Topics to be covered

• CPU operation
• Memory space
• Software overview
• Peripheral overview
– Interrupt
– Timers
– Parallel port inputs and outputs
– Serial port
• low power special modes of
operation Noise
8051 Microcontroller Sukesh Rao Muligar, Asst. Prof. - ECE 1
 Introduction
to
Microcontrollers
8051 Microcontroller Sukesh Rao Muligar, Asst. Prof. - ECE 2
 Why do we need to learn
Microprocessors/controllers?

• The microprocessor is the core of computer systems.

• Nowadays many communication, digital entertainment,

portable devices, are controlled by them.

• A designer should know what types of components he

needs, ways to reduce production costs and product
reliable.

8051 Microcontroller Sukesh Rao Muligar, Asst. Prof. - ECE 3

 The necessary tools for a
microprocessor/controller
1. CPU: Central Processing Unit
2. I/O: Input /Output
3. Bus: Address bus & Data bus
4. Memory: RAM & ROM
5. Timer
6. Interrupt
7. Serial Port
8. Parallel Port

8051 Microcontroller Sukesh Rao Muligar, Asst. Prof. - ECE 4

 Microprocessors
• CPU for Computers
• No RAM, ROM, I/O on CPU chip itself
• Example: Intel's x86, Motorola’s 680x0

8051 Microcontroller Sukesh Rao Muligar, Asst. Prof. - ECE 5

 Microcontroller
• A smaller computer
• On-chip RAM, ROM, I/O ports...
• Example: Motorola’s 6811, Intel’s 8051, Zilog’s Z8 and PIC

8051 Microcontroller Sukesh Rao Muligar, Asst. Prof. - ECE 6

 Microprocessor vs. Microcontroller
Microprocessor Microcontroller
• CPU is stand-alone, RAM, • CPU, RAM, ROM, I/O and
ROM, I/O, timer are separate timer are all on a single chip

• Designer can decide on the • Fix amount of on-chip ROM,

amount of ROM, RAM and I/O RAM, I/O ports
ports.
• For applications in which cost,
• Expansive power and space are critical

• Versatility • Not Expansive

• General-purpose • Single-purpose

8051 Microcontroller Sukesh Rao Muligar, Asst. Prof. - ECE 7

 Microcontrollers for Embedded Systems
• Home
– Appliances, intercom, telephones, security systems, garage door openers,
answering machines, fax machines, home computers, TVs, cable TV tuner,
VCR, camcorder, remote controls, video games, cellular phones, musical
instruments, sewing machines, lighting control, paging, camera, pinball
machines, toys, exercise equipment etc.

• Office
– Telephones, computers, security systems, fax machines, microwave, copier,
laser printer, color printer, paging etc.

• Auto
– Trip computer, engine control, air bag, ABS, instrumentation, security
system, transmission control, entertainment, climate control, cellular
phone, keyless entry

8051 Microcontroller Sukesh Rao Muligar, Asst. Prof. - ECE 8

 Choosing a Microcontroller
• 8-bit microcontrollers
– Motorola’s 6811
– Intel’s 8051
– Zilog’s Z8
– Microchip’s PIC

• There are also 16-bit and 32-bit microcontrollers

made by various chip makers

8051 Microcontroller Sukesh Rao Muligar, Asst. Prof. - ECE 9

 Criteria for Choosing a Microcontroller
• Meeting the computing needs of the task at hand
efficiently and cost effectively
– Speed
– Packaging
– Power consumption
– The amount of RAM and ROM on chip
– The number of I/O pins and the timer on chip
– How easy to upgrade to higher performance or lower power-
consumption versions
– Cost per unit

8051 Microcontroller Sukesh Rao Muligar, Asst. Prof. - ECE 10

 Criteria for Choosing a Microcontroller
• Availability of software development tools, such as
compilers, assemblers, and debuggers

• Wide availability and reliable sources of the

microcontroller
– The 8051 family has the largest number of diversified (multiple
source) suppliers
• Intel (original)
• Atmel
• Philips/Signetics
• AMD
• Infineon (formerly Siemens)
• Matra
• Dallas Semiconductor/Maxim
8051 Microcontroller Sukesh Rao Muligar, Asst. Prof. - ECE 11
 8051 CPU Operation
1. Features
2. Pin Diagram
3. Block Diagram

8051 Microcontroller Sukesh Rao Muligar, Asst. Prof. - ECE 12

 8051 Microcontroller
• Intel introduced 8051, referred as MCS- 51, in
1981.

• The 8051 is an 8-bit processor

– The CPU can work on only 8 bits of data at a time

• The 8051 became widely popular after allowing

other manufactures to make and market any
flavor of the 8051.

8051 Microcontroller Sukesh Rao Muligar, Asst. Prof. - ECE 13

 8051 Family
• The 8051 is a subset of the 8052
• The 8031 is a ROM-less 8051
– Add external ROM to it
– You lose two ports, and leave only 2 ports for I/O operations

8051 Microcontroller Sukesh Rao Muligar, Asst. Prof. - ECE 14

 8051 Features
• 64KB Program Memory address space
• 64KB Data Memory address space
• 4K bytes of on-chip Program Memory
• 128 bytes of on-chip Data RAM
• 32 bidirectional and individually addressable 1/0 lines
• Two 16-bit timer/counters
• Full duplex UART
• 6-source/5-vector interrupt structure with two priority
levels
• On-chip clock oscillator
8051 Microcontroller Sukesh Rao Muligar, Asst. Prof. - ECE 15
 Pin Diagram of the 8051

8051 Microcontroller Sukesh Rao Muligar, Asst. Prof. - ECE 16

 General Block Diagram of 8051

Interrupt 4K Timer 0
128 B
Control ROM RAM Timer 1

CPU

Bus Serial
OSC 4 I/O Ports
Control Port

TXD RXD
8051 Microcontroller P0
Sukesh Rao Muligar, P1 - ECE
Asst. Prof. P2 P3 17
 Detailed Block Diagram

8051 Microcontroller Sukesh Rao Muligar, Asst. Prof. - ECE 18

 8051
Memory Space
8051 Microcontroller Sukesh Rao Muligar, Asst. Prof. - ECE 19
 8051 Memory Structure

External

External
60K

64K 64K

SFR
EXT INT 4K
128
EA = 0 EA = 1

Program Memory Data Memory

8051 Microcontroller Sukesh Rao Muligar, Asst. Prof. - ECE 20
 Internal RAM Structure

Direct
Addressing
Only
SFR [ Special Function
Direct & Registers]
Indirect
Addressing
128 Byte Internal RAM

8051 Microcontroller Sukesh Rao Muligar, Asst. Prof. - ECE 21

 Special Function Registers [SFR]

8051 Microcontroller Sukesh Rao Muligar, Asst. Prof. - ECE 22

 Program Status Word [PSW]

C AC F0 RS1 RS0 OV F1 P
Carry Parity
Auxiliary Carry User Flag 1
User Flag 0 Register Bank Select Overflow

8051 Microcontroller Sukesh Rao Muligar, Asst. Prof. - ECE 23

 8051 instructions that affects flag

8051 Microcontroller Sukesh Rao Muligar, Asst. Prof. - ECE 24

 128 Byte RAM
• There are 128 bytes of RAM in the 8051.
– Assigned addresses 00 to 7FH General Purpose
Area
• The 128 bytes are divided into 3 different
groups as follows:
BIT Addressable
1. A total of 32 bytes from locations 00 to 1F Area
hex are set aside for register banks and the 128 BYTE
stack. INTERNAL RAM
Reg Bank 3
2. A total of 16 bytes from locations 20H to 2FH
Reg Bank 2
are set aside for bit-addressable read/write Register Banks
memory. Reg Bank 1
3. A total of 80 bytes from locations 30H to 7FH Reg Bank 0
are used for read and write storage, called
scratch pad.
8051 Microcontroller Sukesh Rao Muligar, Asst. Prof. - ECE 25
 8051 RAM with addresses

8051 Microcontroller Sukesh Rao Muligar, Asst. Prof. - ECE 26

 8051 Register Bank Structure

Bank 3 R0 R1 R2 R3 R4 R5 R6 R7
Bank 2 R0 R1 R2 R3 R4 R5 R6 R7
Bank 1 R0 R1 R2 R3 R4 R5 R6 R7
Bank 0 R0 R1 R2 R3 R4 R5 R6 R7

8051 Microcontroller Sukesh Rao Muligar, Asst. Prof. - ECE 27

 8051 Register Banks with address

8051 Microcontroller Sukesh Rao Muligar, Asst. Prof. - ECE 28

 8051 Programming Model

8051 Microcontroller Sukesh Rao Muligar, Asst. Prof. - ECE 29

 8051 Stack
• The stack is a section of RAM used by the CPU to store
information temporarily.
– This information could be data or an address

• The register used to access the stack is called the SP

(stack pointer) register
– The stack pointer in the 8051 is only 8 bit wide, which means
that it can take value of 00 to FFH
– When the 8051 is powered up, the SP register contains value
07
– RAM location 08 is the first location begin used for the stack by
the 8051
8051 Microcontroller Sukesh Rao Muligar, Asst. Prof. - ECE 30
 8051 Stack
• The storing of a CPU register in the stack is called a PUSH
– SP is pointing to the last used location of the stack
– As we push data onto the stack, the SP is incremented by one
– This is different from many microprocessors

• Loading the contents of the stack back into a CPU

register is called a POP
– With every pop, the top byte of the stack is copied to the
register specified by the instruction and the stack pointer is
decremented once

8051 Microcontroller Sukesh Rao Muligar, Asst. Prof. - ECE 31

 Bit Addressable & Byte Addressable

8051 Microcontroller Sukesh Rao Muligar, Asst. Prof. - ECE 32

 Single bit Instructions

8051 Microcontroller Sukesh Rao Muligar, Asst. Prof. - ECE 33

 Bit Addressable Programming
• Example: Find out to which by each of the following bits
belongs. Give the address of the RAM byte in hex
(a) SETB 42H, (b) CLR 67H, (c) CLR 0FH (d) SETB 28H, (e) CLR 12, (f) SETB 05

8051 Microcontroller Sukesh Rao Muligar, Asst. Prof. - ECE 34

 8051 Peripheral Overview
1. Io PORTs
2. Timers
3. Serial Port
4. Interrupts
8051 Microcontroller Sukesh Rao Muligar, Asst. Prof. - ECE 35
 Input-output(IO) Ports
8051 has 4 IO ports •
 PORT0
PORT1
PORT2
PORT3

8051 Microcontroller Sukesh Rao Muligar, Asst. Prof. - ECE 36

 Configuring port as OUTPUT or INPUT
• By default every IO port pin is at logic 1.
OUTPUT
• There is no conditional logic need be loaded to
configure a port as OUTPUT
• Required logic can be loaded to the port using the
instruction with any addressing mode
INPUT
• Any port to be configured as INPUT must be pulled up to
logic 1.

8051 Microcontroller Sukesh Rao Muligar, Asst. Prof. - ECE 37

 Internal block of an IO PORT

8051 Microcontroller Sukesh Rao Muligar, Asst. Prof. - ECE 38

 Writing IO
PORT
Logic 0

Writing
logic 1

8051 Microcontroller Sukesh Rao Muligar, Asst. Prof. - ECE 39

Reading IO PORT pin

Condition : IO PIN must be at Logic HIGH

8051 Microcontroller Sukesh Rao Muligar, Asst. Prof. - ECE 40

 8051
TIMERS
8051 Microcontroller Sukesh Rao Muligar, Asst. Prof. - ECE 41
 8051 Timer/Counter

OSC ÷12
C /T  0 TLx THx TFx
(8 Bit) (8 Bit) (1 Bit)
C /T 1

T PIN
INTERRUPT
TR

Gate

INT PIN

8051 Microcontroller Sukesh Rao Muligar, Asst. Prof. - ECE 42

 TMOD Register

GATE:
When set, timer/counter x is enabled, if INTx pin is high
and TRx is set.
When cleared, timer/counter x is enabled, if TRx bit set.

C/T*:
When set, counter operation (input from Tx input pin).
When cleared, timer operation (input from internal clock).

8051 Microcontroller Sukesh Rao Muligar, Asst. Prof. - ECE 43

 TMOD Register

The TMOD byte is not bit addressable.

8051 Microcontroller Sukesh Rao Muligar, Asst. Prof. - ECE 44

 TCON Register

8051 Microcontroller Sukesh Rao Muligar, Asst. Prof. - ECE 45

 8051 Timer Modes
8051 TIMERS

Timer 0 Timer 1

Mode 0 Mode 0

Mode 1 Mode 1

Mode 2 Mode 2

Mode 3

8051 Microcontroller Sukesh Rao Muligar, Asst. Prof. - ECE 46

 TIMER 0
OSC ÷12
C /T  0
TL0 TH0 TF0
C /T 1

T 0PIN
INTERRUPT
TR0

Gate

INT 0 PIN

8051 Microcontroller Sukesh Rao Muligar, Asst. Prof. - ECE 47

 TIMER 0 – Mode 0
13 Bit Timer / Counter

OSC ÷12
C /T  0 TL0 TH0 INTERRUPT
C /T 1 TF0
T 0PIN
(5 Bit) (8 Bit)
TR0

Gate

INT 0 PIN

Maximum Count = 1FFFh (0001111111111111)

8051 Microcontroller Sukesh Rao Muligar, Asst. Prof. - ECE 48
 TIMER 0 – Mode 1
16 Bit Timer / Counter

OSC ÷12
C /T  0 TL0 TH0 INTERRUPT
C /T 1 TF0
T 0PIN
(8 Bit) (8 Bit)
TR0

Gate

INT 0 PIN

Maximum Count = FFFFh (1111111111111111)

8051 Microcontroller Sukesh Rao Muligar, Asst. Prof. - ECE 49
 TIMER 0 – Mode 2
8 Bit Timer / Counter with AUTORELOAD

OSC ÷12
C /T  0 TL0 TH0 INTERRUPT
C /T 1 TF0
T 0PIN
(8 Bit) (8 Bit)
TR0

Gate Reload
INT 0 PIN

TH0
(8 Bit)

Maximum Count = FFh (11111111)

8051 Microcontroller Sukesh Rao Muligar, Asst. Prof. - ECE 50
 TIMER 0 – Mode 3
Two - 8 Bit Timer / Counter

OSC ÷12
C /T  0 TL0 INTERRUPT
C /T 1 TF0
T 0PIN
(8 Bit)
TR0

Gate

INT 0 PIN

OSC ÷12 TH0 INTERRUPT

TF1
(8 Bit)

TR1
8051 Microcontroller Sukesh Rao Muligar, Asst. Prof. - ECE 51
 TIMER 1
OSC ÷12
C /T  0
TL1 TH1 TF1
C /T 1

T1PIN
INTERRUPT
TR1

Gate

INT1 PIN

8051 Microcontroller Sukesh Rao Muligar, Asst. Prof. - ECE 52

 TIMER 1 – Mode 0
13 Bit Timer / Counter

OSC ÷12
C /T  0 TL1 TH1 INTERRUPT
C /T 1 TF1
T1PIN
(5 Bit) (8 Bit)
TR1

Gate

INT1 PIN

Maximum Count = 1FFFh (0001111111111111)

8051 Microcontroller Sukesh Rao Muligar, Asst. Prof. - ECE 53
 TIMER 1 – Mode 1
16 Bit Timer / Counter

OSC ÷12
C /T  0 TL1 TH1 INTERRUPT
C /T 1 TF1
T1PIN
(8 Bit) (8 Bit)
TR1

Gate

INT1 PIN

Maximum Count = FFFFh (1111111111111111)

8051 Microcontroller Sukesh Rao Muligar, Asst. Prof. - ECE 54
 TIMER 1 – Mode 2
8 Bit Timer / Counter with AUTORELOAD

OSC ÷12
C /T  0 TL1 TH1 INTERRUPT
C /T 1 TF1
T1PIN
(8 Bit) (8 Bit)
TR1

Gate Reload
INT1 PIN

TH1
(8 Bit)

Maximum Count = FFh (11111111)

8051 Microcontroller Sukesh Rao Muligar, Asst. Prof. - ECE 55
 Programming Timers
• Example: Indicate which mode and which timer are
selected for each of the following.
(a) MOV TMOD, #01H (b) MOV TMOD, #20H (c) MOV
TMOD, #12H

• Solution: We convert the value from hex to binary.

(a) TMOD = 00000001, mode 1 of timer 0 is selected.
(b) TMOD = 00100000, mode 2 of timer 1 is selected.
(c) TMOD = 00010010, mode 2 of timer 0, and mode 1 of timer 1
are selected.

8051 Microcontroller Sukesh Rao Muligar, Asst. Prof. - ECE 56

 Programming Timers
• Find the timer’s clock frequency and its period for
various 8051-based system, with the crystal frequency
11.0592 MHz when C/T bit of TMOD is 0.

• Solution:

1/12 × 11.0529 MHz = 921.6 MHz;

T = 1/921.6 kHz = 1.085 us

8051 Microcontroller Sukesh Rao Muligar, Asst. Prof. - ECE 57

 8051
Serial
8051 Microcontroller
Port
Sukesh Rao Muligar, Asst. Prof. - ECE 58
 Basics of Serial Communication
• Computers transfer data in two ways:
– Parallel: Often 8 or more lines (wire conductors) are used to
transfer data to a device that is only a few feet away.

– Serial: To transfer to a device located many meters away, the

serial method is used. The data is sent one bit at a time.

8051 Microcontroller Sukesh Rao Muligar, Asst. Prof. - ECE 59

 Basics of Serial Communication
• Serial data communication uses two methods
– Synchronous method transfers a block of data at a time

– Asynchronous method transfers a single byte at a time

• There are special IC’s made by many manufacturers for

serial communications.
– UART (universal asynchronous Receiver transmitter)

– USART (universal synchronous-asynchronous Receiver-

transmitter)

8051 Microcontroller Sukesh Rao Muligar, Asst. Prof. - ECE 60

 Asynchronous – Start & Stop Bit
• Asynchronous serial data communication is widely used
for character-oriented transmissions
– Each character is placed in between start and stop bits, this is
called framing.
– Block-oriented data transfers use the synchronous method.

• The start bit is always one bit, but the stop bit can be
one or two bits

• The start bit is always a 0 (low) and the stop bit(s) is 1

(high)
8051 Microcontroller Sukesh Rao Muligar, Asst. Prof. - ECE 61
 Asynchronous – Start & Stop Bit

8051 Microcontroller Sukesh Rao Muligar, Asst. Prof. - ECE 62

 Data Transfer Rate
• The rate of data transfer in serial data communication is
stated in bps (bits per second).

• Another widely used terminology for bps is baud rate.

– It is modem terminology and is defined as the number of
signal changes per second
– In modems, there are occasions when a single change of signal
transfers several bits of data

• As far as the conductor wire is concerned, the baud rate

and bps are the same.

8051 Microcontroller Sukesh Rao Muligar, Asst. Prof. - ECE 63

 8051 Serial Port
• Synchronous and Asynchronous
• SCON Register is used to Control
• Data Transfer through TXd & RXd pins
• Some time - Clock through TXd Pin
• Four Modes of Operation:

Mode 0 :Synchronous Serial Communication

Mode 1 :8-Bit UART with Timer Data Rate
Mode 2 :9-Bit UART with Set Data Rate
Mode 3 :9-Bit UART with Timer Data Rate

8051 Microcontroller Sukesh Rao Muligar, Asst. Prof. - ECE 64

 Registers related to Serial
Communication

1. SBUF Register

2. SCON Register

3. PCON Register

8051 Microcontroller Sukesh Rao Muligar, Asst. Prof. - ECE 65

 SBUF Register
• SBUF is an 8-bit register used solely for serial communication.
• For a byte data to be transferred via the TxD line, it must be
placed in the SBUF register.
• The moment a byte is written into SBUF, it is framed with the
start and stop bits and transferred serially via the TxD line.
• SBUF holds the byte of data when it is received by 8051 RxD
line.
• When the bits are received serially via RxD, the 8051 deframes
it by eliminating the stop and start bits, making a byte out of
the data received, and then placing it in SBUF.

8051 Microcontroller Sukesh Rao Muligar, Asst. Prof. - ECE 66

 SBUF Register
• Sample Program:

8051 Microcontroller Sukesh Rao Muligar, Asst. Prof. - ECE 67

 SCON Register

SM0 SM1 SM2 REN TB8 RB8 TI RI

Set when a Cha-

Set to Enable ractor received
Serial Data
reception Set when Stop bit Txed

Enable Multiprocessor 9th Data Bit 9th Data Bit

Communication Mode Sent in Mode 2,3 Received in Mode 2,3

8051 Microcontroller Sukesh Rao Muligar, Asst. Prof. - ECE 68

 8051 Serial Port – Mode 0
The Serial Port in Mode-0 has the following features:

1. Serial data enters and exits through RXD

2. TXD outputs the clock

3. 8 bits are transmitted / received

4. The baud rate is fixed at (1/12) of the oscillator frequency

8051 Microcontroller Sukesh Rao Muligar, Asst. Prof. - ECE 69

 8051 Serial Port – Mode 1
The Serial Port in Mode-1 has the following features:

1. Serial data enters through RXD

2. Serial data exits through TXD
3. On receive, the stop bit goes into RB8 in SCON
4. 10 bits are transmitted / received
1. Start bit (0)
2. Data bits (8)
3. Stop Bit (1)
5. Baud rate is determined by the Timer 1 over flow rate.

8051 Microcontroller Sukesh Rao Muligar, Asst. Prof. - ECE 70

 8051 Serial Port – Mode 2
The Serial Port in Mode-2 has the following features:

1. Serial data enters through RXD

2. Serial data exits through TXD
3. 9th data bit (TB8) can be assign value 0 or 1
4. On receive, the 9th data bit goes into RB8 in SCON
5. 11 bits are transmitted / received
1.Start bit (0)
2.Data bits (9)
3.Stop Bit (1)
6. Baud rate is programmable

8051 Microcontroller Sukesh Rao Muligar, Asst. Prof. - ECE 71

 8051 Serial Port – Mode 3
The Serial Port in Mode-3 has the following features:

1. Serial data enters through RXD

2. Serial data exits through TXD
3. 9th data bit (TB8) can be assign value 0 or 1
4. On receive, the 9th data bit goes into RB8 in SCON
5. 11 bits are transmitted / received
1.Start bit (0)
2.Data bits (9)
3.Stop Bit (1)
6. Baud rate is determined by Timer 1 overflow rate.

8051 Microcontroller Sukesh Rao Muligar, Asst. Prof. - ECE 72

 Programming Serial Data Transmission
1. TMOD register is loaded with the value 20H, indicating the use of timer
1 in mode 2 (8-bit auto-reload) to set baud rate.
2. The TH1 is loaded with one of the values to set baud rate for serial data
transfer.
3. The SCON register is loaded with the value 50H, indicating serial mode
1, where an 8- bit data is framed with start and stop bits.
4. TR1 is set to 1 to start timer 1
5. TI is cleared by CLR TI instruction
6. The character byte to be transferred serially is written into SBUF
register.
7. The TI flag bit is monitored with the use of instruction JNB TI, xx to see
if the character has been transferred completely.
8. To transfer the next byte, go to step 5
8051 Microcontroller Sukesh Rao Muligar, Asst. Prof. - ECE 73
 Programming Serial Data Reception
1. TMOD register is loaded with the value 20H, indicating the use of timer 1
in mode 2 (8-bit auto-reload) to set baud rate.
2. TH1 is loaded to set baud rate
3. The SCON register is loaded with the value 50H, indicating serial mode 1,
where an 8- bit data is framed with start and stop bits.
4. TR1 is set to 1 to start timer 1
5. RI is cleared by CLR RI instruction
6. The RI flag bit is monitored with the use of instruction JNB RI, xx to see if
an entire character has been received yet
7. When RI is raised, SBUF has the byte, its contents are moved into a safe
place.
8. To receive the next character, go to step 5.

8051 Microcontroller Sukesh Rao Muligar, Asst. Prof. - ECE 74

 Doubling Baud Rate
• There are two ways to increase the baud rate of data
transfer
1. By using a higher frequency crystal
2. By changing a bit in the PCON register

• PCON register is an 8-bit register.

•When 8051 is powered up, SMOD is zero

•We can set it to high by software and thereby double the baud rate.

8051 Microcontroller Sukesh Rao Muligar, Asst. Prof. - ECE 75

 Doubling Baud Rate (cont…)

8051 Microcontroller Sukesh Rao Muligar, Asst. Prof. - ECE 76

 8051
Interrupts
8051 Microcontroller Sukesh Rao Muligar, Asst. Prof. - ECE 77
 INTERRUPTS
• An interrupt is an external or internal event that
interrupts the microcontroller to inform it that a device
needs its service

• A single microcontroller can serve several devices by two

ways:
1. Interrupt
2. Polling

8051 Microcontroller Sukesh Rao Muligar, Asst. Prof. - ECE 78

 Interrupt Vs Polling
1. Interrupts
– Whenever any device needs its service, the device notifies the
microcontroller by sending it an interrupt signal.
– Upon receiving an interrupt signal, the microcontroller
interrupts whatever it is doing and serves the device.
– The program which is associated with the interrupt is called the
interrupt service routine (ISR) or interrupt handler.
2. Polling
– The microcontroller continuously monitors the status of a
given device.
– When the conditions met, it performs the service.
– After that, it moves on to monitor the next device until every
one is serviced.
8051 Microcontroller Sukesh Rao Muligar, Asst. Prof. - ECE 79
 Interrupt Vs Polling
• The polling method is not efficient, since it wastes much of
the microcontroller’s time by polling devices that do not
need service.
• The advantage of interrupts is that the microcontroller can
serve many devices (not all at the same time).
• Each devices can get the attention of the microcontroller
based on the assigned priority.
• For the polling method, it is not possible to assign priority
since it checks all devices in a round-robin fashion.

• The microcontroller can also ignore (mask) a device request

for service in Interrupt.
8051 Microcontroller Sukesh Rao Muligar, Asst. Prof. - ECE 80
 Steps in Executing an Interrupt
1. It finishes the instruction it is executing and saves the address of
the next instruction (PC) on the stack.
2. It also saves the current status of all the interrupts internally (i.e:
not on the stack).
3. It jumps to a fixed location in memory, called the interrupt
vector table, that holds the address of the ISR.
4. The microcontroller gets the address of the ISR from the
interrupt vector table and jumps to it.
5. It starts to execute the interrupt service subroutine until it
reaches the last instruction of the subroutine which is RETI
(return from interrupt).
6. Upon executing the RETI instruction, the microcontroller returns
to the place where it was interrupted.
8051 Microcontroller Sukesh Rao Muligar, Asst. Prof. - ECE 81
 Six Interrupts in 8051
Six interrupts are allocated as follows:
1. Reset – power-up reset.

2. Two interrupts are set aside for the timers.

– one for timer 0 and one for timer 1

3. Two interrupts are set aside for hardware external

interrupts.
– P3.2 and P3.3 are for the external hardware interrupts INT0
(or EX1), and INT1 (or EX2)

4. Serial communication has a single interrupt that

belongs to both receive and transfer.
8051 Microcontroller Sukesh Rao Muligar, Asst. Prof. - ECE 82
 What events can trigger Interrupts?
• We can configure the 8051 so that any of the following
events will cause an interrupt:

– Timer 0 Overflow.
– Timer 1 Overflow.
– Reception/Transmission of Serial Character.
– External Event 0.
– External Event 1.

• We can configure the 8051 so that when Timer 0

Overflows or when a character is sent/received, the
appropriate interrupt handler routines are called.
8051 Microcontroller Sukesh Rao Muligar, Asst. Prof. - ECE 83
 8051 Interrupt Vectors

8051 Microcontroller Sukesh Rao Muligar, Asst. Prof. - ECE 84

 8051 Interrupt related Registers
• The various registers associated with the use of
interrupts are:

– TCON - Edge and Type bits for External Interrupts 0/1

– SCON - RI and TI interrupt flags for RS232

– IE - Enable interrupt sources

– IP - Specify priority of interrupts

8051 Microcontroller Sukesh Rao Muligar, Asst. Prof. - ECE 85

 Enabling and Disabling an Interrupt
• Upon reset, all interrupts are disabled (masked),
meaning that none will be responded to by the
microcontroller if they are activated.

• The interrupts must be enabled by software in order for

the microcontroller to respond to them.

• There is a register called IE (interrupt enable) that is

responsible for enabling (unmasking) and disabling
(masking) the interrupts.

8051 Microcontroller Sukesh Rao Muligar, Asst. Prof. - ECE 86

 Interrupt Enable (IE) Register

--

• EA : Global enable/disable.
• --- : Reserved for additional interrupt hardware.

MOV IE,#08h • ES : Enable Serial port interrupt.

or
SETB ET1
• ET1 : Enable Timer 1 control bit.
• EX1 : Enable External 1 interrupt.
• ET0 : Enable Timer 0 control bit.
• EX0 : Enable External 0 interrupt.
8051 Microcontroller Sukesh Rao Muligar, Asst. Prof. - ECE 87
 Enabling and Disabling an Interrupt
• Example: Show the instructions to (a) enable the serial interrupt,
timer 0 interrupt, and external hardware interrupt 1 and (b)
disable (mask) the timer 0 interrupt, then (c) show how to disable
all the interrupts with a single instruction.
• Solution:
– (a) MOV IE,#10010110B ;enable serial, timer 0, EX1
• Another way to perform the same manipulation is:
– SETB IE.7 ;EA=1, global enable
– SETB IE.4 ;enable serial interrupt
– SETB IE.1 ;enable Timer 0 interrupt
– SETB IE.2 ;enable EX1
– (b) CLR IE.1 ;mask (disable) timer 0 interrupt only
– (c) CLR IE.7 ;disable all interrupts
8051 Microcontroller Sukesh Rao Muligar, Asst. Prof. - ECE 88
 Interrupt Priority
• When the 8051 is powered up, the priorities are assigned according
to the following.

• In reality, the priority scheme is nothing but an internal polling

sequence in which the 8051 polls the interrupts in the sequence
listed and responds accordingly.

8051 Microcontroller Sukesh Rao Muligar, Asst. Prof. - ECE 89

 Interrupt Priority
• We can alter the sequence of interrupt priority by assigning a
higher priority to any one of the interrupts by programming a
register called IP (interrupt priority).
• To give a higher priority to any of the interrupts, we make the
corresponding bit in the IP register high.

8051 Microcontroller Sukesh Rao Muligar, Asst. Prof. - ECE 90

 Interrupt Priority (IP) Register

Reserved PS PT1 PX1 PT0 PX0

Serial Port
INT 0 Pin
Timer 1 Pin

INT 1 Pin Timer 0 Pin

Priority bit=1 assigns high priority

Priority bit=0 assigns low priority
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 8051 Software Overview
1. Addressing Modes
2. Instruction Set
3. Programming

8051 Microcontroller Sukesh Rao Muligar, Asst. Prof. - ECE 92

 8051 Addressing Modes
• The CPU can access data in various ways, which are
called addressing modes

1. Immediate
2. Register
3. Direct
4. Register indirect
5. External Direct

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 Immediate Addressing Mode
• The source operand is a constant.
• The immediate data must be preceded by the pound sign, “#”
• Can load information into any registers, including 16-bit DPTR
register
– DPTR can also be accessed as two 8-bit registers, the high byte DPH and
low byte DPL

8051 Microcontroller Sukesh Rao Muligar, Asst. Prof. - ECE 94

 Register Addressing Mode
• Use registers to hold the data to be manipulated.

• The source and destination registers must match in size.

MOV DPTR,A will give an error

• The movement of data between Rn registers is not allowed

MOV R4,R7 is invalid
8051 Microcontroller Sukesh Rao Muligar, Asst. Prof. - ECE 95
 Direct Addressing Mode
• It is most often used the direct addressing mode to
access RAM locations 30 – 7FH.

• The entire 128 bytes of RAM can be accessed.

• Contrast this with immediate addressing mode, there is

no “#” sign in the operand.

8051 Microcontroller Sukesh Rao Muligar, Asst. Prof. - ECE 96

 SFR Registers & their Addresses
MOV 0E0H,#55H ;is the same as
MOV A,#55H ;which means load 55H into A (A=55H)

MOV 0F0H,#25H ;is the same as

MOV B,#25H ;which means load 25H into B (B=25H)

MOV 0E0H,R2 ;is the same as

MOV A,R2 ;which means copy R2 into A

MOV 0F0H,R0 ;is the same as

MOV B,R0 ;which means copy R0 into B

8051 Microcontroller Sukesh Rao Muligar, Asst. Prof. - ECE 97

 SFR Addresses ( 1 of 2 )

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 SFR Addresses ( 2 of 2 )

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 Example

8051 Microcontroller Sukesh Rao Muligar, Asst. Prof. - ECE 100

 Stack and Direct Addressing Mode
• Only direct addressing mode is allowed for pushing or
popping the stack.

• PUSH A is invalid.

• Pushing the accumulator onto the stack must be coded

as PUSH 0E0H.

8051 Microcontroller Sukesh Rao Muligar, Asst. Prof. - ECE 101

 Register Indirect Addressing Mode
• A register is used as a pointer to the data.
• Only register R0 and R1 are used for this purpose.
• R2 – R7 cannot be used to hold the address of an
operand located in RAM.
• When R0 and R1 hold the addresses of RAM locations,
they must be preceded by the “@” sign.

8051 Microcontroller Sukesh Rao Muligar, Asst. Prof. - ECE 102

 Register Indirect Addressing Mode
• Write a program to copy the value 55H into RAM memory locations 40H
to 41H using (a) direct addressing mode, (b) register indirect addressing
mode without a loop, and (c) with a loop.

8051 Microcontroller Sukesh Rao Muligar, Asst. Prof. - ECE 103

 Register Indirect Addressing Mode
• The advantage is that it makes accessing data dynamic
rather than static as in direct addressing mode.

• Looping is not possible in direct addressing mode.

• Write a program to clear 16 RAM locations starting at

RAM address 60H.

8051 Microcontroller Sukesh Rao Muligar, Asst. Prof. - ECE 104

 External Direct
• External Memory is accessed.

• There are only two commands that use External Direct

addressing mode:
– MOVX A, @DPTR
MOVX @DPTR, A

• DPTR must first be loaded with the address of external

memory.

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 8051 Instruction Set

8051 Microcontroller Sukesh Rao Muligar, Asst. Prof. - ECE 106

 MOV Instruction
• MOV destination, source ; copy source to destination.

• MOV A,#55H ;load value 55H into reg. A

MOV R0,A ;copy contents of A into R0
;(now A=R0=55H)
MOV R1,A ;copy contents of A into R1
;(now A=R0=R1=55H)
MOV R2,A ;copy contents of A into R2
;(now A=R0=R1=R2=55H)
MOV R3,#95H ;load value 95H into R3
;(now R3=95H)
MOV A,R3 ;copy contents of R3 into A
;now A=R3=95H

8051 Microcontroller Sukesh Rao Muligar, Asst. Prof. - ECE 107

 ADD Instruction
• ADD A, source ;ADD the source operand to the
accumulator

• MOV A, #25H ;load 25H into A

MOV R2,#34H ;load 34H into R2
ADD A,R2 ;add R2 to accumulator
;(A = A + R2)

8051 Microcontroller Sukesh Rao Muligar, Asst. Prof. - ECE 108

 Structure of Assembly Language
ORG 0H ;start (origin) at location 0
MOV R5,#25H ;load 25H into R5
MOV R7,#34H ;load 34H into R7
MOV A,#0 ;load 0 into A
ADD A,R5 ;add contents of R5 to A
;now A = A + R5
ADD A,R7 ;add contents of R7 to A
;now A = A + R7
ADD A,#12H ;add to A value 12H
;now A = A + 12H
HERE: SJMP HERE ;stay in this loop
END ;end of asm source file

8051 Microcontroller Sukesh Rao Muligar, Asst. Prof. - ECE 109

 Data Types & Directives

ORG 500H
DATA1: DB 28 ;DECIMAL (1C in Hex)
DATA2: DB 00110101B ;BINARY (35 in Hex)
DATA3: DB 39H ;HEX
ORG 510H
DATA4: DB “2591” ; ASCII NUMBERS
ORG 518H
DATA6: DB “My name is Joe” ;ASCII CHARACTERS

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 ADD Instruction and PSW

8051 Microcontroller Sukesh Rao Muligar, Asst. Prof. - ECE 111

 ADD Instruction and PSW

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 Multiplication of Unsigned Numbers
MUL AB ; A  B, place 16-bit result in B and A

MOV A,#25H ;load 25H to reg. A

MOV B,#65H ;load 65H in reg. B
MUL AB ;25H * 65H = E99 where B = 0EH and A = 99H

Table 6-1:Unsigned Multiplication Summary (MUL AB)

Multiplication Operand 1 Operand 2 Result

byte  byte A B A=low byte,

B=high byte

8051 Microcontroller Sukesh Rao Muligar, Asst. Prof. - ECE 113

 Division of Unsigned Numbers
DIV AB ; divide A by B

• MOV A,#95H ;load 95 into A

• MOV B,#10H ;load 10 into B
• DIV AB ;now A = 09 (quotient) and B = 05 (remainder)

Table 6-2:Unsigned Division Summary (DIV AB)

Division Numerator Denominator Quotient Remainder
byte / byte A B A B

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 Checking an input bit
JNB (jump if no bit) ; JB (jump if bit = 1)

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 Switch Register Banks

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 Pushing onto Stack

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 Popping from Stack

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 Looping

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 Loop inside a Loop (Nested Loop)

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 8051 Conditional Jump Instructions

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 Conditional Jump Example

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 Conditional Jump Example

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 Unconditional Jump Instructions
• All conditional jumps are short jumps
– Target address within -128 to +127 of PC

• LJMP (long jump): 3-byte instruction

– 2-byte target address: 0000 to FFFFH
– Original 8051 has only 4KB on-chip ROM

• SJMP (short jump): 2-byte instruction

– 1-byte relative address: -128 to +127

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 Call Instructions

• LCALL (long call): 3-byte instruction

– 2-byte address
– Target address within 64K-byte range

• ACALL (absolute call): 2-byte instruction

– 11-bit address
– Target address within 2K-byte range

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