDATA
~
~
0RG 200H
MYDATA:DB “IRAN”

Register Addressing – either source or
destination is one of CPU register
MOV R0,A
MOV A,R7
ADD A,R4
ADD A,R7
MOV DPTR,#25F5H
MOV R5,DPL
MOV R,DPH
Note that MOV R4,R7 is incorrect

Direct Mode – specify data by its 8-bit address
Usually for 30h-7Fh of RAM
Mov a, 70h ; copy contents of RAM at 70h to a
Mov R0,40h ; copy contents of RAM at 70h to a
Mov 56h,a ; put contents of a at 56h to a
Mov 0D0h,a ; put contents of a into PSW

Direct Mode – play with R0-R7 by direct address
MOV A,4 ≡ MOV A,R4
MOV A,7 ≡ MOV A,R7
MOV 7,2 ≡ MOV R7,R6
MOV R2,#5 ;Put 5 in R2
MOV R2,5 ;Put content of RAM at 5 in R2

Register Indirect – the address of the source or
destination is specified in registers
Uses registers R0 or R1 for 8-bit address:
mov psw, #0 ; use register bank 0
mov r0, #0x3C
mov @r0, #3 ; memory at 3C gets #3
; M[3C] 🡨 3
Uses DPTR register for 16-bit addresses:
mov dptr, #0x9000 ; dptr 🡨 9000h
movx a, @dptr ; a 🡨 M[9000]
Note that 9000 is an address in external memory

Register Indexed Mode – source or destination
address is the sum of the base address and
the accumulator(Index)
Base address can be DPTR or PC
mov dptr, #4000h
mov a, #5
movc a, @a + dptr ;a 🡨 M[4005]

 A register can be accessed by direct and register
mode
 This 3 instruction has same function with different
code
0703 E500 mov a,00h
0705 8500E0 mov acc,00h
0708 8500E0 mov 0e0h,00h
 Also this 3 instruction
070B E9 mov a,r1
070C 89E0 mov acc,r1
070E 89E0 mov 0e0h,r1

 B – always direct mode - except in MUL & DIV
0703 8500F0 mov b,00h
0706 8500F0 mov 0f0h,00h
0709 8CF0 mov b,r4
070B 8CF0 mov 0f0h,r4
 P0~P3 – are direct address
0704 F580 mov p0,a
0706 F580 mov 80h,a
0708 859080 mov p0,p1
 Also other SFRs (pcon, tmod, psw,….)

Op code Direct address
Op code Immediate data
immediate addressing
add a,#3dh ;machine code=243d
Direct addressing
mov r3,0E8h ;machine code=ABE8

 Stack-oriented data transfer
– Only one operand (direct addressing)
– SP is other operand – register indirect - implied
 Direct addressing mode must be used in Push and
Pop
mov sp, #0x40 ; Initialize SP
push 0x55 ; SP 🡨 SP+1, M[SP] 🡨 M[55]
; M[41] 🡨 M[55]
pop b ; b 🡨 M[55]
Note: can only specify RAM or SFRs (direct mode) to push or pop.
Therefore, to push/pop the accumulator, must use acc, not a

 Therefore
Push a ;is invalid
Push r0 ;is invalid
Push r1 ;is invalid
push acc ;is correct
Push psw ;is correct
Push b ;is correct
Push 13h
Push 0
Push 1
Pop 7
Pop 8
Push 0e0h ;acc
Pop 0f0h ;b

 Serial communication means transfer data bit by
bit serially at a time, where as in parallel
communication, the number of bits that can be
transferred at a time depends upon the number
of data lines available for communication.
 Two methods of serial communication are
 Synchronous Communication: Transfer of bulk
data in framed structure at a time
 Asynchronous Communication: Transfer of a byte
data in framed structure at a time
 8051 has built in UART with RXD (serial data
receive pin) and TXD (serial data transmit pin) on
PORT3.0 and PORT3.1 respectively.

 Asynchronous serial communication is widely used for
byte oriented transmission.
 Frame structure in Asynchronous communication:
 START bit: It is a bit with which serial communication
start and it is always low.
 Data bits packet: Data bits can be 5 to 9 bits packet.
Normally we use 8 data bit packet, which is always sent
after START bit.
 STOP bit: This is one or two bits. It is sent after data
bits packet to indicate end of frame. Stop bit is always
logic high.
 In asynchronous serial communication frame, first
START bit followed by data byte and at last STOP bit,
forms a 10-bit frame. Sometimes last bit is also used
as parity bit.

Data transmission rate is measured in bits per
second (bps). In binary system it is also called
as baud rate (number of signal changes per
second).
Standard baud rates supported are 1200,
2400, 4800, 19200, 38400, 57600, and
115200. Normally most of the time 9600 bps
is used when speed is not a big issue.

SBUF: Serial Buffer Register
This is the serial communication data register
used to transmit or receive data through it.
SCON: Serial Control Register
Serial control register SCON is used to set
serial communication operation modes. Also
it is used to control transmit and receive
operations.

Mode SM0 SM1 Mode
0 0 0
1/12 of Osc frequency shift register mode fixed
baud rate
1 0 1 8-bit UART with timer 1 determined baud rate
2 1 0 9-bit UART with 1/32 of Osc fixed baud rate
3 1 1 9-bit UART with timer 1 determined baud rate
Bit 7:6 - SM0:SM1: Serial Mode Specifier
Normally mode-1 (SM0 =0, SM1=1) is used with 8 data bits, 1 start bit and
1 stop bit.

 Bit 5 - SM2: for Multiprocessor Communication
 This bit enables multiprocessor communication feature in mode 2 & 3.
 Bit 4 - REN: Receive Enable
 1 = Receive enable
 0 = Receive disable
 Bit 3 - TB8: 9th Transmit Bit
 This is 9th bit which is to be transmitted in mode 2 & 3 (9-bit mode)
 Bit 2 - RB8: 9th Receive Bit
 This is 9th received bit in mode 2 & 3 (9-bit mode), where as in mode 1, if SM2 =
0 then RB8 hold stop bit that received
 Bit 1 - TI: Transmit Interrupt Flag
 This bit indicates transmission is complete and gets set after transmitting the
byte from buffer. Normally TI (Transmit Interrupt Flag) is set by hardware at the
end of 8th bit in mode 0 and at the beginning of stop bit in other modes.
 Bit 0 – RI: Receive Interrupt Flag
 This bit indicates reception is complete and gets set after receiving the complete
byte in buffer. Normally RI (Receive Interrupt Flag) is set by hardware in receiving
mode at the end of 8th bit in mode 0 and at the stop bit receive time in other
modes.


The 8051 has two counters/timers which can
be used either as timer to generate a time
delay or as counter to count events
happening outside the microcontroller.
he 8051 has two timers: timer0 and timer1.
They can be used either as timers or as
counters. Both timers are 16 bits wide. Since
the 8051 has an 8-bit architecture, each 16-
bit is accessed as two separate registers of
low byte and high byte.

Timer0 registers is a 16 bits register and
accessed as low byte and high byte. The low
byte is referred as a TL0 and the high byte is
referred as TH0. These registers can be
accessed like any other registers.

Timer1 registers is also a 16 bits register and
is split into two bytes, referred to as TL1 and
TH1.

 This is an 8-bit register which is used by both timers 0 and
1 to set the various timer modes. In this TMOD register, lower
4 bits are set aside for timer0 and the upper 4 bits are set
aside for timer1
 In upper or lower 4 bits, first bit is a GATE bit. Every timer
has a means of starting and stopping. Some timers do this by
software, some by hardware, and some have both software
and hardware controls.
 The hardware way of starting and stopping the timer by an
external source is achieved by making GATE=1 in the TMOD
register. And if we change to GATE=0 then we do no need
external hardware to start and stop the timers.

 The second bit is C/T bit and is used to decide
whether a timer is used as a time delay generator
or an event counter. If this bit is 0 then it is used
as a timer and if it is 1 then it is used as a
counter.
 In upper or lower 4 bits, the last bits third and
fourth are known as M1 and M0 respectively.
These are used to select the timer mode.
 M0 M1 Mode Operating Mode
 0 0 0 13-bit timer mode,
 0 1 1 16-bit timer mode,
 1 0 2 8-bit auto reload
 1 1 3 Spilt timer mode.

TCON is 8-bit control register and contains timer and interrupt flags.
Bit 7 - TF1: Timer1 Overflow Flag
1 = Timer1 overflow occurred (i.e. Timer1 goes to its max and roll over back to
zero).
0 = Timer1 overflow not occurred.
It is cleared through software. In Timer1 overflow interrupt service routine, this bit
will get cleared automatically while exiting from ISR.
Bit 6 - TR1: Timer1 Run Control Bit
1 = Timer1 start.
0 = Timer1 stop.
It is set and cleared by software.
Bit 5 – TF0: Timer0 Overflow Flag
1 = Timer0 overflow occurred (i.e. Timer0 goes to its max and roll over back
to zero).
0 = Timer0 overflow not occurred.
It is cleared through software. In Timer0 overflow interrupt service routine, this bit
will get cleared automatically while exiting from ISR.

 Bit 4 – TR0: Timer0 Run Control Bit
 1 = Timer0 start.
 0 = Timer0 stop.
 It is set and cleared by software.
 Bit 3 - IE1: External Interrupt1 Edge Flag
 1 = External interrupt1 occurred.
 0 = External interrupt1 Processed.
 It is set and cleared by hardware.
 Bit 2 - IT1: External Interrupt1 Trigger Type Select Bit
 1 = Interrupt occur on falling edge at INT1 pin.
 0 = Interrupt occur on low level at INT1 pin.
 Bit 1 – IE0: External Interrupt0 Edge Flag
 1 = External interrupt0 occurred.
 0 = External interrupt0 Processed.
 It is set and cleared by hardware.
 Bit 0 – IT0: External Interrupt0 Trigger Type Select Bit
 1 = Interrupt occur on falling edge at INT0 pin.
 0 = Interrupt occur on low level at INT0 pin.

Interrupts in 8051 microcontroller are more
desirable to reduce the regular status
checking of the interfaced devices or inbuilt
devices.
Interrupt is an event that temporarily
suspends the main program, passes the
control to a special code section, executes
the event-related function and resumes the
main program flow where it had left off.

Interrupts are of different types like software
and hardware, maskable and non-maskable,
fixed and vector interrupts, and so on.
 Interrupt Service Routine (ISR) comes into the
picture when interrupt occurs, and then tells
the processor to take appropriate action for
the interrupt, and after ISR execution, the
controller jumps into the main program.

 8051 has 5 sources of interrupts
– Timer 0 overflow(T0)
– Timer 1 overflow(T1)
– External Interrupt 0(INT0)
– External Interrupt 1(INT1)
– Serial Port events(TI/RI)
– The Timer and Serial interrupts are internally
generated by the microcontroller, whereas the external
interrupts are generated by additional interfacing
devices or switches that are externally connected to
the microcontroller. These external interrupts can be
edge triggered or level triggered. When an interrupt
occurs, the microcontroller executes the interrupt
service routine so that memory location corresponds to
the interrupt that enables it. The Interrupt
corresponding to the memory location is given in the
interrupt vector table below.

 What if two interrupt sources interrupt at the same time?
 The interrupt with the highest PRIORITY gets serviced first.
 All interrupts have a default priority order.
 Priority can also be set to “high” or “low”.

This register is responsible for enabling and
disabling the interrupt. It is a bit addressable
register in which EA must be set to one for
enabling interrupts. The corresponding bit in
this register enables particular interrupt like
timer, external and serial inputs. In the below
IE register, bit corresponding to 1 activates
the interrupt and 0 disables the interrupt.

▪EA : Global enable/disable.
▪--- : Undefined.
▪ET2 :Enable Timer 2 interrupt.
▪ES :Enable Serial port interrupt.
▪ET1 :Enable Timer 1 interrupt.
▪EX1 :Enable External 1 interrupt.
▪ET0 : Enable Timer 0 interrupt.
▪EX0 : Enable External 0 interrupt.

 It is also possible to change the priority levels of
the interrupts by setting or clearing the
corresponding bit in the Interrupt priority (IP)
register as shown in the figure.
 This allows the low priority interrupt to interrupt
the high-priority interrupt, but prohibits the
interruption by another low-priority interrupt.
Similarly, the high-priority interrupt cannot be
interrupted.
 If these interrupt priorities are not programmed,
the microcontroller executes in predefined
manner and its order is INT0, TF0, INT1, TF1, and
SI.

 Operation:MOV
 Function:Move memory
 Syntax:MOV operand1,operand2
 Description: MOV copies the value
of operand2 into operand1. The value
of operand2 is not affected.
 Both operand1 and operand2 must be in Internal
RAM.
 No flags are affected unless the instruction is
moving the value of a bit into the carry bit in
which case the carry bit is affected or unless the
instruction is moving a value into the PSW register
(which contains all the program flags).

No flags are affected unless the instruction is
moving the value of a bit into the carry bit in
which case the carry bit is affected or unless
the instruction is moving a value into the PSW
register (which contains all the program
flags).
MOV @R0,#data
MOV @R0,A
MOV A,#data
MOV A,@R1
MOV A,R0

two way data transfer
XCH a, 30h ; a 🡨 🡨 M[30]
XCH a, R0 ; a 🡨 🡨 R0
XCH a, @R0 ; a 🡨 🡨 M[R0]
XCHD a, R0 ; exchange “digit”
R0[7..4] R0[3..0]
a[7..4] a[3..0]
Only 4 bits exchanged

 transfers between individual bits.
 Carry flag (C) (bit 7 in the PSW) is used as a single-
bit accumulator
 RAM bits in addresses 20-2F are bit addressable
mov C, P0.0
mov C, 67h
mov C, 2ch.7

SFRs with addresses
ending in 0 or 8
are bit-
addressable.
(80, 88, 90, 98, etc)
Notice that all 4
parallel I/O ports
are bit addressable.

 Subroutines allow us to have "structured" assembly language
programs.
 This is useful for breaking a large design into manageable
parts.
 It saves code space when subroutines can be called many
times in the same program.
Main: ...
acall sublabel
...
...
sublabel: ...
...
ret
the subroutine

square: push b
mov b,a
mul ab
pop b
ret
 8 byte and 11 machine cycle
 SP is initialized to 07 after reset.(Same address as R7)
 With each push operation 1st , pc is increased
 When using subroutines, the stack will be used to store the PC, so it is
very important to initialize the stack pointer. Location 2Fh is often
used.
mov SP, #2Fh

mov a,#0aah
Back1:mov p0,a
lcall delay1
cpl a
sjmp back1
Delay1:mov r0,#0ffh;1cycle
Here: djnz r0,here ;2cycle
ret ;2cycle
end
Delay=1+255*2+2=513 cycle
Delay2:
mov r6,#0ffh
back1: mov r7,#0ffh ;1cycle
Here: djnz r7,here ;2cycle
djnz r6,back1;2cycle
ret ;2cycle
end
Delay=1+(1+255*2+2)*255+2
=130818 machine cycle

Mnemonic Description
ADD A, byte add A to byte, put result in A
ADDC A, byte add with carry
SUBB A, byte subtract with borrow
INC A increment A
INC byte increment byte in memory
INC DPTR increment data pointer
DEC A decrement accumulator
DEC byte decrement byte
MUL AB multiply accumulator by b register
DIV AB divide accumulator by b register
DA A decimal adjust the accumulator

add a, byte ; a 🡨 a + byte
addc a, byte ; a 🡨 a + byte + C
These instructions affect 3 bits in PSW:
C = 1 if result of add is greater than FF
AC = 1 if there is a carry out of bit 3
OV = 1 if there is a carry out of bit 7, but not from bit 6, or
visa versa.

mov a, #3Fh
add a, #D3h
 What is the value of the
C, AC, OV flags after
the second instruction
is executed?
0011 1111
1101 0011
0001 0010
C = 1
AC = 1
OV = 0

0111 1111 (positive 127)
0111 0011 (positive 115)
1111 0010 (overflow
cannot represent 242 in 8
bits 2’s complement)
2’s complement:
0000 0000 00 0
…
0111 1111 7F 127
1000 0000 80 -128
…
1111 1111 FF -1
1000 1111 (negative 113)
1101 0011 (negative 45)
0110 0010 (overflow)
0011 1111 (positive)
1101 0011 (negative)
0001 0010 (never overflows)

; Computes Z = X + Y
; Adds values at locations 78h and 79h and puts them in 7Ah
;--------------------------------------------------------------
----
X equ 78h
Y equ 79h
Z equ 7Ah
;--------------------------------------------------------------
---
org 00h
ljmp Main
;--------------------------------------------------------------
---
org 100h
Main:
mov a, X
add a, Y
mov Z, a
end

; Computes Z = X + Y (X,Y,Z are 16 bit)
;------------------------------------------------------
------------
X equ 78h
Y equ 7Ah
Z equ 7Ch
;------------------------------------------------------
-----------
org 00h
ljmp Main
;------------------------------------------------------
-----------
org 100h
Main:
mov a, X
add a, Y
mov Z, a
mov a, X+1
adc a, Y+1
mov Z+1, a
end

SUBB A, byte subtract with borrow
Example:
SUBB A, #0x4F ;A 🡨 A – 4F – C
Notice that
There is no subtraction WITHOUT borrow.
Therefore, if a subtraction without borrow is desired,
it is necessary to clear the C flag.
Example:
Clr c
SUBB A, #0x4F ;A 🡨 A – 4F

 The increment and decrement instructions do NOT affect
the C flag.
 Notice we can only INCREMENT the data pointer, not
decrement.
INC A increment A
INC byte increment byte in memory
INC DPTR increment data pointer
DEC A decrement accumulator
DEC byte decrement byte

Assume 16-bit word in R3:R2
mov a, r2
add a, #1 ; use add rather than increment to affect C
mov r2, a
mov a, r3
addc a, #0 ; add C to most significant byte
mov r3, a

When multiplying two 8-bit numbers, the size of the
maximum product is 16-bits
FF x FF = FE01
(255 x 255 = 65025)
MUL AB ; BA 🡨 A *
B
Note : B gets the High byte
A gets the Low byte

Integer Division
DIV AB ; divide A by B
A 🡨 Quotient(A/B)
B 🡨 Remainder(A/B)
OV - used to indicate a divide by zero condition.
C – set to zero

DA a ; decimal adjust a
Used to facilitate BCD addition.
Adds “6” to either high or low nibble after an addition
to create a valid BCD number.
Example:
mov a, #23h
mov b, #29h
add a, b ; a 🡨 23h + 29h = 4Ch (wanted 52)
DA a ; a 🡨 a + 6 = 52

❑ Bitwise logic operations
❖ (AND, OR, XOR, NOT)
❑ Clear
❑ Rotate
❑ Swap
Logic instructions do NOT affect the flags in PSW

ANL 🡨 AND
ORL 🡨 OR
XRL 🡨 XOR
CPL 🡨 Complement
Examples:
0000111
1
1010110
0
ANL
0000111
1
1010110
0
ORL
0000111
1
1010110
0
XRL
1010110
0
CPL
0000110
0
1010111
1
1010001
1
0101001
1

a, byte
direct, reg. indirect, reg,
immediate
byte, a
direct
byte, #constant
a ex: cpl a
ANL – AND
ORL – OR
XRL – eXclusive oR
CPL – Complement

 Force individual bits low, without affecting other
bits.
anl PSW, #0xE7 ;PSW AND 11100111
 Force individual bits high.
orl PSW, #0x18 ;PSW OR 00011000
 Complement individual bits
xrl P1, #0x40 ;P1 XRL 01000000

CLR - clear
RL – rotate left
RLC – rotate left through Carry
RR – rotate right
RRC – rotate right through Carry
SWAP – swap accumulator nibbles

CLR A
CLR byte (direct mode)
CLR Ri (register mode)
CLR @Ri (register indirect mode)

Rotate instructions operate only on a
RL a
Mov a,#0xF0 ; a🡨 11110000
RR a ; a🡨 11100001
RR a
Mov a,#0xF0 ; a🡨 11110000
RR a ; a🡨 01111000

RRC a
mov a, #0A9h ; a 🡨 A9
add a, #14h ; a 🡨 BD (10111101), C🡨 0
rrc a ; a 🡨 01011110, C🡨 1
RLC a
mov a, #3ch ; a 🡨 3ch(00111100)
setb c ; c 🡨 1
rlc a ; a 🡨 01111001, C🡨 1
C
C

Note that a shift left is the same as
multiplying by 2, shift right is divide by 2
mov a, #3 ; A🡨 00000011 (3)
clr C ; C🡨 0
rlc a ; A🡨 00000110 (6)
rlc a ; A🡨 00001100 (12)
rrc a ; A🡨 00000110 (6)

SWAP a
mov a, #72h ; a 🡨 27h
swap a ; a 🡨 27h

 Some logic operations can be used with single bit
operands
ANL C, bit
ORL C, bit
CLR C
CLR bit
CPL C
CPL bit
SETB C
SETB bit
 “bit” can be any of the bit-addressable RAM
locations or SFRs.

Program segment to multiply by 2 and add 1.

Unconditional jumps (“go to”)
Conditional jumps
Call and return

SJMP <rel addr> ; Short jump,
relative address is 8-bit 2’s complement number,
so jump can be up to 127 locations forward, or 128
locations back.
LJMP <address 16> ; Long jump
AJMP <address 11> ; Absolute jump to
anywhere within 2K block of program memory
JMP @A + DPTR ; Long indexed
jump

Start: mov C, p3.7
mov p1.6, C
sjmp Start
Microcontroller application programs are almost always infinite loops!

Memory specific NOT Re-locatable (machine code)
org 8000h
Start: mov C, p1.6
mov p3.7, C
ljmp Start
end
Re-locatable (machine code)
org 8000h
Start: mov C, p1.6
mov p3.7, C
sjmp Start
end

Mov dptr,#jump_table
Mov a,#index_number
Rl a
Jmp @a+dptr
...
Jump_table: ajmp case0
ajmp case1
ajmp case2
ajmp case3

 These instructions cause a jump to occur only if a
condition is true. Otherwise, program execution
continues with the next instruction.
loop: mov a, P1
jz loop ; if a=0, goto loop,
; else goto next instruction
mov b, a
 There is no zero flag (z)
 Content of A checked for zero on time

JZ <rel addr> Jump if a = 0
JNZ <rel addr> Jump if a != 0
JC <rel addr> Jump if C = 1
JNC <rel addr> Jump if C != 1
JB <bit>, <rel addr> Jump if bit = 1
JNB <bit>,<rel addr> Jump if bit != 1
JBC <bir>, <rel addr> Jump if bit =1, &clear
bit
CJNE A, direct, <rel addr> Compare A and memory,
jump if not equal

jz led_off
Setb P1.6
sjmp skipover
led_off: clr P1.6
mov A, P0
skipover:
if (a = 0) is true
send a 0 to LED
else
send a 1 to LED

CJNE A, #data <rel addr> Compare A and data, jump
if not equal
CJNE Rn, #data <rel addr> Compare Rn and data,
jump if not equal
CJNE @Rn, #data <rel addr> Compare Rn and memory,
jump if not equal
DJNZ Rn, <rel addr> Decrement Rn and then
jump if not zero
DJNZ direct, <rel addr> Decrement memory and
then jump if not zero

For A = 0 to 4 do
{…}
clr a
loop: ...
...
inc a
cjne a, #4, loop
For A = 4 to 0 do
{…}
mov R0, #4
loop: ...
...
djnz R0, loop

mov a,#50h
mov b,#00h
cjne a,#50h,next
mov b,#01h
next: nop
end
mov a,#25h
mov r0,#10h
mov r2,#5
Again: mov @ro,a
inc r0
djnz r2,again
end
mov a,#0h
mov r4,#12h
Back: add a,#05
djnz r4,back
mov r5,a
end
mov a,#0aah
mov b,#10h
Back1:mov r6,#50
Back2:cpl a
djnz r6,back2
djnz b,back1
end

Call is similar to a jump, but
– Call pushes PC on stack before branching
acall <address ll> ; stack 🡨 PC
; PC 🡨 address 11 bit
lcall <address 16> ; stack 🡨 PC
; PC 🡨 address 16 bit

Return is also similar to a jump, but
– Return instruction pops PC from stack to get
address to jump to
ret ; PC 🡨 stack

 Assembly language programming
 Data Transfer operations
 Input/Output operations

 The assembly language is a fully hardware
related programming language.
 The embedded designers must have
sufficient knowledge on hardware of
particular processor or controllers before
writing the program.
 The assembly language is developed by
mnemonics; therefore, users cannot
understand it easily to modify the program.

 The assembly code must be written in upper
case letters
 The labels must be followed by a colon
(label:)
 All symbols and labels must begin with a
letter
 All comments are typed in lower case
 The last line of the program must be the END
directive

 The assembling directives give the directions
to the CPU. The
8051 microcontroller consists of various
kinds of assembly directives to give the
direction to the control unit. The most useful
directives are 8051 programming, such as:
 ORG
 DB
 EQU
 END

 ORG(origin): This directive indicates the start of the
program. This is used to set the register address during
assembly. For example; ORG 0000h tells the compiler all
subsequent code starting at address 0000h.
 Syntax: ORG 0000h
 DB(define byte): The define byte is used to allow a string
of bytes. For example, print the “EDGEFX” wherein each
character is taken by the address and finally prints the
“string” by the DB directly with double quotes.
 Syntax:
 ORG 0000h
 MOV a, #00h
————-
DB”EDGEFX”

 EQU (equivalent): The equivalent directive is used to
equate address of the variable.
 Syntax:
 reg equ,09h
—————–
—————–
MOV reg,#2h
 END:The END directive is used to indicate the end of the
program.
 Syntax:
 reg equ,09h
 —————–
—————–
MOV reg,#2h
END

 An Assembly language program consists of,
among other things, a series of lines of
Assembly language instructions.
 An Assembly language instruction consists
of a mnemonic, optionally followed by one or
two operands.
 The operands are the data items being
manipulated, and the mnemonics are the
commands to the CPU, telling it what to do
with those items.

 MOV destination, source. Data movement in the internal RAM. This type of
instructions supported by virtually all addresses, direct, indirect, recording and
immediate.
 MOV A,P0 ; Mueve el contenido del puerto 0 al acumulador
MOV R1,A ; Mueve el contenido del Acumulador al registro 1
 MOVX. Data movement in the external RAM (XRAM). This type of motion only
supports indirect addressing, register 8bit by R0 or R1 and 16-bit register via the
DPTR.
 MOV DPTR,#2000H ; Mover al registro apuntador DPTR el dato inmediato 2000H
(dirección)
MOVX A,@DPTR ; Mover el contenido de la memoria que apunta el DPTR (2000H)
al Acumulador
 MOVC. Allows movement of the accumulator ROM. By this statement can make the
manipulation or movement of tables from the program memory.
 XCH. Swaps the contents of the accumulator and the internal RAM.
 XCHD. Swaps the contents of the first 4 bits of the Accumulator with internal RAM.
 PUSH and POP. To transfer data to the stack.

 Computers transfer data in two ways: parallel and serial.
 In parallel data transfers, often 8 or more lines (wire conductors)
are used to transfer data to a device that is only a few feet away.
Examples of parallel transfers are printers and hard disks; each
uses cables with many wire strips. Although in such cases a lot
of data can be transferred in a short amount of time by using
many wires in parallel, the distance cannot be great.
 To transfer to a device located many meters away, the serial
method is used. In serial communication, the data is sent one bit
at a time, in contrast to parallel communication, in which the
data is sent a byte or more at a time.
 Serial communication of the 8051 is the topic of this chapter.
The 8051 has serial communication capability built into it,
thereby making possible fast data transfer using only a few
wires.

 When a microprocessor communicates with the
outside world, it provides the data in byte-sized
chunks. In some cases, such as printers, the
information is simply grabbed from the 8-bit data
bus and presented to the 8-bit data bus of the
printer.
 This can work only if the cable is not too long, since
long cables diminish and even distort signals.
Furthermore, an 8-bit data path is expensive.
 For these reasons, serial communication is used for
transferring data between two systems located at
distances of hundreds of feet to millions of miles
apart. Figure 10-1 diagrams serial versus parallel
data transfers.

The fact that serial communication uses a single data line instead of
the 8-bit data line of parallel communication not only makes it much
cheaper but also enables two computers located in two different cities
to communicate over the telephone
.For serial data communication to work, the byte of data must be
converted to serial bits using a parallel-in-serial-out shift register;
then it can be transmitted over a single data line. This also means
that at the receiving end there must be a serial-in-parallel-out shift
register to receive the serial data and pack them into a byte.

 Serial data communication uses two methods,
asynchronous and synchronous.
The synchronous method transfers a block of
data (characters) at a time, while
the 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. For this
reason, there are special 1C chips made by
many manufacturers for serial data
communications

. These chips are commonly referred to as UART (universal
asynchronous receiver-transmitter) and USART (universal
synchronous-asynchronous receiver-transmitter).

 Data transfer instructions are responsible for
transferring data between various memory storing
elements like registers, RAM, and ROM. The
execution time of these instructions varies based on
how complex an operation they have to perform.
 In the table given in next slides, we have listed all
the data transfer instruction. In the table [A]=
Accumulator; [Rn]=Register in RAM; DPTR=Data
Pointer; PC=Program Counter
 Lets take all these Data Transfer instructions one by
one.

 1) MOV Instruction-The MOV instruction has two
operands, the source, and the destination. The second
operand is the source, whereas the first one is the
destination. This instruction uses various addressing
modes to move data in the RAM space of the
microcontroller.
• Examples-MOV A, R0 //Moves data from the register
R0 to the accumulator
• MOV R0,50H //Moves data stored in memory location
50H to Ro
• MOV A,@R0 //Uses data stored in R0 register as an
address and moves the data at that location to the
accumulator

OPCODE OPERAND DESCRIPTION
MOV A,Rn Moves data from
registers in register
banks of RAM to
accumulator
MOV A, Address Moves data from an
address in the RAM
space to the
accumulator
MOV A,@Rn Uses data stored in a
register as an address
and moves the data at
that address to the
accumulator

MOV A,#Data Moves data given by
programmer directly
to the accumulator
MOV Rn,A Moves data from the
accumulator to
registers in register
bank
MOV Rn,Address Moves data from an
address in the RAM
space to a register in
the register banks

MOV Rn,#Data Moves data given by a
programmer directly
to a register in the
register banks
MOV Address,A Moves data to an
address in the RAM
space from the
Accumulator
MOV Address,Rn Moves data to an
address in the RAM
space from a register
in the register banks

MOV Address,Address Moves data from one
address to the other
MOV Address,@Ri Uses data stored in a
register as an address
and moves the data at
that address to a
register in the register
bank
MOV Address,#Data Moves data given by
the programmer
directly to an address

MOV @Rn,A Moves data from the
accumulator to an
address which is
stored in a register
MOV @Rn,Address Moves data from an
address to an address
which is stored in a
register
MOV @Rn,#Data Moves data given by
the programmer to an
address which is
stored in the register

 MOVC Instruction
 MOVC instruction is responsible for moving data
from the Program memory (Flash memory) to
the RAM for processing it.
 Example
• MOVC A,@A+DPTR
• MOVC A,@A+PC
 The table for this instruction is on the next
slide

MOVC A, @A+DPTR Moves data to
accumulator from a
address stored in the
memory location
(internal ROM) at
A+DPTR
MOVC A, @A+PC Moves data to
accumulator from a
address stored in the
memory
location(internal ROM)
at A+PC

 MOVX Instruction
 The 8051 microcontroller in most cases has an
on-chip 4K flash memory, but due to its 16-bit
address bus, it can access 64k memory locations.
Due to this reason, the 8051 can be interfaced
with external memory using ports 0 and 2. To
access data in this external memory, the MOVX
instruction is used
 The Table for this instruction starts from the next
slide.

MOVX A, @Rn Moves data to
accumulator from a
memory location
(External ROM)
MOVX @Rn,A Moves data to
Memory location
(External ROM) from a
register in the register
bank
MOVX A, @DPTR Moves data to
accumulator from a
memory location
(External ROM)
pointed by the Data
Pointer
MOVX @DPTR,A Moves data to
Memory location
(External ROM)

 Stack operations
 The RAM of the 8051 microcontroller is home to a set of 32 general-
purpose registers (00H-1FH). These registers are 8 bit wide and are
bundled in groups of 8 forming four register banks. Stack operations
can be used to place data into these registers in an efficient manner.
These stack operations use special commands (PUSH, POP) to place
and extract data from these general purposes registers.
 The PUSH operation
 The PUSH operation is used to place data into the stack. When this
command is given the value of address stored in the stack pointer is
increased by one. After incrementing the address in the stack pointer,
data is placed at that memory location. For example, when the 8051 is
powered up, it holds the address 07H. When it receives the first PUSH
instruction, the address is updated to 08H, and data is stored in that
location.

 Example (R6 contains 80H and stack pointer points at
07H)
 PUSH 6; This instruction moves data stored in register R6
to 08H
 The POP operation
 The POP operation is used to extract data that is stored in
the stack. This operation is the complete opposite of the
PUSH operation. It extracts the data from the location
which the stack pointer points to and then decreases the
value of the SP by 1.
 Example (Stack pointer points at memory location 08H
which contains 50H)
 POP 6; register R6 now contains the data 50H and the
stack pointer points to 07H

PUSH Rn Places value at the top
of the stack
POP Rn Extracts the data from
the top of the stack

 Exchange Instructions
 This operation is used to exchange data between
the source and the destination operands.
 Example
• XCH A, R0; exchanges the data stored in the
accumulator and R0
• XCHD A,@R0; Exchanges the lower four bits of a
memory location stored in a register with the
accumulator

XCH A,Rn Exchanges the value
between a register
and the accumulator
XCH A,Address Exchanges the value
between the
accumulator and a
memory location in
the RAM
XCHD A,@Rn Exchanges the value
between the
accumulator and a
memory location
stored in the register
XCHD A,@Rn Exchanges the lower
four bits of a memory
location stored in a

 The 8051 has four important ports. Port 0, Port 1, Port 2 and Port 3. These
ports allow the microcontroller to connect with the outside world. The four
ports of 8051 microcontrollers have certain specific functions and
corresponding features. In this post, we will have a look at the purpose of
each of these ports.
 What are the features of the four ports of 8051?
• Each port has 8 pins. Thus the four ports jointly comprise 32 pins.
• All ports are bidirectional.
• They are constructed with a D type output latch. They have output drivers
and input buffers.
• We can modify their functions using software and hardware that they
connect to.
• All the ports are configured as input ports on Reset.
• To configure ports as an input port 1 must be written to that port
• To configure it as an output port 0 must be written to it.

• Port 0 Features
• Address is 80H
• Construction: Port 0 has a D-type latch, unidirectional buffer, and 2
FETs at each pin. It does not have an internal pull-up resistor. An
external pull-up resistor is needed when Port 0 is defined as an output
port.
• Port 0 of the 8051 has two main functions: To be used as a simple
input-output port and to access external memory in conjunction with
Port 2.
 Functions of Port 0
 Simple I/O port:
 When we use Port 0 as an input port, the internal latch should know
that it’s being used for input, and thus, a digital 1 (FFH) is written at the
port address of 80H. This turns off the transistors causing the pin to
float in high impedance state connecting it to the input buffer. We can
read data from ‘Read Pin Data’/’Read Latch Bit.’

 When we use Port 0 as an output port, the latch
programmed to 0 will turn on. Consequently, the FET
will connect to GND. We will require an external pull
up resistor(10k Ohm) here to give a logic ‘1’ for using
Port 0 as an output port.
 When the 8051 wants to access external memory, the
address for the memory generates due to Port 0 and
Port 2. We get the lower half of the address from Port
0 and the upper half from Port 2. This is done using
ALE pulses, which help to latch the address to the
external bus. Once done, the Port 0 goes back to
being an input port to read data from that memory.

 Working of port 0
 As mentioned above port zero has a lot up its sleeve, from reading data to addresses it
does a lot of things for the microcontroller. Therefore it is imperative for us to get a deeper
understanding of the workings of this port.
 To configure port 0 as an input port the internal bus writes 1 to the D flip flop and the
control pin is set to 0(Upper FET is OFF). The mux is connected to Q'(0) of the D flip flop
as the control pin is 0. Due to this, the pin is connected to the input buffer which can be
read to get the input data.
 To use the port as an output port 0 is written to the D flip flop with the control signal being
set to 0. This enables the lower FET and disables the upper FET due to this the pin gets
connected to the ground and a zero is written to the output device. To write a 1 to the
external device the microcontroller writes 1 to the D flip flop which drives the pin to a high
impedance state as it is not connected to either VCC or ground. To solve this problem a
pull-up resistor is connected to the output pin which pulls the value to 5v or logic 1.
 For reading Addresses or data from external memory the Control bit is set to set to 1 which
connects the Mux to Data/address pin. The ALE pin is used to latch the address and once
that is done the port is used for data transfer.

 What are the features and functions of Port 1 in 8051?
 Features of Port 1:
• Address is 90H
• Construction: Port 1 has one D latch, two unidirectional buffers, 1
FET, and one internal pull-up resistor at each pin.
• It has only one function – to act as an Input-Output port.
 The function of Port 1 – I/O port:
 When Port 1 is functioning in the capacity of an input port, a
digital ‘1’ (FFH) is written to the latch. At 90H. This turns off the
transistor, and the pin floats in a high impedance state.
Consequently, it connects to the input buffer.
 When Port 1 is functioning in the capacity of an output port, the
latch is given a ‘LOW’ signal (00H). This turns the FER (Field
Effect Transistor) o. The pull-up resistor is OFF, and the port is
used as an output port.

 Features of Port 2
• Address is 10H
• Construction: Port 2 has a D type latch, 1 FET, an internal pull-up resistor, two
unidirectional buffers, and a Control Logic block.
• Its main functions are kind of similar to those of Port 0. It can be used as an
input-output port. And can access external memory in conjunction with Port 0.
 I/O port:
 Quite similar to Port 0. The only difference here is that in Port 2, we use one
FET with an internal pull-up resistor instead of the two FETs we saw in Port 0.
 Memory Access:
 Port 2 is used in conjunction with Port 0 to generate the upper address of the
external memory location that needs to be accessed. However, one key
difference is that it doesn’t need to turnaround and get a 1 in the latch
immediately for input as in Port 0. It can remain stable.

 Features of Port 3
• Address is B0H
• Construction: The third Port of 8051 has a D-type latch. In
addition to that, it has three unidirectional buffers. A FET with an
internal pull-up resistor. Additionally, it also has a NAND gate
connected to the FET.
• Port 3 performs two main functions, as we will see below.
 I/O port
 Just like Port 2, Port 3 can function as an input-output port.
 Alternate SFR function
 The input to SFR 1, we get the output of latch as 1, which turns
on the NAND gate, and depending on the value of ‘Alternate
Output Pin,’ FET will be wither ON/OFF.

RXD: this is used for a serial input port
TXD: this is used for serial output port
INT0: this used for an external interrupt 0
INT1: this used for external interrupt 1
T0: Timer 0 external input
T1: Timer 1 external input
WR: external data memory write strobe
RD: external data memory Read strobe

INSTRUCTION EXAMPLE EXPLANATION
MOV MOV A,Port Moves data from a given
port to the accumulator
JNB JNB
Port,Address
Checks the value in the input
buffer. If the value is zero
then it transfers the control
to the given address
JB JB Port,Address Checks the value in the input
buffer. If the value is not
zero then it transfers the
control to the given address
CJNE CJNE
A,Port,Address
Checks the value in the input
buffer of a port. Compares it
to the value in the
accumulator. If the values
are not the same then the
control is transferred to a
given address

ANL ANL P1, A Performs logical AND between
the value stored in the latch of
the port and the accumulator
ORL ORL P1, A Performs logical OR between
the port and the accumulator.
After this, it writes the new
value to the latch
XRL XRL P1, A Performs logical XOR between
the port and the accumulator.
After this, it writes the new
value to the latch
JBC JBC,Port,Add
ress
If the value at a given port is 1
then it jumps to a given
address and then clears the
latch

CPL CPL,Port Complements the
data in the latch
MOV MOV Port,C Reads the value at the
latch of a given port
and then transfers it
to the carry flag
CLR CLR Port Clears the value
stored at the latch of
a port
SETB SETB Port Sets the value of a
latch

Design and Interfacing with
8051

 keypad interface
 7- segment interface
 LCD Interfacing
 Stepper motor Interfacing

 At the lowest level, keyboards are organized in a
matrix of rows and columns.
 The CPU accesses both rows and columns through
ports; therefore, with two 8-bit ports, an 8 x 8 matrix
of keys can be connected to a microprocessor.
 When a key is pressed, a row and a column make a
contact; otherwise, there is no
 In such systems, it is the function of programs stored
in the EPROM of the microcontroller to scan the keys
continuously, identify which one has been activated,
and present it to the motherboard. In this section we
look at the mechanism by which the 8051 scans and
identifies the key.

 4 x 4 matrix connected to two ports. The rows
are connected to an output port and the columns
are connected to an input port.
 If no key has been pressed, reading the input
port will yield 1 s for all columns since they are
all connected to high (Vcc).
 If all the rows are grounded and a key is
pressed, one of the columns will have 0 since the
key pressed provides the path to ground.
 It is the function of the microcontroller to scan
the keyboard continuously to detect and identify
the key pressed.

 Keypad is used as an input device to read the key
pressed by user and to process it.
 4x4 keypad consists of 4 rows and 4 columns.
Switches are placed between the rows and
columns. A key press establishes a connection
between corresponding row and column between
which the switch is placed.
 To read the key press, we need to configure the
rows as outputs and columns as inputs.
 Columns are read after applying signals to the
rows in order to determine whether or not a key
is pressed and if pressed, which key is pressed.

 Display units are the most important output
devices in embedded projects and electronics
products. 16x2 LCD is one of the most used
display unit. 16x2 LCD means that there are
two rows in which 16 characters can be
displayed per line, and each character takes
5X7 matrix space on LCD.

 LCD 16x2 is 16 pin device which has 8 data pins (D0-
D7) and 3 control pins (RS, RW, EN). The remaining 5
pins are for supply and backlight for the LCD.
 The control pins help us configure the LCD in
command mode or data mode. They also help
configure read mode or write mode and also when to
read or write.
 LCD 16x2 can be used in 4-bit mode or 8-bit mode
depending on the requirement of the application. In
order to use it we need to send certain commands to
the LCD in command mode and once the LCD is
configured according to our need, we can send the
required data in data mode.

 For initializing the LCD, the following are the
steps that are given below and these steps
are the same for almost all the applications.
 Send 38H to the 8-bit data line for
initialization
 Send 0FH for making LCD ON, cursor ON,
cursor blinking ON
 Send 06H for incrementing cursor position
 Send 01H for clearing the display and return
the cursor

 The logic state of these pins that make the
module to determine whether a given data
input is a data or command to be displayed.
 Make R/W low
 Make RS=1, if the data byte is a data to be
displayed and make
 RS=0, if the data byte is a command.
 Place data byte on the data register
 Then pulse E from high to low
 Repeat the above steps for sending other
data

 Seven segment displays internally consist of 8
LEDs. In these LEDs, 7 LEDs are used to indicate
the digits 0 to 9 and single LED is used for
indicating decimal point. Generally seven
segments are two types, one is common cathode
and the other is common anode.
 In common cathode, all the cathodes of LEDs are
tied together and labeled as com. and the anode
are left alone. In common anode, seven segment
display all the anodes are tied together and
cathodes are left freely. Below figure shows the
internal connections of seven segment Display.

 A stepper motor is a brushless and synchronous
motor which divides the complete rotation into
number of steps. Each stepper motor will have some
fixed step angle and motor rotates at this angle.
 To interface a Stepper Motor with 8051 two different
drivers: L293D and ULN 2003 are required.
 The main principle of these circuits is to rotate the
stepper motor step wise at a particular step angle.
 The step size of the motor is determined by the
number of phases and the number of teeth on the
rotor. Step size is the angular displacement of the
rotor in one step.

 P1.0, P1.1, P1.2 and P1.3 pins are used for controlling the
phases A1, A2, A3 and A4 of the stepper motor respectively.
 ULN2003 is used for driving the individual phases of
the stepper motor. ULN2003 is used for driving high current
loads such as relays and motors.
 ULN2003 has 8 individual channels each with 1A capacity. The
channels can be paralleled to increase the current capacity. Each
channels are fitted with individual freewheeling diodes.
 The ULN2003 is operated in current sinking mode. Each channel
is activated by giving a logic LOW at the corresponding input.
 For example if we make pin 1 of ULN2003 LOW, phase A1 of the
stepper motor gets switched ON.

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