Eee iv-microcontrollers [10 es42]-notes

Microcontrollers 10ES42
Department of EEE, SJBIT Page 1
MICROCONTROLLERS
(Common to EC/TC/EE/IT/BM/ML)
Sub Code: 10ES42 IA Marks: 25
Hrs/ Week: 04 Exam Hours: 03
Total Hrs. 52 Exam Marks: 100
PART-A
UNIT 1:
Microprocessors and microcontroller. Introduction, Microprocessors and Microcontrollers, RISC
& CISC CPU Architectures, Harvard & Von- Neumann CPU architecture, Computer software.
The 8051 Architecture: Introduction, Architecture of 8051, Pin diagram of 8051, Memory
organization, External Memory interfacing, Stacks.
6 Hrs
UNIT 2:
Addressing Modes: Introduction, Instruction syntax, Data types, Subroutines, Addressing modes:
Immediate addressing , Register addressing, Direct addressing, Indirect addressing, relative
addressing, Absolute addressing, Long addressing, Indexed addressing, Bit inherent addressing,
bit direct addressing. Instruction set: Instruction timings, 8051 instructions: Data transfer
instructions, Arithmetic instructions, Logical instructions, Branch instructions, Subroutine
instructions, Bit manipulation instruction. 6 Hrs
UNIT 3:
8051 programming: Assembler directives, Assembly language programs and Time delay
calculations.
6 Hrs
UNIT 4:
8051 Interfacing and Applications: Basics of I/O concepts, I/O Port Operation, Interfacing 8051
to LCD, Keyboard, parallel and serial ADC, DAC, Stepper motor interfacing and DC motor
interfacing and programming
7 Hrs
PART-B
UNIT 5:
8051 Interrupts and Timers/counters: Basics of interrupts, 8051 interrupt structure, Timers and
Counters, 8051 timers/counters, programming 8051 timers in assembly and C.
6 Hrs

UNIT 6:
8051 Serial Communication: Data communication, Basics of Serial Data Communication, 8051
Serial Communication, connections to RS-232, Serial communication Programming in assembly
and C.
8255A Programmable Peripheral Interface:, Architecture of 8255A, I/O addressing,, I/O devices
interfacing with 8051 using 8255A.
6 Hrs
Course Aim – The MSP430 microcontroller is ideally suited for development of low-power
embedded systems that must run on batteries for many years. There are also applications where
MSP430 microcontroller must operate on energy harvested from the environment. This is possible
due to the ultra-low power operation of MSP430 and the fact that it provides a complete system
solution including a RISC CPU, flash memory, on-chip data converters and on-chip peripherals.
UNIT 7:
Motivation for MSP430microcontrollers – Low Power embedded systems, On-chip peripherals
(analog and digital), low-power RF capabilities. Target applications (Single-chip, low cost, low
power, high performance system design).
2 Hrs
MSP430 RISC CPU architecture, Compiler-friendly features, Instruction set, Clock system,
Memory subsystem. Key differentiating factors between different MSP430 families.
2 Hrs
Introduction to Code Composer Studio (CCS v4). Understanding how to use CCS for Assembly,
C, Assembly+C projects for MSP430 microcontrollers. Interrupt programming.
3 Hrs
Digital I/O – I/O ports programming using C and assembly, Understanding the muxing scheme of
the MSP430 pins.
2 Hrs
UNIT 8:
On-chip peripherals. Watchdog Timer, Comparator, Op-Amp, Basic Timer, Real Time Clock
(RTC), ADC, DAC, SD16, LCD, DMA.
2 Hrs

Using the Low-power features of MSP430. Clock system, low-power modes, Clock request
feature, Low-power programming and Interrupt.
2 Hrs
Interfacing LED, LCD, External memory. Seven segment LED modules interfacing. Example –
Real-time clock.
2 Hrs
Case Studies of applications of MSP430 - Data acquisition system, Wired Sensor network,
Wireless sensor network with Chipcon RF interfaces.
3 Hrs
TEXT BOOKS:
1. “The 8051 Microcontroller and Embedded Systems – using assembly and C ”-, Muhammad Ali
Mazidi and Janice Gillespie Mazidi and Rollin D. McKinlay; PHI, 2006 / Pearson, 2006
2. “MSP430 Microcontroller Basics”, John Davies, Elsevier, 2010 (Indian edition available)
REFERENCE BOOKS:
1. “The 8051 Microcontroller Architecture, Programming & Applications”, 2e Kenneth
J.Ayala , Penram International, 1996 / Thomson Learning 2005.
2. “The 8051 Microcontroller”, V.Udayashankar and MalikarjunaSwamy, TMH, 2009
3. MSP430 Teaching CD-ROM, Texas Instruments, 2008 (can be requested
http://www.uniti.in )
4. Microcontrollers: Architecture, Programming, Interfacing and System Design”,Raj Kamal,
“Pearson Education, 2005

INDEX SHEET
Unit TOPIC PAGE NO.
1 Microprocessors and microcontroller: Introduction,
6 to 30
Microprocessors and Microcontrollers, RISC & CISC CPU
Architectures, Harvard & Von- Neumann CPU architecture,
Computer software.
The 8051 Architecture: Introduction, Architecture of 8051,
Pin diagram of 8051, Memory organization, External
Memory interfacing, Stacks.
2
Addressing Modes: Introduction, Instruction syntax, Data
types, Subroutines,
31 to 59
Addressing modes: Immediate addressing, Register
addressing, Direct addressing, Indirect addressing, relative
addressing, Absolute addressing, Long addressing, Indexed
addressing, Bit inherent addressing, and bit direct addressing.
Instruction set: Instruction timings, 8051 instructions: Data
transfer instructions, Arithmetic instructions, Logical
instructions, Branch instructions, Subroutine instructions, Bit
manipulation instruction
3
8051 programming: Assembler directives, Assembly
language programs and Time delay calculations.
60 to 62
4
8051 Interfacing and Applications: Basics of I/O concepts,
I/O Port Operation, Interfacing 8051 to LCD, Keyboard,
parallel and serial ADC, DAC, Stepper motor interfacing and
DC motor interfacing and programming
63 to 90
5
8051 Interrupts and Timers/counters: Basics of interrupts,
8051 interrupt structure, Timers and Counters, 8051
timers/counters, programming 8051 timers in assembly and
C.
6
8051 Serial Communication: Data communication, Basics of
Serial Data Communication, 8051 Serial Communication,
connections to RS-232, Serial communication Programming
in assembly and C.
8255A Programmable Peripheral Interface:, Architecture of
8255A, I/O addressing,, I/O devices interfacing with 8051
using 8255A.
91 to 101
7
Motivation for MSP430microcontrollers – Low Power
embedded systems, On-chip peripherals (analog and digital),
low-power RF capabilities. Target applications (Single-chip,
low cost, low power, high performance system design).
101 to 110
MSP430 RISC CPU architecture, Compiler-friendly features,
Instruction set, Clock system, Memory subsystem. Key
differentiating factors between different MSP430 families.
Introduction to Code Composer Studio (CCS v4).
Understanding how to use CCS for Assembly, C,

Assembly+C projects for MSP430 microcontrollers. Interrupt
programming.
Digital I/O – I/O ports programming using C and assembly,
Understanding the muxing scheme of the MSP430 pins.
8
On-chip peripherals. Watchdog Timer, Comparator, Op-
Amp, Basic Timer, Real Time Clock (RTC), ADC, DAC,
SD16, LCD, DMA.
Using the Low-power features of MSP430. Clock
system, low-power modes, Clock request feature, Low-power
programming and Interrupt.
Interfacing LED, LCD, External memory. Seven
segment LED modules interfacing. Example – Real-time
clock.
Case Studies of applications of MSP430 -
Data acquisition system, Wired Sensor network, Wireless
sensor network with Chipcon RF interfaces.

UNIT 1:
Microprocessors and microcontroller.
Introduction, Microprocessors and Microcontrollers, RISC & CISC CPU Architectures, Harvard
& Von- Neumann CPU architecture, Computer software.
The 8051 Architecture: Introduction, Architecture of 8051, Pin diagram of 8051, Memory
organization, External Memory interfacing, Stacks.
Computer: A computer is a multipurpose programmable machine that reads binary
instructions from its memory , accepts binary data as input ,processes the data according to those
instructions and provides results as output. It is a programmable device made up of both
hardware and software. The various components of the computer are called hardware. A set of
instructions written for the computer to solve a specific task is called program and collection of
programs is called software .
The computer hardware consists of four main components. The central processing unit which
acts as computer‟s brain. Input unit through which program and data can be entered to computer,
output unit on which the results of the computations can be displayed. Memory in which data and
programs are stored.
Fig 1. Block diagram of a microcomputer
A computer that is designed using a microprocessor as its CPU , is known as a microcomputer.
Microprocessor or „Computer on Chip‟ first became a commercial reality in 1971 with the
introduction of the 4 bit 4004 by Intel. A byproduct of Microprocessor development was
Microcontroller. The same fabrication technology and programming concept that make the
general purpose microprocessor also yielded the Microcontroller.

Microprocessors:
A microprocessor is a general purpose digital computer central processing unit (CPU). Although
known as a „Computer on Chip‟ the Microprocessor in no sense a complete digital computer.
Block diagram of a Microprocessor CPU which contains ALU; Program counter (PC), a stack
pointer (SP) ,some working registers , a clock timing circuit and interrupt circuit s is shown in the
following figure
Fig.2.Block Diagram of a Microprocessor
To make a computer microcomputer one must add memory usually RAM and ROM,
memory decoders , an oscillator and a number of Input ,Output devices such as serial and parallel
ports. In addition special purpose devices such as interrupt handler and counters may be added to
relieve the CPU from time consuming counting or timing cores. When the Microcomputer is
equipped with mass storage devices , I/O peripherals such as a key board and a display CRT it
yields a small computer that can be applied to a range of general purpose applications.
The hardware design of a microprocessor is arranged such that a very small or very large
system can be configured around the CPU as the application demands as shown in Fig1. The
prime use of the Microprocessor is to read data , perform extensive calculations on that data, and
store those calculations in a mass storage device or display the results for human use. The
programs used by microprocessor are stored in the mass storage device and loaded into RAM as
user directs. A few microprocessor program are stored in ROM . The ROM based programs are
primarily small fixed programs that operate peripherals and other fixed devices that are connected
to the system.
Microcontroller: A Microcontroller is a programmable digital processor with necessary
peripherals. Both microcontrollers and microprocessors are complex sequential digital circuits
meant to carry out job according to the program / instructions. Sometimes analog input/output
interface makes a part of microcontroller circuit as mixed mode(both analog and digital) in nature.
A microcontroller can be compared to a Swiss knife with multiple functions incorporated
in the same Integrated Circuits. Block diagram of a typical Microcontroller which is a true
Arithmetic and Logic
Unit
Accumulator
Working Register
Program Counter
Clock Circuit
Stack Pointer
Interrupt Circuits

computer on a chip is shown below. The design incorporates all the features found in
microprocessor CPU : ALU,PC, SP and registers. It also has other features needed to make a
complete computer: ROM, RAM, Parallel I/O, serial I/O, Counters and clock circuits. Like the
microprocessor , a microcontroller is a general purpose device, but one that is meant to read
data, perform limited calculations on that data and control its environment based on those
calculations. The prime use of microcontroller is to control the operation of a machine using a
fixed program that is stored in ROM and that does not change over the lifetime of the system.
Fig3. Block diagram of a single chip computer
Complex Instruction Set Computer (CISC):
Memory in those days was expensive. Bigger programs required more storage which
included more money . There was a need to reduce the number of instructions per program .
This was achieved by having multiple operations within single instruction. Multiple
operations lead to many different kinds of instructions .Access to memory in turn makes the
instruction length variable and fetch-decode execute time unpredictable – making it more
complex. Thus hardware was made to understand the complexity of instruction set. The
computer having such instruction set was named as Complex Instruction Set Computer
(CISC). Intel 8051 is an example for CISC architecture.
Reduced Instruction Set Computer (RISC):
In applications which require more of input , output related operations having few simple
instructions that are of the same length allows memory access only with explicit load and
store instructions. Hence each instruction performs less work but instruction execution time
among different instructions is consistent. This would lead to instruction execution by
hardware including multiple number of registers inside CPU. The computer using such
instructions is called Reduced Instruction Set Computer (RISC). PIC microcontroller
manufactured by Microchip Company is an example for RISC architecture.
Vonneumann (Princeton) and Harvard Architecture :
Intel‟s 8051 employs Harvard architecture. A microcontroller has some embedded
peripherals and Input/Output (I/O) devices. The data transfer to these devices takes place
through I/O registers.
In a microprocessor, input /output (I/O) devices are externally interfaced and are mapped
either to memory address (memory mapped I/O) or a separate I/O address space (I/O mapped
I/O). There are two possible architectures one is Princeton (Von Neumann) and another is

Harvard .I/O Registers space in Princeton architecture have only one memory interface for
program memory (ROM) and data memory (RAM). One option is to map the I/O Register as a
part of data memory or variable RAM area ( memory mapped I/O). Alternatively a separate
I/O register space can be assigned (I/O Mapped I/O) . Both the arrangements are shown in
Fig.4.
Fig 4. Input/Output Registers in Princeton Architecture
As shown in Fig 4. Program memory and Data memory are together in both the arrangements.
The Princeton or Von neumann architecture one bus is used to carry the address and data with
an appropriate multiplexing technique ,which in turn reduces the cost. But Harvard
architecture which 8051 employs has separate Data memory and separate Code or Program
memory . The Fig. 5 and Fig .6 show the need for separate address and data bus for each
Program and Data memory in Harvard architecture. Since there are separate bus for access the
operation of fetching the code and data can happen simultaneously which increases the speed
of operation of execution inside CPU.
Fig. 5.Organization of I/O registers in Harvard Architecture

In Fig. 5, the first option is difficult to implement as there is no means to write to program
ROM area. It is also complicated to have a separate I/O space as shown in (3). Hence the
second option where I/O registers are placed in the register space is widely used in Harvard
architecture.
Fig6. Harvard Architecture
Computer Software: A set of instructions written in a specific sequence for computer to
solve a specific task is called a program, and software is collection of programs. The program
stored in the computer memory in the form of 0s and 1sand it is called as machine level
instructions. Since it would be difficult to remember machine codes in the form of binary
numbers an intermediate level of language for programming, between higher and machine
level was developed and is known as assembly level language . Assembly language programs
are written using assembly instructions known as mnemonics.
For example in CLR A, instruction CLR means clear and A means accumulator. The program
mnemonics are converted to machine codes in the form of binary by a software called
Assembler.
The Assembly language programming requires a detailed knowledge of the architecture
with which the program is executed. In order to overcome the drawback of assembly language
programming Higher level language like C,C++ are introduced where an interpreter or a
compiler takes care of translating a higher level source code into machine codes.
Development/Classification of microcontrollers : Microcontrollers have gone through a
silent evolution (invisible). The evolution can be rightly termed as silent as the impact or
application of a microcontroller is not well known to a common user, although
microcontroller technology has undergone significant change since early 1970's. Development
of some popular microcontrollers is given as follows.
Intel 4004
4 bit (2300 PMOS trans, 108
kHz)
1971
Intel 8048 8 bit 1976
Intel 8031 8 bit (ROM-less) .
Intel 8051 8 bit (Mask ROM) 1980
C
P
U
Data
Memory
Program
Memory
Data
Addres
s
Addres
s
Data

Microchip PIC16C64 8 bit 1985
Motorola 68HC11 8 bit (on chip ADC) .
Intel 80C196 16 bit 1982
Atmel AT89C51 8 bit (Flash memory) .
Microchip PIC 16F877 8 bit (Flash memory + ADC) .
We use more number of microcontrollers compared to microprocessors. Microprocessors are
primarily used for computational purpose, whereas microcontrollers find wide application in
devices needing real time processing and control. Application of microcontrollers are numerous.
Starting from domestic applications such as in washing machines, TVs, air conditioners,
microcontrollers are used in automobiles, process control industries , cell phones, electrical
drives, robotics and in space applications.
Fig. 7. Internal Structure of a typical Microcontroller
The one we are studying is a 8 bit Embedded Microcontroller introduced by Intel, 8051.
8051 ARCHITECTURE:

Fig 8. Block diagram of 8051 Microcontroller
Salient Features
• Eight bit CPU with registers A (Accumulator) and B
• Sixteen bit Program counter (PC) and a data pointer (DPTR)
• 8 Bit Program Status Word (PSW)
• 8 Bit Stack Pointer
• 4K Code Memory
• Internal Memory of 128 Bytes
• 32 I/O Pins arranged as 4 , 8 Bit ports
• Two 16 Bit Timer/Counter :T0, T1
• Full Duplex serial data receiver/transmitter
• Control Registers : TCON,TMOD,SCON,PCON,IP and IE
• Two External and Internal Interrupt sources
• Oscillator and clock circuits
The programming model of 8051 shows the 8051 as the collection of 8 and 16 bit registers and 8
bit memory locations. These registers and memory locations can be made to operate using
software instructions that are incorporated as part of the program instructions. The pin
configuration of 8051 is shown in Fig.9.
ALU PSW
A B
SFR
GPR & RAM
ROM
PC DPTR
DPH
DPL
Port
0
Port
2
Port
3
Port
1
I/O
A0-A7
D0-D7
I/O
I/O
A8-A15
I/O
INT
CNTR
SERIAL
RD/WR

Fig.9 Pin configuration of 8051
8051 Clock and Instruction Cycle:
The heart of 8051 is the circuitry that generates the clock pulses by which all internal operations
are synchronised. Pins XTAL1 and XTAL2 are provided for connecting resonator to form an
oscillator. The crystal frequency is the basic internal frequency of the microcontroller. 8051 is
designed to operate between 1MHz to 16MHz and generally operates with a crystal frequency
11.04962 MHz.
The oscillator formed by the crystal , capacitor and an on-chip inverter generates a pulse train at
the frequency of the crystal. The clock frequency f establishes the smallest interval to accomplish
any simple instruction. The time taken to complete any instruction is called as machine cycle or
instruction cycle. In 8051 one instruction cycle consists of 6 states or 12 clock cycles, instruction
cycle is also referred as Machine
Microcontroller Chips :
Broad Classification of different microcontroller chips could be as follows:
Embedded (Self -Contained) 8 - bit Microcontroller
16 to 32 Microcontrollers
Digital Signal Processors

cycle.
Fig. 10 Instruction cycle of 8051(Instruction cycle has six states (S 1 - S 6 ). Each state has two
pulses (P1 and P2))
Processor Architectures:
Fig 11.Basic 8051 Architecture
Internal Memory:
A functioning computer memory for program code bytes , commonly in ROM, and RAM
memory for variable data that can be altered as the program runs.. Additional memory can be
added externally using suitable circuits.
Unlike microcontrollers with Von- Neumann architectures, which can use a single memory
address for either program code or data, but not for both, the 8051 has Harvard architecture which
uses the same address in different memories for code and data The internal circuitry accesses the
current memory based on the nature of operation in the program.
Internal RAM: The 128 bytes internal RAM is organized into 3 distinct areas.
1. 32 bytes from address 00h to 1fh that make up 32 working registers organized as 4
memory banks of 8 registers each. The 4 register banks are numbered 0 to 3 and are made

up of 8 registers named R0 to R7. Each register can be addressed by name or by its RAM
addresses. Thus R0 of bank3 is R0 (if bank3 is selected) or address 18h (where bank3 is
selected). Bits RS0 and RS1 in the PSW determine which bank of registers is currently in
use at any time when program is running. Register banks not selected can be used as
general purpose RAM. Bank0 is selected by default on reset..
2. A bit addressable area of 16 bytes occupies RAM byte addresses 20h to 2fh, forming total
of 128 bits. An addressable bit may be specified by its bit address of 00h to 7fh or 8 bits
may form any byte address from 20h to 2fh.For example bit address 4fh is also bit 7 of
byte address 29h. Addressable bits are useful when the program need only remember a
binary event.
3. A general purpose RAM area above the bit area from 30h to 7f h, addressable as byte.
Fig.12. Internal RAM structure
The Stack and Stack pointer:
The stack refers to an area of internal RAM that is used in conjunction with certain opcodes
to store and retrieve data quickly. The 8 bit Stack Pointer (SP) register is used by the 8051 to
hold internal RAM address that is called the top of the stack. The address in SP register is the
location in internal RAM where the last byte of the data was stored by stack operation.
When data is to be placed on the stack , the SP increments before storing data on the stack so
that the stack grows up as data is stored. Whenever data is retrieved from the stack, the byte is

read from the stack and then the SP decrements to point to the next available byte of stored
data.
Operation of the Stack and Stack Pointer: Operation of the stack is shown in the above
figure. The SP is set to 07 when the 8051 is reset and can be changed to any internal RAM
address by the programmer. The stack is limited in height to the size of internal RAM. The
stack can overwrite valuable data in register banks, bit addressable RAM and scratched pad
RAM areas.It is programmer‟s responsibility to make it sure that the stack does not grow
beyond predefined bounds. The stack is normally placed high in the internal RAM by an
appropriate choice of the number placed in SP register, to avoid conflict with registers or
RAM.
Special Function Registers (SFRs):
The 8051 operations that do not use the internal RAM addresses from 00h to 7fh are done by
a group of specific internal registers each called a specific function register (SFR) which may
be addressed much like internal RAM using addresses from 80h to ffh.
Some SFRs are also bit addressable as is the case for the bit area of RAM. This feature allows
the programmer the programmer to change only what needs to be altered leaving the
remaining bits in that SFR unchanged. Not all of the addresses from 80h to ffh are used for
SFRs . Only the addressed ones can be used in programming SFRs and equivalent internal
RAM addresses are shown in Fig.10.
SFR Map: The set of Special Function Registers (SFRs) contain important registers such as
Accumulator, Register B, I/O Port latch registers, Stack pointer, Data Pointer, Processor
Status Word (PSW) and various control registers. Some of these registers are bit addressable
(they are marked with a * in the Fig. 13 below). The detailed map of various registers is
shown in the following figure.
The PC is not part of the SFR 0e0h or 8ch. and has no internal RAM address. SFRs are
named in certain opcodes by their function names as A, TH0 and can also be referred by their
addresses such as
Address
F8H
F0H B*
E8H
E0H ACC*
D8H
D0H PSW*
C8H (T2CON)* (RCAP2L) (RCAP2H) (TL2) (TH2)
C0H
B8H IP*
B0H P3*
A8H IE*
A0H P2*
98H SCON* SBUF

90H P1*
88H TCON* TMOD TL0 TL1 TH0 TH1
80H P0* SP DPL DPH PCON
Fig.13 Special Function Registers and the addresses
Internal ROM
8051 is organized so that data memory and program code memory can be two entirely
different physical memory entities. Each has the same address ranges. The internal program
ROM occupies code address space 000h to 0fffh. The PC is normally used to address program
code bytes from address 0000h to ffffh. Program addresses higher than offfh which exceed the
internal ROM capacity will cause the 8051 to automatically fetch code bytes from external
memory, addresses 00h to ffffh by connecting the external access pin (EA) to ground.
I/O Port pins, Ports and Circuits:One major feature of a microcontroller is versatility built
into the I/O circuits that connect the 8051 to the outside world. Out of 40 pins 24 pins may
each be used for one of two entirely different functions yielding a total pin configuration of
64.But the port pins have been multiplexed to perform different functions to make 8051 as 40
Pin IC.
The port pin circuitry is as shown below.
Fig. 14 Port -0
Port -0 has 8 pins (P0.0-P0.7).The structure of a Port-0 pin is shown in fig 13.Port-0 can be
configured as a normal bidirectional I/O port or it can be used for address/data interfacing for

accessing external memory. When control is '1', the port is used for address/data interfacing.
When the control is '0', the port can be used as a normal bidirectional I/O port.
Let us assume that control is '0'. When the port is used as an input port, '1' is written to the
latch. In this situation both the output MOSFETs are 'off'. Hence the output pin floats. This
high impedance pin can be pulled up or low by an external source. When the port is used as an
output port, a '1' written to the latch again turns 'off' both the output MOSFETs and causes the
output pin to float. An external pull-up is required to output a '1'. But when '0' is written to the
latch, the pin is pulled down by the lower MOSFET. Hence the output becomes zero.
When the control is '1', address/data bus controls the output driver MOSFETs. If the
address/data bus (internal) is '0', the upper MOSFET is 'off' and the lower MOSFET is 'on'.
The output becomes '0'. If the address/data bus is '1', the upper transistor is 'on' and the lower
transistor is 'off'. Hence the output is '1'. Hence for normal address/data interfacing (for
external memory access) no pull-up resistors are required. Port-0 latch is written to with 1's
when used for external memory access.
Port-1 Pin Structure:
Port-1 has 8 pins (P1.1-P1.7) .The structure of a port-1 pin is shown in fig 15
Fig 15. Port 1 Structure
Port-1 does not have any alternate function i.e. it is dedicated solely for I/O interfacing. When
used as output port, the pin is pulled up or down through internal pull-up. To use port-1 as
input port, '1' has to be written to the latch. In this input mode when '1' is written to the pin by
the external device then it reads fine. But when '0' is written to the pin by the external device
then the external source must sink current due to internal pull-up. If the external device is not
able to sink the current the pin voltage may rise, leading to a possible wrong reading.

Port-2 has 8-pins (P2.0-P2.7) . The structure of a port-2 pin is shown in fig 14.
Fig. 16.PORT 2 Pin Structure
Port-2 is used for higher external address byte or a normal input/output port. The I/O
operation is similar to Port-1. Port-2 latch remains stable when Port-2 pin are used for external
memory access. Here again due to internal pull-up there is limited current driving capability.

Fig. 17.PORT 3 Pin Structure:
Each pin of Port-3 can be individually programmed for I/O operation or for alternate function.
The alternate function can be activated only if the corresponding latch has been written to '1'.
To use the port as input port, '1' should be written to the latch. This port also has internal pull-
up and limited current driving capability.
Alternate functions of Port-3 pins –
Note:
1. Port 1, 2, 3 each can drive 4 LS TTL inputs.
2. Port-0 can drive 8 LS TTL inputs in address /data mode. For digital output port, it needs
external pull-up resistors.
3. Ports-1,2and 3 pins can also be driven by open-collector or open-drain outputs.
Each Port 3 bit can be configured either as a normal I/O or as a special function bit. Reading a
port (port-pins) versus reading a latch. There is a subtle difference between reading a latch
and reading the output port pin.
P3.0 RxD
P3.1 TxD
P3.2
P3.3
P3.4 T0
P3.5 T1
P3.6
P3.7

The status of the output port pin is sometimes dependant on the connected load. For instance
if a port is configured as an output port and a '1' is written to the latch, the output pin should
also show '1'. If the output is used to drive the base of a transistor, the transistor turns 'on'. If
the port pin is read, the value will be '0' which is corresponding to the base-emitter voltage of
the transistor. Reading a latch: Usually the instructions that read the latch, read a value,
possibly change it, and then rewrite it to the latch. These are called "read-modify-write"
instructions. Examples of a few instructions are-
ORL P2, A; P2 <-- P2 or A
MOV P2.1, C; Move carry bit to PX.Y bit
In this the latch value of P2 is read, is modified such that P2.1 is the same as Carry and is
then written back to P2 latch.
Reading a Pin: Examples of a few instructions that read port pin, are-
MOV A, P0; Move port-0 pin values to A
MOV A, P1; Move port-1 pin values to A
Connecting External Memory: The following figure shows the connection between an
8051 and external memory
Interfacing External Memory: The system designer is not limited by the amount of
internal ROM and RAM available on chip. Two separate external memory spaces are
made available by the 16 bit Program Counter PC and DPTR and by different control pins
for enabling the external ROM and RAM chips.
Internal control entry accesses the correct physical memory , depending on the machine
cycle state and opcode being executed . There are several reasons for adding external
memory, particularly Program Memory, when applying the 8051 in a system. When
project is in the prototype stage, having a masked internal ROM for each program “try” is
prohibitive. To help the programmer the manufacturers make available an EPROM
version, the 8751, which has 4K of on-chip EPROM that may be programmed and erased
as needed as the program is developed
If external program/data memory are to be interfaced, they are interfaced in the following
way.

Fig.18.Diagram for Interfacing of External Memory
External program memory is fetched if either of the following two conditions are satisfied.
External program memory is fetched if either of the following two conditions are satisfied.
1. Enable Address) is low. The microcontroller by default starts searching for program
from external program memory.
2. PC is higher than FFFH for 8051 or 1FFFH for 8052.
3. tells the outside world whether the external memory fetched is program memory or
data memory. is user configurable. is processor controlled.
Accessing external memory: Access to external program memory uses the signal
(Program store enable) as the read strobe. Access to external data memory uses
(alternate function of P3.7 and P3.6).
For external program memory, always 16 bit address is used. For example –Access to external
data memory can be either 8-bit address or 16-bit address - 8-bit address- MOVX A, @Rp
where Rp is either R0 or R1
MOVX @Rp, A
16 bit address- MOVX A,@DPTR
MOV X @DPTR, A.The external memory access in 8051 can be shown by a schematic
diagram as given in fig 19.

Fig 19. Schematic diagram of external memory access
If an 8-bit external address is used for data memory (i.e. MOVX @Rp) then the content of Port-2
SFR remains at Port-2 pins throughout the external memory cycle. This facilitates memory paging
as the upper 8 bit address remains fixed.
During any access to external memory, the CPU writes FFH to Port-0 latch (SFR). If the user
writes to Port-0 during an external memory fetch, the incoming byte is corrupted.
External program memory is accessed under the following condition.
1. Whenever is low, or whenever PC contains a number higher than 0FFFH (for 8051) or
1FFF (for 8052).
Some typical use of code/program memory access: External program memory can be not only
used to store the code, but also for lookup table of various functions required for a particular
application. Mathematical functions such as Sine, Square root, Exponential, etc. can be stored in
the program memory (Internal or external) and these functions can be accessed using MOVC
instruction.
Timers / Counters :
8051 has two 16-bit programmable UP timers/counters. They can be configured to operate either
as timers or as event counters. The names of the two counters are T0 and T1 respectively. The
timer content is available in four 8-bit special function registers, viz, TL0,TH0,TL1 and TH1
respectively.
In the "timer" function mode, the counter is incremented in every machine cycle. Thus, one can
think of it as counting machine cycles. Hence the clock rate is 1/12 th
of the oscillator frequency.

In the "counter" function mode, the register is incremented in response to a 1 to 0 transition at its
corresponding external input pin (T0 or T1). It requires 2 machine cycles to detect a high to low
transition. Hence maximum count rate is 1/24 th
of oscillator frequency.
The operation of the timers/counters is controlled by two special function registers, TMOD and
TCON respectively.
Timer Mode control (TMOD) Special Function Register:
TMOD register is not bit addressable.
TMOD Address: 89 H
Various bits of TMOD are described as follows –Gate: This is an OR Gate enabled bit which
controls the effect of on START/STOP of Timer. It is set to one ('1') by the program to
enable the interrupt to start/stop the timer. If TR1/0 in TCON is set and signal on pin is high
then the timer starts counting using either internal clock (timer mode) or external pulses (counter
mode).
It is used for the selection of Counter/Timer mode.
Mode Select Bits:
M1 and M0 are mode select bits.
Timer/ Counter control logic:

Fig .20. Timer/Counter Control Logic
Timer control (TCON) Special function register:
TCON is bit addressable. The address of TCON is 88H. It is partly related to Timer and partly to
interrupt.
Fig. 20. TCON Register
The various bits of TCON are as follows. TF1 : Timer1 overflow flag. It is set when timer rolls
from all 1s to 0s. It is cleared when processor vectors to execute ISR located at address 001BH.
TR1:Timer1 run control bit. Set to1tostartthe timer / counter.
TF0:Timer0overflowflag.(SimilartoTF1)TR0:Timer0 run control bit.
IE1 : Interrupt1 edge flag. Set by hardware when an external interrupt edge is detected. It is
interrupt is processed.
IE0:Interrupt0edgeflag.(SimilartoIE1)
IT1 : Interrupt1 type control bit. Set/ cleared by software to specify falling edge / low level
triggered external interrupt.
IT0 : Interrupt0 type control bit. (Similar to IT1)
As mentioned earlier, Timers can operate in four different modes. They are as follows
Timer Mode-0:
In this mode, the timer is used as a 13-bit UP counter as follows.

Fig. 21. Operation of Timer on Mode-0
The lower 5 bits of TLX and 8 bits of THX are used for the 13 bit count.Upper 3 bits of TLX are
ignored. When the counter rolls over from all 0's to all 1's, TFX flag is set and an interrupt is
generated.
The input pulse is obtained from the previous stage. If TR1/0 bit is 1 and Gate bit is 0, the counter
continues counting up. If TR1/0 bit is 1 and Gate bit is 1, then the operation of the counter is
controlled by input. This mode is useful to measure the width of a given pulse fed to
input.
Timer Mode-1:
This mode is similar to mode-0 except for the fact that the Timer operates in 16-bit mode.
.
Fig .22of Timer in Mode 1
Timer Mode-2: (Auto-Reload Mode)
This is a 8 bit counter/timer operation. Counting is performed in TLX while THX stores a
constant value. In this mode when the timer overflows i.e. TLX becomes FFH, it is fed with the
value stored in THX. For example if we load THX with 50H then the timer in mode 2 will count
from 50H to FFH. After that 50H is again reloaded. This mode is useful in applications like fixed
time sampling.
Fig .23. Operation of Timer in Mode 2
Timer Mode-3:

Timer 1 in mode-3 simply holds its count. The effect is same as setting TR1=0. Timer0 in mode-3
establishes TL0 and TH0 as two separate counters.
Fig. 24. Operation of Timer in Mode 3
Control bits TR1 and TF1 are used by Timer-0 (higher 8 bits) (TH0) in Mode-3 while TR0 and
TF0 are available to Timer-0 lower 8 bits(TL0).
Serial Interface
The serial port of 8051 is full duplex, i.e., it can transmit and receive simultaneously. The register
SBUF is used to hold the data. The special function register SBUF is physically two registers.
One is, write-only and is used to hold data to be transmitted out of the 8051 via TXD. The other
is, read-only and holds the received data from external sources via RXD. Both mutually exclusive
registers have the same address 099H.
Serial Port Control Register (SCON)
Register SCON controls serial data communication.
Address: 098H (Bit addressable)
Mode select bits

SM2:multi processor communication bit
REN: Receive enable bit
TB8: Transmitted bit 8 (Normally we have 0-7 bits transmitted/received)
RB8: Received bit 8
TI: Transmit interrupt flag
RI: Receive interrupt flag
Power Mode control Register
Register PCON controls processor powerdown, sleep modes and serial data bandrate. Only one
bit of PCON is used with respect to serial communication. The seventh bit (b7)(SMOD) is used to
generate the baud rate of serial communication.
Address: 87H
SMOD: Serial baud rate modify bit
GF1: General purpose user flag bit 1
PD: Power down bit
IDL: Idle mode bit
Data Transmission :Transmission of serial data begins at any time when data is written to SBUF.
Pin P3.1 (Alternate function bit TXD) is used to transmit data to the serial data network. TI is set
to 1 when data has been transmitted. This signifies that SBUF is empty so that another byte can be
sent
Data Reception: Reception of serial data begins if the receive enable bit is set to 1 for all modes.
Pin P3.0 (Alternate function bit RXD) is used to receive data from the serial data network.
Receive interrupt flag, RI, is set after the data has been received in all modes. The data gets stored
in SBUF register from where it can be read
Serial Data Transmission Modes:
Mode-0: In this mode, the serial port works like a shift register and the data transmission works
synchronously with a clock frequency of fosc /12. Serial data is received and transmitted through
RXD. 8 bits are transmitted/ received aty a time. Pin TXD outputs the shift clock pulses of
frequency fosc /12, which is connected to the external circuitry for synchronization. The shift
frequency or baud rate is always 1/12 of the oscillator frequency

Fig .25. Data transmission/reception in Mode-0
Mode-1 (standard UART mode) :
In mode-1, the serial port functions as a standard Universal Asynchronous Receiver Transmitter
(UART) mode. 10 bits are transmitted through TXD or received through RXD. The 10 bits
consist of one start bit (which is usually '0'), 8 data bits (LSB is sent first/received first), and a
stop bit (which is usually '1'). Once received, the stop bit goes into RB8 in the special function
register SCON. The baud rate is variable.
The following figure shows the way the bits are transmitted/ received.
Fig .26. Data transmission format in UART mode
Bit time= 1/fbaud
In receiving mode, data bits are shifted into the receiver at the programmed baud rate. The data
word (8-bits) will be loaded to SBUF if the following conditions are true.
1. RI must be zero. (i.e., the previously received byte has been cleared from SBUF)
Mode bit SM2 = 0 or stop bit = 1.
After the data is received and the data byte has been loaded into SBUF, RI becomes one.
Mode-1 baud rate generation:

Timer-1 is used to generate baud rate for mode-1 serial communication by using overflow flag of
the timer to determine the baud frequency. Timer-1 is used in timer mode-2 as an auto-reload 8-
bit timer. The data rate is generated by timer-1 using the following formula.
Where, SMOD is the 7th
bit of PCON register
fosc is the crystal oscillator frequency of the microcontroller
It can be noted that fosc/ (12 X [256- (TH1)]) is the timer overflow frequency in timer mode-2,
which is the auto-reload mode.
If timer-1 is not run in mode-2, then the baud rate is,
Timer-1 can be run using the internal clock,
fosc/12 (timer mode) or from any external source via pin T1 (P3.5) (Counter mode).
Example: If standard baud rate is desired, then 11.0592 MHz crystal could be selected. To get a
standard 9600 baud rate, the setting of TH1 is calculated as follows.
Assuming SMOD to be '0'
Or,
Or,
In mode-1, if SM2 is set to 1, no receive interrupt (RI) is generated unless a valid stop bit is
received.
Interrupts:
8051 provides 5 vectored interrupts. They are-
1.
2. TF0
3.
4. TF1
5. RI/TI
Out of these, and are external interrupts whereas Timer and Serial port interrupts are
generated internally. The external interrupts could be negative edge triggered or low level triggered.
All these interrupt, when activated, set the corresponding interrupt flags. Except for serial interrupt,
the interrupt flags are cleared when the processor branches to the Interrupt Service Routine (ISR).
The external interrupt flags are cleared on branching to Interrupt Service Routine (ISR), provided
the interrupt is negative edge triggered. For low level triggered external interrupt as well as for
serial interrupt, the corresponding flags have to be cleared by software by the programmer.

UNIT 2:
Addressing Modes: Introduction, Instruction syntax, Data types, Subroutines, Addressing modes:
Immediate addressing , Register addressing, Direct addressing, Indirect addressing, relative
addressing, Absolute addressing, Long addressing, Indexed addressing, Bit inherent addressing,
bit direct addressing. Instruction set: Instruction timings, 8051 instructions: Data transfer
instructions, Arithmetic instructions, Logical instructions, Branch instructions, Subroutine
instructions, Bit manipulation instruction.
Instruction set of 8051
1. Data transfer instructions
a. MOV <dest-byte>,<src-byte>-
Function: Move byte variable
Description: The byte variable indicated by the second operand is copied into the
location specified by the first operand. The source byte is not affected. No other
register or flag is affected.
1. mov direct , A
2. mov A, @Ri
3. mov A, Rn
4. mov direct, direct
5. mov A, #data
EX: MOV 30h, A
MOV A,@R0 ; moves the content of memory pointed to by Ro into A
MOV A, R1; ;moves the content of Register R1 to Accumulator A
MOV 20h,30h;moves the content of memory location 30h to 20h
MOV A,#45h;moves 45h to Accumulator A
MOV <dest-bit>,<src-bit>
Function: Move bit data
Description: MOV <dest-bit>,<src-bit> copies the Boolean variable indicated by the
second operand into the location specified by the first operand. One of the operands must
be the carry flag; the other may be any directly addressable bit. No other register or flag
is affected.
Example: MOV P1.3,C; moves the carry bit to 3rd
bit of port1
C. MOV DPTR,#data16

Function: Load Data Pointer with a 16-bit constant
Description: MOV DPTR,#data16 loads the Data Pointer with the 16-bit constant indicated. The
16-bit constant is loaded into the second and third bytes of the instruction. The second byte (DPH)
is the high-order byte, while the third byte
(DPL) holds the lower-order byte. No flags are affected.
This is the only instruction which moves 16 bits of data at once.
Example: The instruction, MOV DPTR, # 4567H
loads the value 4567H into the Data Pointer. DPH holds 45H, and DPL holds 67H.
d. MOVC A,@A+ <base-reg>
Function: Move Code byte
Description: The MOVC instructions load the Accumulator with a code byte or constant from
program memory. The address of the byte fetched is the sum of the original unsigned 8-bit
Accumulator contents and the contents of a 16-bit base register, which may be either the Data
Pointer or the PC. In the latter case, the PC is incremented to the address of the following
instruction before being added with the Accumulator; otherwise the base register is not altered.
Sixteen-bit addition is performed so a carry-out from the low-order eight bits may propagate
through higher-order bits. No flags are affected.
e. MOVC A,@A+PC
(PC) (PC) + 1
(A) ((A) + (PC))
f. MOVX <dest-byte>,<src-byte>
Function: Move External
Description: The MOVX instructions transfer data between the Accumulator and a byte of
external data memory, which is why “X” is appended to MOV. There are two types of
instructions, differing in whether they provide an 8-bit or 16-bit indirect address to the external
data RAM.
In the first type, the contents of R0 or R1 in the current register bank provide an 8-bit address
multiplexed with data on P0. Eight bits are sufficient for external I/O expansion decoding or for a
relatively small RAM array. For somewhat larger arrays, any output port pins can be used to
output higher-order address bits. These pins are
controlled by an output instruction preceding the MOVX.

In the second type of MOVX instruction, the Data Pointer generates a 16-bit address. P2 outputs
the high-order eight address bits (the contents of DPH), while P0 multiplexes the low-order eight
bits (DPL) with data. The P2 Special Function Register retains its previous contents, while the P2
output buffers emit the contents of DPH.
This form of MOVX is faster and more efficient when accessing very large data arrays (up to 64K
bytes), since no additional instructions are needed to set up the output ports.
It is possible to use both MOVX types in some situations. A large RAM array with its high-order
address lines driven by P2 can be addressed via the Data Pointer, or with code to output high-
order address bits to P2, followed by a MOVX instruction using R0 or R1.
Example: An external 256 byte RAM using multiplexed address/data lines is connected to the
8051 Port 0. Port 3 provides control lines for the external RAM. Ports 1 and 2 are used for normal
I/O. Registers 0 and 1 contain 12H and 34H. Location 34H of the external RAM holds the value
56H. The instruction sequence,
MOVX A,@R1
MOVX @R0,A
copies the value 56H into both the Accumulator and external RAM location 12H.
MOVX A,@DPTR
((DPTR))
PUSH direct
Function: Push onto stack
Description: The Stack Pointer is incremented by one. The contents of the indicated variable is
then copied into the internal RAM location addressed by the Stack Pointer. No flags are affected.
Example: On entering an interrupt routine, the Stack Pointer contains 09H. The Data Pointer
holds the value 0123H. The
following instruction sequence,
PUSH DPL
PUSH DPH
leaves the Stack Pointer set to 0BH and stores 23H and 01H in internal RAM locations 0AH and
0BH,
respectively.

POP direct
Function: Pop from stack.
Description: The contents of the internal RAM location addressed by the Stack Pointer is read,
and the Stack Pointer is decremented by one. The value read is then transferred to the directly
addressed byte indicated. No flags are affected.
Example: The Stack Pointer originally contains the value 32H, and internal RAM locations 30H
through 32H contain the values 20H, 23H, and 01H, respectively. The following instruction
sequence,
POP DPH
POP DPL
leaves the Stack Pointer equal to the value 30H and sets the Data Pointer to 0123H.
2. Arithmetic Group of Instructions
a. ADD A,<src-byte>
Function: Add
Description: ADD adds the byte variable indicated to the Accumulator, leaving the result in the
Accumulator. The carry and auxiliary-carry flags are set, respectively, if there is a carry-out from
bit 7 or bit 3, and cleared otherwise. When adding unsigned integers, the carry flag indicates an
overflow occurred.
OV is set if there is a carry-out of bit 6 but not out of bit 7, or a carry-out of bit 7 but not bit 6;
otherwise, OV is cleared. When adding signed integers, OV indicates a negative number
produced as the sum of two positive operands, or a positive sum from two negative operands.
Four source operand addressing modes are allowed: register, direct, register-indirect, or
immediate.
Example: The Accumulator holds 0C3H (1100001lB), and register 0 holds 0AAH (10101010B).
The following instruction,
ADD A,R0
leaves 6DH (01101101B) in the Accumulator with the AC flag cleared and both the carry flag and
OV set to 1.
ADD A, direct

(A)
ADD A, @Ri
(A) (A) + data
ADDC A, <src-byte>
Function: Add with Carry
Description: ADDC simultaneously adds the byte variable indicated, the carry flag and the
Accumulator contents, leaving the result in the Accumulator. The carry and auxiliary-carry flags
are set respectively, if there is a carry-out from bit 7 or bit 3, and cleared otherwise. When adding
unsigned integers, the carry flag indicates an overflow occurred.
OV is set if there is a carry-out of bit 6 but not out of bit 7, or a carry-out of bit 7 but not out of bit
6; otherwise OV is cleared. When adding signed integers, OV indicates a negative number
produced as the sum of two positive operands or a positive sum from two negative operands.
Four source operand addressing modes are allowed: register, direct, register-indirect, or
immediate.
Example: The Accumulator holds 0C3H (11000011B) and register 0 holds 0AAH (10101010B)
with the carry flag set. The following instruction,
ADDC A,R0
leaves 6EH (01101110B) in the Accumulator with AC cleared and both the Carry flag and OV set
to 1.
ADDC A,Rn Operation: ADDC
(A)
ADDC A, direct Operation: ADDC
(A) (A) + (C) + (direct)
ADDC A, @Ri Operation: ADDC
(A) + (C) + ((Ri))
ADDC A, #data Operation: ADDC (A) (A) + (C) + #data
SUBB A,<src-byte>
Function: Subtract with borrow

Description: SUBB subtracts the indicated variable and the carry flag together from the
Accumulator, leaving the result in the Accumulator. SUBB sets the carry (borrow) flag if a
borrow is needed for bit 7 and clears C otherwise. (If C was set before executing a SUBB
instruction, this indicates that a borrow was needed for the previous step in a
multiple-precision subtraction, so the carry is subtracted from the Accumulator along with the
source operand.)
AC is set if a borrow is needed for bit 3 and cleared otherwise. OV is set if a borrow is needed
into bit 6, but not into bit 7, or into bit 7, but not bit 6.
When subtracting signed integers, OV indicates a negative number produced when a negative
value is subtracted from a positive value, or a positive result when a positive number is subtracted
from a negative number.
The source operand allows four addressing modes: register, direct, register-indirect, or immediate.
Example: The Accumulator holds 0C9H (11001001B), register 2 holds 54H (01010100B), and
the carry flag is set. The instruction,
SUBB A,R2
will leave the value 74H (01110100B) in the accumulator, with the carry flag and AC cleared but
OV set.
Instructions OpCode Bytes Flags
SUBB A,#data 0x94 2 C, AC, OV
SUBB A,iram addr 0x95 2 C, AC, OV
SUBB A,@R0 0x96 1 C, AC, OV
SUBB A,@R1 0x97 1 C, AC, OV
SUBB A,R0 0x98 1 C, AC, OV
SUBB A,R1 0x99 1 C, AC, OV
SUBB A,R2 0x9A 1 C, AC, OV
SUBB A,R3 0x9B 1 C, AC, OV
SUBB A,R4 0x9C 1 C, AC, OV
SUBB A,R5 0x9D 1 C, AC, OV
SUBB A,R6 0x9E 1 C, AC, OV

SUBB A,R7 0x9F 1 C, AC, OV
SUBB A,Rn
Operation: SUBB
(A) - (C) - (Rn)
SUBB A, direct
Operation: SUBB
(A) - (C) - (direct)
SUBB A,@Ri
Operation: SUBB
(A) - (C) - ((Ri))
SWAP A
Function: Swap nibbles within the Accumulator
Description: SWAP A interchanges the low- and high-order nibbles (four-bit fields) of the
Accumulator (bits 3 through 0 and bits 7 through 4). The operation can also be thought of as a 4-
bit rotate instruction. No flags are affected.
Example: The Accumulator holds the value 0C5H (11000101B). The instruction,
SWAP A
leaves the Accumulator holding the value 5CH (01011100B
Operation: SWAP
(A3-0) D (A7-4)
XCH A,<byte>
Function: Exchange Accumulator with byte variable
Description: XCH loads the Accumulator with the contents of the indicated variable, at the same
time writing the original Accumulator contents to the indicated variable. The source/destination
operand can use register, direct, or register-indirect addressing.

Example: R0 contains the address 20H. The Accumulator holds the value 3FH (0011111lB).
Internal RAM location 20H holds the value 75H (01110101B).
XCH A,@R0
leaves RAM location 20H holding the values 3FH (00111111B) and 75H (01110101B) in the
accumulator.
XCHD A,@Ri
Function: Exchange Digit
Description: XCHD exchanges the low-order nibble of the Accumulator (bits 3 through 0),
generally representing a hexadecimal or BCD digit, with that of the internal RAM location
indirectly addressed by the specified register.
The high-order nibbles (bits 7-4) of each register are not affected. No flags are affected.
Example: R0 contains the address 20H. The Accumulator holds the value 36H (00110110B).
Internal RAM location 20H holds the value 75H (01110101B).
XCHD A,@R0
leaves RAM location 20H holding the value 76H (01110110B) and 35H (00110101B) in the
Accumulator.
CPL A
Function: Complement Accumulator
Description: CPLA logically complements each bit of the Accumulator (one‟s complement). Bits
which previously contained a 1 are changed to a 0 and vice-versa. No flags are affected.
Example: The Accumulator contains 5CH (01011100B).
CPL A
leaves the Accumulator set to 0A3H (10100011B).
CPL bit

Function: Complement bit
Description: CPL bit complements the bit variable specified. A bit that had been a 1 is changed
to 0 and vice-versa. No other flags are affected. CLR can operate on the carry or any directly
addressable bit.
Example: Port 1 has previously been written with 5BH (01011101B). The following instruction
sequence, CPL P1.1CPL
P1.2 leaves the port set to 5BH (01011011B).
DA A
Function: Decimal-adjust Accumulator for Addition
Description: DA A adjusts the eight-bit value in the Accumulator resulting from the earlier
addition of two variables (each in packed-BCD format), producing two four-bit digits. Any ADD
or ADDC instruction may have been used to perform the addition.
If Accumulator bits 3 through 0 are greater than nine or if the AC flag is one, six is added to the
Accumulator producing the proper BCD digit in the low-order nibble. This internal addition sets
the carry flag if a carry-out of the low-order four-bit field propagates through all high-order bits,
but it does not clear the carry flag otherwise.
If the carry flag is now set, or if the four high-order bits now exceed nine, these high-order bits
are incremented by six, producing the proper BCD digit in the high-order nibble. Again, this sets
the carry flag if there is a carry-out of the high-order bits, but does not clear the carry. The carry
flag thus indicates if the sum of the original two BCD variables is greater than 100, allowing
multiple precision decimal addition. OV is not affected.
DEC byte
Function: Decrement
Description: DEC byte decrements the variable indicated by 1. An original value of 00H
underflows to 0FFH. No flags are affected.
Example: Register 0 contains 7FH (01111111B). Internal RAM locations 7EH and 7FH contain
00H and 40H, respectively.
The following instruction sequence,
DEC @R0
DEC R0
DEC @R0

leaves register 0 set to 7EH and internal RAM locations 7EH and 7FH set to 0FFH and 3FH.
DEC A
DEC Rn
DEC direct
DEC @Ri
DIV AB
Function: Divide
Description: DIV AB divides the unsigned eight-bit integer in the Accumulator by the unsigned
eight-bit integer in register B.
The Accumulator receives the integer part of the quotient; register B receives the integer
remainder. The carry and OV flags are cleared.
Exception: if B had originally contained 00H, the values returned in the Accumulator and B-
register are undefined and the overflow flag are set. The carry flag is cleared in any case.
Example: The Accumulator contains 251 (0FBH or 11111011B) and B contains 18 (12H or
00010010B). The following instruction,
DIV AB
leaves 13 in the Accumulator (0DH or 00001101B) and the value 17 (11H or 00010001B) in B,
since
251 = (13 x 18) + 17. Carry and OV are both cleared.
INC <byte>
Function: Increment
Description: INC increments the indicated variable by 1. An original value of 0FFH overflows to
00H. No flags are affected.
Example: Register 0 contains 7EH (011111110B). Internal RAM locations 7EH and 7FH contain
0FFH and 40H,
respectively. The following instruction sequence,
INC @R0

INC R0
INC @R0
leaves register 0 set to 7FH and internal RAM locations 7EH and 7FH holding 00H and 41H,
respectively.
INC A
Operation: INC
(A)
INC DPTR
Function: Increment Data Pointer
Description: INC DPTR increments the 16-bit data pointer by 1. A 16-bit increment (modulo
216) is performed, and an overflow of the low-order byte of the data pointer (DPL) from 0FFH to
00H increments the high-order byte (DPH).
No flags are affected.
This is the only 16-bit register which can be incremented.
Example: Registers DPH and DPL contain 12H and 0FEH, respectively. The following
instruction sequence,
INC DPTR
INC DPTR
INC DPTR
changes DPH and DPL to 13H and 01H.
MUL AB
Function: Multiply
Description: MUL AB multiplies the unsigned 8-bit integers in the Accumulator and register B.
The low-order byte of the 16-bit product is left in the Accumulator, and the high-order byte in B.
If the product is greater than 255 (0FFH), the overflow flag is set; otherwise it is cleared. The
carry flag is always cleared.
Example: Originally the Accumulator holds the value 80 (50H). Register B holds the value 160
(0A0H). The instruction,

MUL AB
will give the product 12,800 (3200H), so B is changed to 32H (00110010B) and the Accumulator
is cleared. The overflow flag is set, carry is cleared.
NOP
Function: No Operation
Description: Execution continues at the following instruction. Other than the PC, no registers or
flags are affected.
Logical instructions
ANL <dest-byte>,<src-byte>
Function: Logical-AND for byte variables
Description: ANL performs the bitwise logical-AND operation between the variables indicated
and stores the results in the destination variable. No flags are affected.
The two operands allow six addressing mode combinations. When the destination is the
Accumulator, the source can use register, direct, register-indirect, or immediate addressing; when
the destination is a direct address, the source can be the Accumulator or immediate data.
.Example: If the Accumulator holds 0C3H (1100001lB), and register 0 holds 55H (01010101B),
then the following instruction,
ANL A,R0
leaves 41H (01000001B) in the Accumulator.
When the destination is a directly addressed byte, this instruction clears combinations of bits in
any RAM location or hardware register. The mask byte determining the pattern of bits to be
cleared would either be a constant contained in the instruction or a value computed in the
Accumulator at run-time. The following instruction,
ANL P1,#01110011B
clears bits 7, 3, and 2 of output port 1.
ANL iram addr,A 0x52 2 None
ANL iram addr,#data 0x53 3 None
ANL A,#data 0x54 2 None

ANL A,iram addr 0x55 2 None
ANL A,@R0 0x56 1 None
ANL A,@R1 0x57 1 None
ANL A,R0 0x58 1 None
ANL A,R1 0x59 1 None
ANL A,R2 0x5A 1 None
ANL A,R3 0x5B 1 None
ANL A,R4 0x5C 1 None
ANL A,R5 0x5D 1 None
ANL A,R6 0x5E 1 None
ANL A,R7 0x5F 1 None
ANL C,bit addr 0x82 2 C
ANL C,/bit addr 0xB0 2 C
ANL A,Rn
Operation: ANL
ANL A,@Ri
Operation: ANL
(A)
ANL direct,#data
Operation: ANL
ORL <dest-byte> <src-byte>
Function: Logical-OR for byte variables

Description: ORL performs the bitwise logical-OR operation between the indicated variables,
storing the results in the destination byte. No flags are affected.
Example: If the Accumulator holds 0C3H (11000011B) and R0 holds 55H (01010101B) then the
following instruction,
ORL A,R0
leaves the Accumulator holding the value 0D7H (1101011lB).
The instruction,
ORL P1,#00110010B
sets bits 5, 4, and 1 of output Port 1.
ORL A, Rn ; or the content of Accumulator and Register Rn and store the
result in Accumulator
ORL A, direct ; or the content of Accumulator and the memory and store the
result in Accumulator
ORL A, @Ri ; or the content of accumulator and the memory location whose
address is specified in Ri
ORL C,<src-bit>
Function: Logical-OR for bit variables
Description: Set the carry flag if the Boolean value is a logical 1; leave the carry in its current
state otherwise. A slash ( / ) preceding the operand in the assembly language indicates that the
logical complement of the addressed bit is used as the source value, but the source bit itself is not
affected. No other flags are affected.
Example:
ORL C, ACC.7 ;OR CARRY WITH THE ACC. BIT 7
ORL C, /OV ;OR CARRY WITH THE INVERSE OF OV.
SETB

Operation: SETB
Function: Set Bit
Syntax: SETB bit addr
Description: Sets the specified bit.
XRL <dest-byte>,<src-byte>
Function: Logical Exclusive-OR for byte variables
Description: XRL performs the bitwise logical Exclusive-OR operation between the indicated
variables, storing the results in the destination. No flags are affected.
The two operands allow six addressing mode combinations. When the destination is the
Accumulator, the source can use register, direct, register-indirect, or immediate addressing; when
the destination is a direct address, the source can be the Accumulator or immediate data.
Example: If the Accumulator holds 0C3H (1100001lB) and register 0 holds 0AAH (10101010B)
then the instruction,
XRL A,R0
leaves the Accumulator holding the value 69H (01101001B).
XRL iram addr,A 0x62 2 None
XRL iram addr,#data 0x63 3 None
XRL A,#data 0x64 2 None
XRL A,iram addr 0x65 2 None
XRL A,@R0 0x66 1 None
XRL A,@R1 0x67 1 None
XRL A,R0 0x68 1 None
XRL A,R1 0x69 1 None
XRL A,R2 0x6A 1 None
XRL A,R3 0x6B 1 None
XRL A,R4 0x6C 1 None
XRL A,R5 0x6D 1 None
XRL A,R6 0x6E 1 None

XRL A,R7 0x6F 1 None
Rotate Instructions
RL A
Function: Rotate Accumulator Left
Description: The eight bits in the Accumulator are rotated one bit to the left. Bit 7 is rotated into
the bit 0 position. No flags are affected.
Example: The Accumulator holds the value 0C5H (11000101B). The following instruction,
RL A
leaves the Accumulator holding the value 8BH (10001011B) with the carry unaffected.
RLC A
Function: Rotate Accumulator Left through the Carry flag
Description: The eight bits in the Accumulator and the carry flag are together rotated one bit to
the left. Bit 7 moves into the carry flag; the original state of the carry flag moves into the bit 0
position. No other flags are affected.
Example: The Accumulator holds the value 0C5H(11000101B), and the carry is zero. The
following instruction,
RLC A
leaves the Accumulator holding the value 8BH (10001010B) with the carry set.
RRC A
Function: Rotate Accumulator Right through Carry flag
Description: The eight bits in the Accumulator and the carry flag are together rotated one bit to
the right. Bit 0 moves into the carry flag; the original value of the carry flag moves into the bit 7
position. No other flags are affected.
Example: The Accumulator holds the value 0C5H (11000101B), the carry is zero. The following
instruction,
RRC A
leaves the Accumulator holding the value 62 (01100010B) with the carry set.

3. Branch instructions
Unconditional Branch Instructions
Operation: AJMP
Function: Absolute Jump Within 2K Block
Syntax:
AJMP code address
AJMP page0 0x01 2 None
AJMP page5 0xA1 2 None
AJMP page6 0xC1 2 None
AJMP page7 0xE1 2 None
Description: AJMP unconditionally jumps to the indicated code address. The new value for the
Program Counter is calculated by replacing the least-significant-byte of the Program Counter with
the second byte of the AJMP instruction, and replacing bits 0-2 of the most-significant-byte of the
Program Counter with 3 bits that indicate the page of the byte following the AJMP instruction.
Bits 3-7 of the most-significant-byte of the Program Counter remain unchanged.
Since only 11 bits of the Program Counter are affected by AJMP, jumps may only be made to
code located within the same 2k block as the first byte that follows AJMP.
Operation: LJMP
Function: Long Jump
Syntax: LJMP code address.
Description: LJMP jumps unconditionally to the specified code address.
Operation: SJMP
Function: Short Jump
Syntax: SJMP reladdr

Description: SJMP jumps unconditionally to the address specified reladdr. Reladdr must be
within -128 or +127 bytes of the instruction that follows the SJMP instruction
Conditional Branch Instructions
Operation: JNC
Function: Jump if Carry Not Set
Syntax: JNC reladdr
Description: JNC branches to the address indicated by reladdr if the carry bit is not set. If the
carry bit is set program execution continues with the instruction following the JNB instruction.
Operation: JC
Function: Jump if Carry Set
Syntax: JC reladdr
Description: JC will branch to the address indicated by reladdr if the Carry Bit is set. If the Carry
Bit is not set program execution continues with the instruction following the JC instruction.
Operation: JNB
Function: Jump if Bit Not Set
Syntax: JNB bit addr,reladdr
Description: JNB will branch to the address indicated by reladdress if the indicated bit is not set.
If the bit is set program execution continues with the instruction following the JNB instruction.
Operation: JB
Function: Jump if Bit Set
Syntax: JB bit addr, reladdr
Description: JB branches to the address indicated by reladdr if the bit indicated by bit addr is set.
If the bit is not set program execution continues with the instruction following the JB instruction.
Operation: JNZ
Function: Jump if Accumulator Not Zero
Syntax: JNZ reladdr
Description: JNZ will branch to the address indicated by reladdr if the Accumulator contains any
value except 0. If the value of the Accumulator is zero program execution continues with the
instruction following the JNZ instruction.
Operation: JZ
Function: Jump if Accumulator Zero

Syntax: JNZ reladdr
Description: JZ branches to the address indicated by reladdr if the Accumulator contains the
value 0. If the value of the Accumulator is non-zero program execution continues with the
instruction following the JNZ instruction.
Operation: DJNZ
Function: Decrement and Jump if Not Zero
Syntax:
DJNZ register, reladdr
DJNZ iram addr,reladdr 0xD5 3 None
DJNZ R0,reladdr 0xD8 2 None
DJNZ R1,reladdr 0xD9 2 None
DJNZ R2,reladdr 0xDA 2 None
DJNZ R3,reladdr 0xDB 2 None
DJNZ R4,reladdr 0xDC 2 None
DJNZ R5,reladdr 0xDD 2 None
DJNZ R6,reladdr 0xDE 2 None
DJNZ R7,reladdr 0xDF 2 None
Description: DJNZ decrements the value of register by 1. If the initial value of register is 0,
decrementing the value will cause it to reset to 255 (0xFF Hex). If the new value of register is not
0 the program will branch to the address indicated by relative addr. If the new value of register is
0 program flow continues with the instruction following the DJNZ instruction.
Operation: CJNE
Function: Compare and Jump If Not Equal
Syntax: CJNE operand1,operand2,reladdr
CJNE A,#data, reladdr 0xB4 3 C
CJNE A,iram addr,reladdr 0xB5 3 C
CJNE @R0,#data,reladdr 0xB6 3 C
CJNE @R1,#data,reladdr 0xB7 3 C

CJNE R0,#data,reladdr 0xB8 3 C
CJNE R1,#data,reladdr 0xB9 3 C
CJNE R2,#data,reladdr 0xBA 3 C
CJNE R3,#data,reladdr 0xBB 3 C
CJNE R4,#data,reladdr 0xBC 3 C
CJNE R5,#data,reladdr 0xBD 3 C
CJNE R6,#data,reladdr 0xBE 3 C
CJNE R7,#data,reladdr 0xBF 3 C
Description: CJNE compares the value of operand1 and operand2 and branches to the indicated
relative address if operand1 and operand2 are not equal. If the two operands are equal program
flow continues with the instruction following the CJNE instruction.
The Carry bit (C) is set if operand1 is less than operand2, otherwise it is cleared.

UNIT 3:
8051 programming: Assembler directives, Assembly language programs and Time delay
calculations.
Introduction:
8051 micro controller has one data type. It is 8-bit and size of each register is also 8-bit. It
is job of programmer to break down data larger than 8 bits (00 to FFH, 0 to 255 in decimal) to be
processed by CPU.
Data byte (DB) directive: The DB directive is most widely used data directive in assembler It is
used to define 8-bit data. When DB is used to define data, the number can be in decimal, binary,
hex or ASCII formats. The assembler will convert the number into hex. The assembler will assign
the ASCII code for the numbers or characters automatically. The DB directive is only directive
that can be used to define ASCII strings larger than two characters Therefore, it should be used
for all ASCII data definitions.
The most widely used Assembler directives are ORG Directive
EQU Directive & END Directive
Delay calculations:
Delay Calculations Code Remarks
50mS
Xtal freq
=11.0592MHz
50mS/1.085uS
= 46082.9
46082.9/255 = 180.7
Its an Odd Value, so
round off the result
i.e. 180.7 to the
nearest even number
i.e 182
46082.9 / x =182 Find value of x
46082.9 / 253 = 182
We’ll load 253 in the
inner most loop
182 / 2 = 91
Always do this, so its
result will be our
outer loop
Delay:Mov R0, #91
Here1:Mov R1, #253
Here:DJNZ R1, here
DJNZ R0, here1
RET
500mS
Xtal freq
=11.0592MHz
500mS/1.085uS =
460829.5
460829.5 / 255 =
1807.1
Odd number does not
account as its not less
than 255, innermost

loop = 255
1807.1 / 255 = 7.086
Round off to nearest
higher even i.e 8
1807.1 / x = 8
1807.1 / 226 = 8 Inner loop = 226
8 / 2 = 4 Outer loop = 4
Delay:Mov R0,#4
Here2:Mov R1, #226
Here1:Mov R2, #255
Here:DJNZ R2, Here
DJNZ R1, Here1
DJNZ R0,Here2
End
1Second
Xtal freq=
20MHz
1 / 0.6uS =
1.6 x 10e6
1.6 x 10e6 / 255 =
6536
Innermost loop= 255
6536 / 255 =
25.6
Not Even so make it
even i.e. 26
6536 / x = 26
6536 / 251 = 26 Inner loop 251
26/2 = 13 Outer loop = 13
Delay:Mov R0, # 13
Here2: Mov R1, # 251
Here1: Mov R2, #255
Here: DJNZ R2, Here
DJNZ R1, Here1
DJNZ R0, Here2
Ret

UNIT 4:
8051 Interfacing and Applications: Basics of I/O concepts, I/O Port Operation, Interfacing
051
to LCD, Keyboard, parallel and serial ADC, DAC, Stepper motor interfacing and DC
motor interfacing and programming.
Objectives:
At the end of this chapter, we will be able to:
List the different devices that can be interfaced with 8051
Understand the working principle.
Develop the following applications using assembly and C
- Stepper motor interface
- DC motor interfacing and PWM
- Digital-to-Analog interfacing
- Analog-to-Digital interfacing
- LCD interface
- Keyboard interface
This chapter basically gives an insight into the study of different interfacings listed above.
Further we will also study and understand their operation that is the working principle. We will
further discuss on how to develop these interfaces using assembly and C.

Stepper Motor Interfacing:
Stepper motor is a widely used device that translates electrical pulses into mechanical movement.
Stepper motor is used in applications such as; disk drives, dot matrix printer, robotics etc,. The
construction of the motor is as shown in figure 1 below.
Figure 1: Structure of stepper motor
It has a permanent magnet rotor called the shaft which is surrounded by a stator. Commonly used
stepper motors have four stator windings that are paired with a center – tapped common. Such
motors are called as four-phase or unipolar stepper motor.
The stator is a magnet over which the electric coil is wound. One end of the coil are connected
commonly either to ground or +5V. The other end is provided with a fixed sequence such that the
motor rotates in a particular direction. Stepper motor shaft moves in a fixed repeatable increment,
which allows one to move it to a precise position. Direction of the rotation is dictated by the stator
poles. Stator poles are determined by the current sent through the wire coils.
Step angle:
Step angle is defined as the minimum degree of rotation with a single step.
No of steps per revolution = 360° / step angle
Steps per second = (rpm x steps per revolution) / 60
Example: step angle = 2°
No of steps per revolution = 180

Switching Sequence of Motor:
As discussed earlier the coils need to be energized for the rotation. This can be done by sending a
bits sequence to one end of the coil while the other end is commonly connected. The bit sequence
sent can make either one phase ON or two phase ON for a full step sequence or it can be a
combination of one and two phase ON for half step sequence. Both are tabulated below.
Full Step:
Two Phase ON
One Phase ON
Half Step (8 – sequence):
The sequence is tabulated as below:
8051 Connection to Stepper Motor: (explanation of the diagram can be done)

Figure 2: 8051 interfaces to stepper motor
The following example 1 to example 6 shown below will elaborate on the discussion done above:
Example 1: Write an ALP to rotate the stepper motor clockwise / anticlockwise
continuously with full step sequence.
Program:
MOV A,#66H
BACK: MOV P1,A
RR A
ACALL DELAY
SJMP BACK
DELAY: MOV R1,#100
UP1: MOV R2,#50
UP: DJNZ R2,UP
DJNZ R1,UP1
RET
Note: motor to rotate in anticlockwise use instruction RL A instead of RR A

Example 2: A switch is connected to pin P2.7. Write an ALP to monitor the status
of the SW. If SW = 0, motor moves clockwise and if SW = 1, motor moves
anticlockwise.
Program:
ORG 0000H
SETB P2.7
MOV A, #66H
MOV P1,A
TURN: JNB P2.7, CW
RL A
ACALL DELAY
MOV P1,A
SJMP TURN
CW: RR A
ACALL DELAY
MOV P1,A
SJMP TURN
DELAY: as previous example
Example 3: Write an ALP to rotate a motor 90° clockwise. Step angle of motor is
2°.
Solution:
Step angle = 2°
Steps per revolution = 180
No of rotor teeth = 45
For 90° rotation the no of steps is 45
Program:
ORG 0000H
MOV A, #66H
MOV R0, #45
BACK: RR A
MOV P1, A
ACALL DELAY
DJNZ R0, BACK
END

Programming Stepper Motor with 8051 C
The following examples 5 and 6 will show the programming of stepper motor using 8051 C.
Example 4: Rotate the stepper motor continuously clockwise using half-step 8-step
sequence. Say the sequence is in ROM locations.
Program:
ORG 0000H
START: MOV R0, #08
MOV DPTR, #HALFSTEP
RPT: CLR A
MOVC A, @A+DPTR
MOV P1, A
ACALL DELAY
INC DPTR
DJNZ R0, RPT
SJMP START
ORG 0200H
HALFSTEP DB 09, 08, 0CH, 04, 06, 02, 03, 01
END
Example 5: Problem definition is same as example 1.
Program:
#include <reg51.h>
void main ()
{
while (1)
{
P1=0x66;
MSDELAY (200);
P1=0x33;
MSDELAY (200);
P1=0x99;
MSDELAY (200);
P1=0xCC;
MSDELAY (200);
}
}
void MSDELAY (unsigned char value)
{
unsigned int x,y;
for(x=0;x<1275;x++)
for(y=0;y<value;y++);
}

Example 6: Problem definition is same as example 2.
Program:
#include <reg51.h>
sbit SW=P2^7;
void main ()
{
SW=1;
while (1)
{
if(SW==0){
P1=0x66;
MSDELAY (100);
P1=0x33;
MSDELAY (100);
P1=0x99;
MSDELAY (100);
P1=0xCC;
MSDELAY (100);
}
else {
P1=0x66;
MSDELAY (100);
P1=0xCC;
MSDELAY (100);
P1=0x99;
MSDELAY (100);
P1=0x33;
MSDELAY (100);
}
void MSDELAY (unsigned char value)
{
unsigned int x,y;
for(x=0;x<1275;x++)
for(y=0;y<value;y++);
}

DC Motor Interfacing with 8051:
The DC motor is another widely used device that translates electrical pulses into mechanical
movement. Motor has 2 leads +ve and – ve , connecting them to a DC voltage supply moves the
motor in one direction. On reversing the polarity rotates the motor in the reverse direction. Basic
difference between Stepper and DC motor is stepper motor moves in steps while DC motor moves
continuously. Another difference is with stepper motor the number of steps can be counted while
it is not possible in DC motor. Maximum speed of a DC motor is indicated in rpm. The rpm is
either with no load it is few thousands to tens of thousands or with load rpm decreases with
increase in load.
Voltage and current rating : Nominal voltage is the voltage for a motor under normal condition. It
ranges from 1V to 150V. As voltage increases, rpm goes up. Current rating is the current
consumption when the nominal voltage is applied with no load that is 25mA to a few amperes. As
load increases, rpm increases, unless voltage or current increases implies torque increases. With
fixed voltage, as load increases, power consumption of a DC motor is increased.
Unidirectional Control:
Figure 3 shows the rotation of the DC motor in clockwise and anticlockwise direction.
Figure 3: DC motor rotation
Bidirectional Control:
(a) Motor not running (b) Clockwise direction

(c) Counter clockwise direction (d) Invalid state (short circuit)
Figure 4: H-Bridge Motor Configuration
Figure 4 shows the H-Bridge motor configuration. It consists of four switches and based on the
closing and opening of these switches the motor either rotates in clockwise or anti-clockwise
direction.
As seen in figure 4a, all the switches are open hence the motor is not running. In b, turning of the
motor is in one direction when the switches 1 and 4 are closed that is clockwise direction.
Similarly, in c the switches 2 and 3 are closed so the motor rotates in anticlockwise direction,
while in figure 4d all the switches are closed which indicates a invalid state or a short circuit.
The interfacing diagram of 8051 to bidirectional motor control can be referred to fig 17-18 from
text prescribed.
Example 6: A switch is connected to pin P2.7. Write an ALP to monitor the status of the
SW. If SW = 0, DC motor moves clockwise and if SW = 1, DC motor moves
anticlockwise.
Program:
ORG 0000H
CLR P1.0
CLR P1.1
CLR P1.2
CLR P1.3
SETB P2.7
MONITOR: JNB P2.7, CLOCK
SETB P1.0
CLR P1.1
CLR P1.2
SETB P1.3
SJMP MONITOR
CLOCK: CLR P1.0
SETB P1.1
SETB P1.2
CLR P1.3
SJMP MONITOR
END

Pulse Width Modulation (PWM):
The speed of the motor depends on 3 parameters: load, voltage and current. For a given load, we
can maintain a steady speed by using PWM. By changing the width of the pulse applied to DC
motor, power provided can either be increased or decreased. Though voltage has fixed amplitude,
has a variable duty cycle. The wider the pulse, higher the speed obtained. One of the reasons as to
why dc motor are referred over ac is, the ability to control the speed of the DC motor using PWM.
The speed of the ac motor is dictated by the ac frequency of voltage applied to the motor and is
generally fixed. Hence, speed of the AC motors cannot be controlled when load is increased.
Figure 5 below shows the pulse width modulation comparison.
Figure 5: PWM comparison
Example 7: A switch is connected to pin P2.7. Write a C to monitor the status of the SW.
If SW = 0, DC motor moves clockwise and if SW = 1, DC motor moves anticlockwise.
Program:
# include <reg51.h>
sbit SW =P2^7;
sbit Enable = P1^0;
sbit MTR_1 = P1^1;
sbit MTR_2 = P1^2;
void main ( )
{
SW=1;
Enable = 0;
MTR_1=0;
MTR_2=0;
while( )
{
Enable =1;
if( SW==1)
{ MTR_1=1;
MTR_2=0;
}
else
{ MTR_1=0;
MTR_2=1;
}}}

The interfacing diagrams for the above examples can be referred to the text.
Digital-to-Analog (DAC) converter:
The DAC is a device widely used to convert digital pulses to analog signals. In this section we
will discuss the basics of interfacing a DAC to 8051.
The two method of creating a DAC is binary weighted and R/2R ladder.
The Binary Weighted DAC, which contains one resistor or current source for each bit of the DAC
connected to a summing point. These precise voltages or currents sum to the correct output value.
This is one of the fastest conversion methods but suffers from poor accuracy because of the high
precision required for each individual voltage or current. Such high-precision resistors and
current-sources are expensive, so this type of converter is usually limited to 8-bit resolution or
less.
Example 8: A switch is connected to pin P2.7. Write an C to monitor the status of the SW.
If SW = 0, DC motor moves 50% duty cycle pulse and if SW = 1, DC motor moves with
25% duty cycle pulse.
Program:
# include <reg51.h>
sbit SW =P2^7;
sbit MTR = P1^0;
void main ( )
{
SW=1;
MTR=0;
while( )
{
if( SW==1)
{ MTR=1;
Msdelay(25);
MTR=0;
Msdelay(75);
}
else
{ MTR=1;
Msdelay(50);
MTR=0;
Msdelay(50);
}
}
}

The R-2R ladder DAC, which is a binary weighted DAC that uses a repeating cascaded structure
of resistor values R and 2R. This improves the precision due to the relative ease of producing
equal valued matched resistors (or current sources). However, wide converters perform slowly
due to increasingly large RC-constants for each added R-2R link.
The first criterion for judging a DAC is its resolution, which is a function of the number of binary
inputs. The common ones are 8, 10, and 12 bits. The number of data bit inputs decides the
resolution of the DAC since the number of analog output levels is equal to 2n
, where n is the
number of data bit inputs.
DAC0808:
The digital inputs are converter to current Iout, and by connecting a resistor to the Iout pin, we can
convert the result to voltage. The total current Iout is a function of the binary numbers at the D0-
D7 inputs of the DAC0808 and the reference current Iref , and is as follows:
Usually reference current is 2mA. Ideally we connect the output pin to a resistor, convert this
current to voltage, and monitor the output on the scope. But this can cause inaccuracy; hence an
opamp is used to convert the output current to voltage. The 8051 connection to DAC0808 is as
shown in the figure 6 below.
Figure 6: 8051 connections to DAC0808
The following examples 9, 10 and 11 will show the generation of waveforms using DAC0808.
Example 9: Write an ALP to generate a triangular waveform.
Program:
MOV A, #00H
INCR: MOV P1, A
INC A
CJNE A, #255, INCR
DECR: MOV P1, A
DEC A
CJNE A, #00, DECR
SJMP INCR
END

Example 10: Write an ALP to generate a sine waveform.
Vout = 5V(1+sinθ)
Solution: Calculate the decimal values for every 10 degree of the sine wave. These
values can be maintained in a table and simply the values can be sent to port P1. The
sinewave can be observed on the CRO.
Program:
ORG 0000H
AGAIN: MOV DPTR, #SINETABLE
MOV R3, #COUNT
UP: CLR A
MOVC A, @A+DPTR
MOV P1, A
INC DPTR
DJNZ R3, UP
SJMP AGAIN
ORG 0300H
SINETABLE DB 128, 192, 238, 255, 238, 192, 128, 64, 17, 0, 17, 64, 128
END
Note: to get a better wave regenerate the values of the table per 2 degree.
Example 10: Write a C program to generate a sine waveform.
Vout = 5V(1+sinθ)
Program:
#include<reg51.h>
sfr dacdata=P1;
void main( )
{
unsigned char sinetable[12]={ 128, 192, 238, 255, 238, 192,
128, 64, 17, 0, 17, 64};
unsigned char x;
while (1)
{
for(x=0;x<12;x++)
{
dacdata = sinetable[x];
}
}
}

Analog-to-digital converter (ADC) interfacing:
ADCs (analog-to-digital converters) are among the most widely used devices for data acquisition.
A physical quantity, like temperature, pressure, humidity, and velocity, etc., is converted to
electrical (voltage, current) signals using a device called a transducer, or sensor. We need an
analog-to-digital converter to translate the analog signals to digital numbers, so microcontroller
can read them.
ADC804 chip:
ADC804 IC is an analog-to-digital converter. It works with +5 volts and has a resolution of 8 bits.
Conversion time is another major factor in judging an ADC. Conversion time is defined as the
time it takes the ADC to convert the analog input to a digital (binary) number. In ADC804
conversion time varies depending on the clocking signals applied to CLK R and CLK IN pins, but
it cannot be faster than 110μs.
Pin Description of ADC804:
Figure 7: Pin out of ADC0804
 CLK IN and CLK R: CLK IN is an input pin connected to an external clock source. To use
the internal clock generator (also called self-clocking), CLK IN and CLK R pins are
connected to a capacitor and a resistor and the clock frequency is determined by:

Typical values are R = 10K ohms and C =150pF. We get f = 606 kHz and the conversion
time is 110μs.
 Vref/2 : It is used for the reference voltage. If this pin is open (not connected), the analog
input voltage is in the range of 0 to 5 volts (the same as the Vcc pin). If the analog input
range needs to be 0 to 4 volts, Vref/2 is connected to 2 volts. Step size is the smallest
change can be discerned by an ADC
Vref/2 Relation to Vin Range
 D0-D7: The digital data output pins. These are tri-state buffered. The converted data is
accessed only when CS =0 and RD is forced low. To calculate the output voltage, use the
following formula
Dout = digital data output (in decimal),
Vin = analog voltage, and
step size (resolution) is the smallest change
 Analog ground and digital ground: Analog ground is connected to the ground of the analog
Vin and digital ground is connected to the ground of the Vcc pin. To isolate the analog
Vin signal from transient voltages caused by digital switching of the output D0 – D7. This
contributes to the accuracy of the digital data output.
 Vin(+) & Vin(-): Differential analog inputs where Vin = Vin (+) – Vin (-). Vin (-) is
connected to ground and Vin (+) is used as the analog input to be converted.
 RD: Is “output enable” a high-to-low RD pulse is used to get the 8-bit converted data out of
ADC804.
 INTR: It is “end of conversion” When the conversion is finished, it goes low to signal the
CPU that the converted data is ready to be picked up.
 WR: It is “start conversion” When WR makes a low-to-high transition, ADC804 starts
converting the analog input value of Vin to an 8- bit digital number.
 CS: It is an active low input used to activate ADC804.

The following steps must be followed for data conversion by the ADC804 chip:
1. Make CS= 0 and send a L-to-H pulse to pin WR to start conversion.
2. Monitor the INTR pin, if high keep polling but if low, conversion is complete, go to next step.
3. Make CS= 0 and send a H-to-L pulse to pin RD to get the data out
Figure 8 shows the read and write timing for ADC804. Figure 9 and 10 shows the self clocking
with the RC component for frequency and the external frequency connected to XTAL2 of 8051.
Figure 8: Read and Write timing for ADC0804
Figure 9: 8051 Connection to ADC0804 with Self-clocking

Figure 10: 8051 Connection to ADC0804 with Clock from XTAL2 of 8051
Now let us see how we write assembly as well as C program for the interfacing diagram shown in
figure 10.
Programming ADC0804 in assembly
MYDATA EQU P1
MOV P1, #0FFH
SETB P2.7
BACK: CLR P2.6
SETB P2.6
HERE: JB P2.7, HERE
CLR P2.5
MOV A, MYDATA
SETB P2.5
SJMP BACK
Programming ADC0804 in C
#include<reg51.h>
Sbit RD=P2^5;
Sbit WR=P2^6;
Sbit INTR=P2^7;
Sfr Mydata=P1;
Void main ( )
{
Unsigned char value;
Mydata =0xFF;
INTR=1;
RD=1;
WR=1;
While (1)
{
WR=0;
WR=1;
While (INTR == 1);
RD=0;
Value =Mydata;
RD=1;
}

ADC0808/0809 chip:
ADC808 has 8 analog inputs. It allows us to monitor up to 8 different transducers using only
single chip. The chip has 8-bit data output just like the ADC804. The 8 analog input channels are
multiplexed and selected according to the values given to the three address pins, A, B, and C. that
is; if CBA=000, CH0 is selected; CBA=011, CH3 is selected and so on. The pin details of
ADC0808 are as shown in the figure 11 below. (Explanation can be done as is with ADC0804).
Figure 11: Pin out of ADC0808
Steps to Program ADC0808/0809
1. Select an analog channel by providing bits to A, B, and C addresses.
2. Activate the ALE pin. It needs an L-to-H pulse to latch in the address.
3. Activate SC (start conversion) by an H-to-L pulse to initiate conversion.
4. Monitor EOC (end of conversion) to see whether conversion is finished.
5. Activate OE (output enable) to read data out of the ADC chip. An H-to-L pulse to the OE pin
will bring digital data out of the chip.
Let us write an assembly and C program for the interfacing of 8051 to ADC0808 as shown in
figure 12
Programming ADC0808/0809 in assembly
MYDATA EQU P1
ORG 0000H
MOV MYDATA, #0FFH
SETB P2.7
CLR P2.4
CLR P2.6
CLR P2.5
BACK: CLR P2.0
CLR P2.1
SETB P2.2
ACALL DELAY
SETB P2.4
ACALL DELAY
SETB P2.6
ACALL DELAY
CLR P2.4

below.(Figure 12 can be referred from the text prescribed.)
Note: replace the assembly instructions with equivalent C statements for programming ADC0808
in C
LCD Interfacing:
LCD is finding widespread use replacing LEDs for the following reasons:
The declining prices of LCD
The ability to display numbers, characters, and graphics
Incorporation of a refreshing controller into the LCD, thereby relieving the CPU of the
task of refreshing the LCD
Ease of programming for characters and graphics
Pin Description:
LCD Command Codes:

LCD timing diagram for reading and writing is as shown in figure 14 and 15.
Figure 14: LCD timing for read

Figure 15: LCD timing for write
Sending Data/ Commands to LCDs with Time Delay:
To send any of the commands to the LCD, make pin RS=0. For data, make RS=1. Then send a
high-to-low pulse to the E pin to enable the internal latch of the LCD. This is shown in the code
below. The interfacing diagram of LCD to 8051 is as shown in the figure 16.
Example 11: Write an ALP to initialize the LCD and display message “YES”. Say the command to be
given is :38H (2 lines ,5x7 matrix), 0EH (LCD on, cursor on), 01H (clear LCD), 06H (shift cursor right),
86H (cursor: line 1, pos. 6)
Program:
;calls a time delay before sending next data/command ;P1.0-P1.7 are connected to LCD data pins D0-D7
;P2.0 is connected to RS pin of LCD ;P2.1 is connected to R/W pin of LCD ;P2.2 is connected to E pin
of LCD
ORG 0H
MOV A,#38H ;INIT. LCD 2 LINES, 5X7 MATRIX
ACALL COMNWRT ;call command subroutine
ACALL DELAY ;give LCD some time
MOV A,#0EH ;display on, cursor on
MOV A,#01 ;clear LCD
MOV A,#06H ;shift cursor right
MOV A,#86H ;cursor at line 1, pos. 6

Figure 16: 8051 Connection to LCD
MOV A,#‟Y‟ ;display letter Y
ACALL DATAWRT ;call display subroutine
MOV A,#‟E‟ ;display letter E
MOV A,#‟S‟ ;display letter S
AGAIN: SJMP AGAIN ;stay here
COMNWRT: ;send command to LCD
MOV P1,A ;copy reg A to port 1
CLR P2.0 ;RS=0 for command
CLR P2.1 ;R/W=0 for write
SETB P2.2 ;E=1 for high pulse
CLR P2.2 ;E=0 for H-to-L pulse
RET
DATAWRT: ;write data to LCD
MOV P1,A ;copy reg A to port 1
SETB P2.0 ;RS=1 for data
CLR P2.1 ;R/W=0 for write
SETB P2.2 ;E=1 for high pulse
CLR P2.2 ;E=0 for H-to-L pulse
RET
DELAY:
MOV R3,#50 ;50 or higher for fast CPUs
HERE2: MOV R4,#255 ;R4 = 255
HERE: DJNZ R4,HERE ;stay until R4 becomes 0
DJNZ R3,HERE2
RET
END

Sending Data/ Commands to LCDs checking the Busy Flag
Example 12: Modify example 11, to check for the busy flag (D7=>P1.7), then send the
command and hence display message “NO”.
;Check busy flag before sending data, command to LCD;p1=data pin ;P2.0 connected to RS
pin ;P2.1 connected to R/W pin ;P2.2 connected to E pin
ORG 0H
MOV A,#38H ;init. LCD 2 lines ,5x7 matrix
ACALL COMMAND ;issue command
MOV A,#0EH ;LCD on, cursor on
MOV A,#01H ;clear LCD command
MOV A,#06H ;shift cursor right
ACALL COMMAND issue command
MOV A,#86H ;cursor: line 1, pos. 6
ACALL COMMAND ;command subroutine
MOV A,#‟N‟ ;display letter N
ACALL DATA_DISPLAY
MOV A,#‟O‟ ;display letter O
ACALL DATA_DISPLAY
HERE: SJMP HERE ;STAY HERE
COMMAND:
ACALL READY ;is LCD ready?
MOV P1,A ;issue command code
CLR P2.0 ;RS=0 for command
CLR P2.1 ;R/W=0 to write to LCD
SETB P2.2 ;E=1 for H-to-L pulse
CLR P2.2 ;E=0,latch in
RET
DATA_DISPLAY:
ACALL READY ;is LCD ready?
MOV P1,A ;issue data
SETB P2.0 ;RS=1 for data
CLR P2.1 ;R/W =0 to write to LCD
SETB P2.2 ;E=1 for H-to-L pulse
CLR P2.2 ;E=0,latch in
RET
READY:
SETB P1.7 ;make P1.7 input port
CLR P2.0 ;RS=0 access command reg
SETB P2.1 ;R/W=1 read command reg ;
BACK:SETB P2.2 ;E=1 for H-to-L pulse
CLR P2.2 ;E=0 H-to-L pulse
JB P1.7,BACK ;stay until busy flag=0
RET
END

Programming LCD in C
Example 13: Write an 8051 C program to send letters „P‟, „I‟, and „C‟ to the LCD using
the busy flag method.
Solution:
#include <reg51.h>
sfr ldata = 0x90; //P1=LCD data pins
sbit rs = P2^0;
sbit rw = P2^1;
sbit en = P2^2;
sbit busy = P1^7;
void main(){
lcdcmd(0x38);
lcdcmd(0x0E);
lcdcmd(0x01);
lcdcmd(0x06);
lcdcmd(0x86); //line 1, position 6
lcddata(„P‟);
lcddata(„I‟);
lcddata(„C‟);
}
void lcdcmd(unsigned char value){
lcdready(); //check the LCD busy flag
ldata = value; //put the value on the pins
rs = 0;
rw = 0;
en = 1; //strobe the enable pin
MSDelay(1);

Keyboard Interfacing:
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 connection between rows and columns. A 4x4 matrix connected to two ports. The rows are
connected to an output port and the columns are connected to an input port.
Scanning and Identifying the Key:
void lcdready(){
busy = 1; //make the busy pin at input
rs = 0;
rw = 1;
while(busy==1){ //wait here for busy flag
en = 0; //strobe the enable pin
MSDelay(1);
en = 1;
}
}
void Msdelay(unsigned int itime){
unsigned int i, j;
for(i=0;i<itime;i++)
for(j=0;j<1275;j++);
}

Figure 17: A 4X4 matrix keyboard
It is the function of the microcontroller to scan the keyboard continuously to detect and identify
the key pressed
To detect a pressed key, the microcontroller grounds all rows by providing 0 to the output
latch, then it reads the columns
If the data read from columns is D3 – D0 =1111, no key has been pressed and the process
continues till key press is detected
If one of the column bits has a zero, this means that a key press has occurred For
example, if D3 – D0 = 1101, this means that a key in the D1 column has been pressed
After detecting a key press, microcontroller will go through the process of identifying the
key
Starting with the top row, the microcontroller grounds it by providing a low to row D0
only. It reads the columns, if the data read is all 1s, no key in that row is activated and the
process is moved to the next row
It grounds the next row, reads the columns, and checks for any zero. This process
continues until the row is identified.
After identification of the row in which the key has been pressed. Find out which column
the pressed key belongs to
Algorithm for detection and identification of key activation goes through the following
stages:
1. To make sure that the preceding key has been released, 0s are output to all rows at once, and
the columns are read and checked repeatedly until all the columns are high
When all columns are found to be high, the program waits for a short amount of time
before it goes to the next stage of waiting for a key to be pressed

2. To see if any key is pressed, the columns are scanned over and over in an infinite loop until one
of them has a 0 on it
Remember that the output latches connected to rows still have their initial zeros (provided
in stage 1), making them grounded
After the key press detection, it waits 20 ms for the bounce and then scans the columns
again
(a) It ensures that the first key press detection was not an erroneous one due a spike noise
(b) The key press. If after the 20-ms delay the key is still pressed, it goes back into the
loop to detect a real key press
3. To detect which row key press belongs to, it grounds one row at a time, reading the columns
each time
If it finds that all columns are high, this means that the key press cannot belong to that
row. Therefore, it grounds the next row and continues until it finds the row the key press
belongs to
Upon finding the row that the key press belongs to, it sets up the starting address for the
look-up table holding the scan codes (or ASCII) for that row
4. To identify the key press, it rotates the column bits, one bit at a time, into the carry flag and
checks to see if it is low
Upon finding the zero, it pulls out the ASCII code for that key from the look-up table
otherwise, it increments the pointer to point to the next element of the look-up table
The flowchart for the above algorithm is as shown below:

Note: The assembly as well as the C program can be written in accordance to the algorithm of the
flowchart shown.

Summary
This chapter gives the details of six different devices that can be interfaced to 8051. These are
widely used in many applications. Initially, we discussed about the stepper motor, giving the
details on the working, sending sequence and hence writing assembly and C program. In
continuation to that we also learnt how to interface DC motor, and DC motors with PWM. The
chapter also covers the study of devices such as DAC, parallel ADC and serial ADC, LCD and
Keyboard along with the interfacing of these devices to 8051. We further, studied how to write
assembly and C program for all the above said interfaces which will help in developing
applications.

Unit: 5: 8051 Interrupts and Timers/counters: Basics of interrupts, 8051 interrupt structure,
Timers and Counters, 8051 timers/counters, programming 8051 timers in assembly and C.
The 8051 has two timerscounters. They can be used either as timers to generate time delay or as
counters to count events happening outside the micro computer. Now we shall see how they are
programmed.
PROGRAMMING 8051TIMERS:
8051 has two timers, timer 0 & timer 1.this module has two 16bit registers.T0 and T1
registers.These registers can be configured to operate either as timers or event counters. In the
„timer‟ function. The register is incremented every machine cycle. Thus, one can think it as
counting machine cycles. Since a machine cycle consists of 12 oscillator
periods, the count rate is 1/12 of the oscillator frequency.
The 16 bit register of T0 / T1 is accessed as low byte and high byte (TH0 / TH1)
:

Interrupts
Concept of Interrupt:
A computer has only two ways to determine the conditions that exist in internal and external
circuits. One method uses software instructions that jump to subroutines on the states of flags and
port pins. The second method responds to hardware signals, called interrupts that force the
program to call a subroutine. Most applications of microcontroller involve responding to events
quickly enough to control the environment that generates the events termed real-time
programming.
Interrupts may be generated by internal chip operation or provided by external sources. Any
interrupt can cause the 8051 to perform a hardware call to an interrupt-handling subroutine that is
located at a predetermined absolute address in program memory.
The 8051 has five interrupts of which three are internally generated namely:
1. Timer 0 overflow: This is indicated by TF0 in TCON, being set
2. Timer 1 overflow: This is indicated by TF1 in TCON, being set
3. Serial port interrupts (RI and TI): Whenever a data byte is received, an interrupt bit, RI
is set to 1 in SCON register. When a data byte is transmitted an interrupt bit TI, is set in
SCON. They are ORed together to provide a single interrupt to the processor. These flags
must be reset by software instruction to enable the next data communication operation.
Two interrupts are triggered by external signals provided by circuitry that is connected to pins
INTO and INT1 (P3.2 and P3.3).
1. External signal at pin INTO (P3.2): When a high-to-low edge signal is received on P3.2,
the external interrupt 0 edge flag IE0 (TCON.1) is set. This flag is cleared when the
processor branches to the subroutine. When the external interrupt signal control bit IT0
(TCON.0) is set to 1 (by program) then interrupt is triggered by falling edge signal. If IT0
is 0, a low-level signal in INTO triggers the interrupt.

2. External signal at pin INT1 (P3.3): Flags IE1 (TCON.3) and IT1 (TCON.2) are similar
to IE0 and IT0 in function.
Each of these interrupts has an address associated where the routine is to be written called as
interrupt service routine addresses. The addresses are listed below:
Interrupt Address called
IE0 0003
TF0 000B
IE1 0013
TF1 001B
Serial 0023
Sequence of events
The sequence of events that take place on the occurrence of an interrupt is as shown below:
An observation made from the above diagram can be explained by the following steps:

1. The programmer enables interrupt circuit action by setting interrupt enable flag bit to 1.
The 8051 has a total of five interrupt sources of each of which may generate an interrupt
signal.
2. External or internal circuit action causes one of interrupt signals to be generated.
3. The CPU finishes the current instruction, pushes the PC on the stack, and replaces the
original PC contents with the address of the first instruction of the program code for the
particular source that caused the interrupt. All the other interrupting source enable bits are
temporarily disabled.
4. The interrupt program executes. While executing, the interrupt program must reset internal
flag that generated the interrupt signals.
5. At the end of the interrupt program, a RETI instruction resets all the interrupt-enable
circuitry and pops the original PC contents from the stack back into the PC. The CPU
resumes executing the interrupted program.
Note that if the interrupting signal is not reset before a RETI instruction, the same interrupt will
occur again. This process will never stop, and the program will loop forever at the interrupt
program location.
The difference between RET and RETI is RET is a return from a function or a subroutine while
RETI is return from an interrupt. The RETI instruction is executed at the end of interrupt
subroutine. After the execution of the RETI instruction the PC address will be restored from the
stack.
Thus the comparison of the call instruction and the interrupt action can be as:
Table 1: Comparison of Call Instruction and Interrupt Action
Call Instruction Interrupt Action
Completely under the control of
programmer
Occurs at any time in the program
Placed in the program by the
programmer
Enabled by the programmer
Execution is determined by where it is
placed in the program
Calls the subroutine at any time and
any place the program is executed
More Details on Interrupts:
The interrupts of 8051 can be programmed and serviced by the microcontroller using the SFRs
Interrupt Enable (IE) and Interrupt Priority (IP).
Interrupt Enable (IE) SFR:
This is a bit addressable SFR with byte address A8H. The bits and addresses are shown in table 2.
The bits are explained below.
Table 2: Interrupt Enable SFR
IE.7 IE.6 IE.5 IE.4 IE.3 IE.2 IE.1 IE.0

EA - ET2 ES ET1 EX1 ET0 EX0
AF AE AD AC AB AA A9 A8
EA: This bit is a global interrupt enable/disable bit. When set to 1, it permits individual interrupts
to be enable by their respective enable bits. When 0, it disables all interrupts.
IE.6: Not implemented
ET2: Reserved for future use.
ES: Enable serial port interrupt. Set to 1 by program to enable serial port interrupt. Cleared to 0 to
disable serial port interrupt.
ET1: Enable (=1)/disable (=0) timer 1 interrupt
EX1: Enable (=1)/disable (=0) external interrupt 1
ET0: Enable (=1)/disable (=0) timer 0 interrupt
ET1: Enable (=1)/disable (=0) external interrupt 0
Interrupt Priority (IP):
This a bit addressable register, with byte address B8H. The addresses are shown in table 3. The
priority of the interrupts is determined by the bits of IP SFR. The bits which are set to 1, have a
high priority and bits with 0 have low priority. Interrupts with high priority can interrupt another
interrupt with low priority. The lower priority interrupt is serviced after higher priority interrupt is
finished.
Table 3: Interrupt Priority (IP)
IP.7 IP.6 IP.5 IP.4 IP.3 IP.2 IP.1 IP.0
- - PT2 PS PT1 PX1 PT0 PX0
- - - BC BB BA B9 B8
IP.7 & IP.6: Not implemented
IP.5: Reserved for future use
PS: Serial port priority interrupt
PT1: Priority of timer 1 interrupt

PX1: Priority of external interrupt 1
PT0: Priority of timer 0 interrupt
PX0: Priority of external interrupt 0
When two or more interrupts have the same priority the microcontroller has its own ranking of
providing service which is a below:
1 External Interrupt 0 (P3.2)
2 Timer 0 Overflow
3 External Interrupt 0 (P3.3)
4 Timer 1 Overflow
5 Serial port

UNIT 6:
8051 Serial Communication: Data communication, Basics of Serial Data Communication, 8051
Serial Communication, connections to RS-232, Serial communication Programming in assembly
and C.
8255A Programmable Peripheral Interface:, Architecture of 8255A, I/O addressing,, I/O
devices interfacing with 8051 using 8255A.
8051 SERIAL COMMUNICATION
Types of Serial Communication
1. Synchronous serial Data Communication
2. Asynchronous Serial Data Communication
Pins TxD (P3.1) and RxD (P3.0) are used for transmitting and receiving the data
serially. Figure below shows synchronous serial data communication which uses a
common clock for synchronization of transmitter and receiver
Serial Interface
The serial port of 8051 is full duplex, i.e., it can transmit and receive simultaneously. The
register SBUF is used to hold the data. The special function register SBUF is physically two
registers. One is, write-only and is used to hold data to be transmitted out of the 8051 via TXD.
The other is, read-only and holds the received data from external sources via RXD. Both
mutually exclusive registers have the same address 099H.
Serial Port Control Register (SCON)
Register SCON controls serial data communication.
Address: 098H (Bit addressable)
Mode select bits
SM0 SM1 MODE
0 0 Mode0
0 1 Mode1
1 0 Mode2
1 1 M de3
SM2: used for multiprocessor communication.
REN: set or cleared by software to enable/disable reception.

TB8: Transmitted bit 8,not widely used.
RB8: Received bit 8.
TI: Transmit interrupt flag –set by the hardware at the beginning of the stop bit in
mode 1, must be cleared by software.
RI: Receive interrupt flag –set by the hardware halfway through the stop bit time in
mode1, must be cleared by software.
SCON Register
Serial control register: SCON
SM0, SM1 : Serial port mode specifier
REN : (Receive enable) set/cleared by software to enable/disable reception.
TI : Transmit interrupt flag.
RI : Receive interrupt flag.
SM2 = RB8 = TB8 =0 (not widely used)
REN (Receive Enable) -SCON.4
Set/cleared by software to enable/disable reception.
REN=1
It enable the 8051 to receive data on the RxD pin of the 8051.
If we want the 8051 to both transfer and receive data, REN must be set to 1.
SETB SCON.4
REN=0
The receiver is disabled.
The 8051 cannot receive data.
CLR SCON.4
SM0, SM1
SM1 and SM0 determine the framing of data.
SCON.6 (SM1) and SCON.7 (SM0)
Only mode 1 is compatible with COM port of PC.
SM1 SM0 Mode Operating Mode Baud Rate
0 0 0 Shift register Fosc./12
0 1 1 8-bit UART Variable by Tmer1
1 0 2 9-bit UART Fosc./64 or Fosc./32
1 1 3 9-bit UART Variable
TB8 (Transfer Bit 8) -SCON.3
• TB8 is used for serial modes 2 and 3.
• The 9th
bit that will be transmitted in mode 2 & 3.
• Set/Cleared by software
RB8 (Receive Bit 8) -SCON.2
In serial mode 1, RB8 gets a copy of the stop bit when an 8-bit data is received.
TI (Transmit Interrupt Flag) -SCON.1
When the 8051 finishes the transfer of the 8-bit character, it raises the TI flag.

TI is raised by hardware at the beginning of the stop bit in mode 1.
Must be cleared by software
RI (Receive Interrupt) -SCON.0
Receive interrupt flag. Set by hardware halfway through the stop bit time in mode 1.
Must be cleared by software.
When the 8051 receives data serially via RxD, it gets rid of the start and stop bits and
place the byte in the SBUF register.
Then 8051 rises RI to indicate that a byte.
RI is raised at the beginning of the stop bit.
Power Mode control Register
Register PCON controls processor power down, sleep modes and serial data baud rate. Only one
bit of PCON is used with respect to serial communication. The seventh bit (b7)(SMOD) is used to
generate the baud rate of serial communication.
Address: 87H
SMOD: Serial baud rate modify bit
PD: Power down bit
IDL: Idle mode bit
Data Transmission
Transmission of serial data begins at any time when data is written to SBUF. Pin P3.1 (Alternate
function bit TXD) is used to transmit data to the serial data network. TI is set to 1 when data has
been transmitted. This signifies that SBUF is empty so that another byte can be sent.
Data Reception
Reception of serial data begins if the receive enable bit is set to 1 for all modes. Pin P3.0
(Alternate function bit RXD) is used to receive data from the serial data network. Receive
interrupt flag, RI, is set after the data has been received in all modes. The data gets stored in
SBUF register from where it can be read.
Serial Data Transmission Modes:
Mode-0: In this mode, the serial port works like a shift register and the data transmission works
synchronously with a clock frequency of fosc /12. Serial data is received and transmitted through
RXD. 8 bits are transmitted/ received aty a time. Pin TXD outputs the shift clock pulses of
frequency fosc /12, which is connected to the external circuitry for synchronization. The shift
frequency or baud rate is always 1/12 of the oscillator frequency.

Fig : Data transmission/reception in Mode-0
In mode-1, the serial port functions as a standard Universal Asynchronous Receiver Transmitter
(UART) mode. 10 bits are transmitted through TXD or received through RXD. The 10 bits consist
of one start bit (which is usually '0'), 8 data bits (LSB is sent first/received first), and a stop bit
(which is usually '1'). Once received, the stop bit goes into RB8 in the special function register
SCON. The baud rate is variable.
The following figure shows the way the bits are transmitted/ received.
Fig : Data transmission format in UART mode
Bit time= 1/fbaud
In receiving mode, data bits are shifted into the receiver at the programmed baud rate. The data
word (8-bits) will be loaded to SBUF if the following conditions are true.
1. RI must be zero. (i.e., the previously received byte has been cleared from SBUF)
2. Mode bit SM2 = 0 or stop bit = 1.
After the data is received and the data byte has been loaded into SBUF, RI becomes one.
Mode-1 baud rate generation:
Timer-1 is used to generate baud rate for mode-1 serial communication by using overflow flag of
the timer to determine the baud frequency. Timer-1 is used in timer mode-2 as an auto-reload 8-
bit timer. The data rate is generated by timer-1 using the following formula.
Where,
SMOD is the 7th
bit of PCON register
fosc is the crystal oscillator frequency of the microcontroller

It can be noted that fosc/ (12 X [256- (TH1)]) is the timer overflow frequency in timer mode-2,
which is the auto-reload mode.
If timer-1 is not run in mode-2, then the baud rate is,
Timer-1 can be run using the internal clock, fosc/12 (timer mode) or from any external source via
pin T1 (P3.5) (Counter mode).
Serial Data Mode-2 - Multiprocessor Mode :
In this mode 11 bits are transmitted through TXD or received through RXD. The various bits are
as follows: a start bit (usually '0'), 8 data bits (LSB first), a programmable 9 th
(TB8 or RB8)bit
and a stop bit (usually '1').
While transmitting, the 9 th
data bit (TB8 in SCON) can be assigned the value '0' or '1'. For
example, if the information of parity is to be transmitted, the parity bit (P) in PSW could be
moved into TB8. On reception of the data, the 9 th
bit goes into RB8 in 'SCON', while the stop bit
is ignored. The baud rate is programmable to either 1/32 or 1/64 of the oscillator frequency.
Mode-3 - Multi processor mode with variable baud rate :
In this mode 11 bits are transmitted through TXD or received through RXD. The various bits are:
a start bit (usually '0'), 8 data bits (LSB first), a programmable 9 th bit and a stop bit (usually '1').
Mode-3 is same as mode-2, except the fact that the baud rate in mode-3 is variable (i.e., just as in
mode-1).
f baud = (2 SMOD
/32) * ( fosc / 12 (256-TH1)) .
This baudrate holds when Timer-1 is programmed in Mode-2.
Programming the 8051 to transfer data serially
Write a program for the 8051 to transfer letter “A” serially at 4800baud, continuously.
MOV TMOD,#20H ;timer 1, mode 2
MOV TH1,#-6 ;4800 baud rate
MOV SCON,#50H ;8-bit,1 stop,REN enabled
SETB TR1 ;start timer 1
AGAIN: MOV SBUF,#”A” ;letter “A” to be transferred
HERE: JNB TI,HERE ;wait for the last bit
CLR TI ;clear TI for next char
SJMP AGAIN ;keep sending A
Write a program to transfer the message “YES” serially at 9600 baud, 8-bit data, 1 stop
bit. Do this continuously.
MOV TMOD,#20H ;timer 1, mode 2
MOV TH1,#-3 ;9600 baud
MOV SCON,#50H
SETB TR1
AGAIN: MOV A,#”Y” ;transfer “Y”

ACALL TRANS
MOV A,#”E” ;transfer “E”
ACALL TRANS
MOV A,#”S” ;transfer “S”
ACALL TRANS
SJMP AGAIN ;keep doing it
;serial data transfer subroutine
TRANS: MOV SBUF,A ;load SBUF
HERE: JNB TI,HERE ;wait for last bit to transfer
CLR TI ;get ready for next byte
RET
Baud Rates in the 8051
• Timer 1, mode 2 (8-bit, auto-reload)
• Define TH1 to set the baud rate.
XTAL = 11.0592 MHz
The system frequency = 11.0592 MHz / 12 = 921.6 kHz
Timer 1 has 921.6 kHz/ 32 = 28,800 Hz as source.
TH1=FDH means that UART sends a bit every 3 timer source.
Baud rate = 28,800/3= 9,600 Hz
Example
With XTAL = 11.0592 MHz, find the TH1 value needed to have the
following baud rates. (a) 9600 (b) 2400 (c) 1200
Solution:
With XTAL = 11.0592 MHz, we have:
The frequency of system clock = 11.0592 MHz / 12 = 921.6 kHz
The frequency sent to timer 1 = 921.6 kHz/ 32 = 28,800 Hz
(a) 28,800 / 3 = 9600 where -3 = FD (hex) is loaded into TH1
(b) 28,800 / 12 = 2400 where -12 = F4 (hex) is loaded into TH1
(c) 28,800 / 24 = 1200 where -24 = E8 (hex) is loaded into TH1
Registers Used in Serial Transfer Circuit
SUBF (Serial data buffer)
SCON (Serial control register)
PCON (Power control register)
SBUF Register
Serial data register: SBUF
MOV SBUF,#‟A‟ ;put char „A‟ to transmit
MOV SBUF,A ;send data from A
MOV A,SUBF ;receive and copy to A
An 8-bit register
Set the usage mode for two timers
For a byte of data to be transferred via the TxD line, it must be placed in the SBUF.
SBUF holds the byte of data when it is received by the 8051‟s RxD line.

Block Diagram of the 8255 Programmable Peripheral Interface (PPI)
Data Bus Buffer
This three-state bi-directional 8-bit buffer is used to interface the 8255 to the system data
bus. Data is transmitted or received by the buffer upon execution of input or output
instructions by the CPU. Control words and status informa-tion are also transferred
through the data bus buffer.
Read/Write and Control Logic
The function of this block is to manage all of the internal and external transfers of both
Data and Control or Status words. It accepts inputs from the CPU Address and Control
busses and in turn, issues commands to both of the Control Groups.
(CS) Chip Select. A "low" on this input pin enables the communcation between the 8255
and the CPU.
(RD) Read. A "low" on this input pin enables 8255 to send the data or status information
to the CPU on the data bus. In essence, it allows the CPU to "read from" the 8255.

(WR) Write. A "low" on this input pin enables the CPU to write data or control words into
the 8255.
(A0 and A1) Port Select 0 and Port Select 1. These input signals, in conjunction with the
RD and WR inputs, control the selection of one of the three ports or the control word
register. They are normally connected to the least significant bits of the address bus (A0
and A1).
(RESET) Reset. A "high" on this input initializes the control register to 9Bh and all ports
(A, B, C) are set to the input mode.
A1 A0 SELECTION
0 0 PORT A
0 1 PORT B
1 0 PORT C
1 1 CONTROL
Group A and Group B Controls
The functional configuration of each port is programmed by the systems software. In
essence, the CPU "outputs" a control word to the 8255. The control word contains
information such as "mode", "bit set", "bit reset", etc., that initializes the functional
configuration of the 8255. Each of the Control blocks (Group A and Group B) accepts
"commands" from the Read/Write Control logic, receives "control words" from the
internal data bus and issues the proper commands to its associated ports.
Ports A, B, and C
The 8255 contains three 8-bit ports (A, B, and C). All can be configured to a wide variety
of functional characteristics by the system software but each has its own special features
or "personality" to further enhances the power and flexibility of the 8255.
Port A One 8-bit data output latch/buffer and one 8-bit data input latch. Both "pull-up"
and "pull-down" bus-hold devices are present on Port A.
Port B One 8-bit data input/output latch/buffer and one 8-bit data input buffer.
Port C One 8-bit data output latch/buffer and one 8-bit data input buffer (no latch for
input). This port can be divided into two 4-bit ports under the mode control. Each 4-bit
port contains a 4-bit latch and it can be used for the control signal output and status
signal inputs in conjunction with ports A and B.

UNIT 7:
Motivation for MSP430microcontrollers – Low Power embedded systems, On-chip peripherals
(analog and digital), low-power RF capabilities. Target applications (Single-chip, low cost, low
power, high performance system design).
2 Hrs
MSP430 RISC CPU architecture, Compiler-friendly features, Instruction set, Clock system,
Memory subsystem. Key differentiating factors between different MSP430 families.
2 Hrs
Introduction to Code Composer Studio (CCS v4). Understanding how to use CCS for Assembly,
C, Assembly+C projects for MSP430 microcontrollers. Interrupt programming.
3 Hrs
Digital I/O – I/O ports programming using C and assembly, Understanding the muxing scheme of
the MSP430 pins.
2 Hrs
Introduction
The MSP430 is a 16-bit microcontroller that has a number of special features not commonly available with
other microcontrollers:
Complete system on-a-chip — includes LCD control, ADC, I/O ports,ROM, RAM, basic timer,
watchdog timer, UART, etc.
Extremely low power consumption — only 4.2 nW per instruction, typical
High speed — 300 ns per instruction @ 3.3 MHz clock, in register and register addressing mode
RISC structure — 27 core instructions
Orthogonal architecture (any instruction with any addressing mode)
Seven addressing modes for the source operand
Four addressing modes for the destination operand
Constant generator for the most often used constants (–1, 0, 1, 2, 4, 8)
Only one external crystal required — a frequency locked loop (FLL) oscillator derives all internal
clocks
Full real-time capability — stable, nominal system clock frequency is available after only six clocks
when the MSP430 is restored from low-power mode (LPM) 3; — no waiting for the main crystal to
begin oscillation and stabilize
The 27 core instructions combined with these special features make it easyto program the MSP430 in
assembler or in C, and provide exceptional flexibility and functionality.

Advantages of the MSP430 Concept
The MSP430 concept differs considerably from other microcontrollers and offers some significant
advantages over more traditional designs.
1.5.1 RISC Architecture without RISC Disadvantages
Typical RISC architectures show their highest performance in calculation- intensive applications in which
several registers are loaded with input data, all calculations are made within the registers, and the results
are stored back into RAM. Memory accesses (using addressing modes) are necessary only for the LOAD
instructions at the beginning and the STORE instructions at the end of the calculations. The MSP430 can
be programmed for such operation, for example,performing a pure calculation task in the floating point
without any I/O accesses. Pure RISC architectures have some disadvantages when running real-time
applications that require frequent I/O accesses, however. Time is lost whenever
an operand is fetched and loaded from RAM, modified, and then stored back into RAM. The MSP430
architecture was designed to include the best of both worlds, taking advantage of RISC features for fast
and efficient calculations, and addressing modes for real-time requirements:
The RISC architecture provides a limited number of powerful instructions, numerous registers, and
single-cycle execution times.
The more traditional microcomputer features provide addressing modes for all instructions. This
functionality is further enhanced with 100% orthogonality, allowing any instruction to be used with
any addressing mode.
1.5.2 Real-Time Capability with Ultra-Low Power Consumption
The design of the MSP430 was driven by the need to provide full real-time capability while still exhibiting
extremely low power consumption. Average power consumption is reduced to the minimum by running the
CPU and certain other functions of the MSP430 only when it is necessary. The rest of the time (the
majority of the time), power is conserved by keeping only selected low-power peripheral functions active.

But to have a true real-time capability, the device must be able to shift from a low-power mode with the
CPU off to a fully active mode with the CPU and all other device functions operating nominally in a very
short time. This was accomplished primarily with the design of the system clock:
No second high frequency crystal is used — inherent delays can range
from 20 ms to 200 ms until oscillator stability is reached
Instead, a sophisticated FLL system clock generator is used — generator output frequency
(MCLK) reaches the nominal frequency within 8 cycles after activation from low power mode 3
(LPM3) or sleep mode. This design provides real-time capability almost immediately after the
device comes out of a LPM — as if the CPU is always active. Only two additional MCLK cycles (2
s @ fC = 1 MHz) are necessary to get the device from LPM3 to the first instruction of the interrupt
handler.
1.5.3 Digitally Controlled Oscillator Stability
The digitally controlled oscillator (DCO) is voltage and temperature dependent, which does not mean that
its frequency is not stable. During the active mode, the integral error is corrected to approximately zero
every 30.5 s . This is accomplished by switching between two different DCO frequencies. One frequency
is higher than the programmed MCLK frequency and the other is lower, causing the errors to essentially
cancel-out. The two DCO frequencies are interlaced as much as possible to provide the smallest possible
error at any given time. See System Clock Generator for more information.
1.5.4 Stack Processing Capability
The MSP430 is a true stack processor, with most of the seven addressing modes implemented for the
stack pointer (SP) as well as the other CPU registers (PC and R4 through R15). The capabilities of the
stack include:
Free access to all items on the stack — not only to the top of the stack (TOS)
Ability to modify subroutine and interrupt return addresses located on the stack
Ability to modify the stored status register of interrupt returns located on the stack
No special stack instructions — all of the implemented instructions may be used for the stack and
the stack pointer
Byte and word capability for the stack
Free mix of subroutine and interrupt handling — as long as no stack modification (PUSH, POP, etc.)
is made, no errors can occur

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