MPI UNIT 5 - (INTERRUPTS OF 8086, INTRODUCTION TO 8051).pptx

Prepared by M.V.Bhuvaneswari, Asst.Prof, CSE,MVGRA
INTERRUPTS OF 8086,
INTRODUCTION TO 8051
UNIT 5

Introduction to 8051

● To make a complete microcomputer system, only microprocessor is not
sufficient.
● It is necessary to add other peripherals such as ROM, read/write memory
(RAM), decoders, drivers, number of input/output devices to make a complete
microcomputer system.
● In addition special purpose devices such as interrupt controller,
programmable timers, programmable i/o devices, DMA controllers may be
added to improve the capability, performance and flexibility of a
microcomputer system.
● The key feature of microprocessor based system is that is possible to design
a system with great flexibility and possible to configure a system as large or
small system by adding suitable peripherals.

● On the other hand the microcontroller incorporates all the features that are
found in microprocessor.
● However it has also added features to make a complete microcomputer
system on its own.
● The microcontroller has built-in ROM, RAM, parallel I/O, serial I/O, counters
and a clock circuit.
● The microcontroller has on-chip (built in) peripheral devices that make
possible to have single chip microcomputer system.

Difference between
Microcontroller
& Microprocessor

8051 Features
● The 8051 is an 8-bit microcontroller designed by Intel.
● It was optimized for 8-bit math and single bit Boolean operations.
● Its family MCS-51 includes 8031, 8051 and 8751 microcontrollers.
● The 8051 is an 8-bit microcontroller with 8 bit data bus and 16-bit address bus.
● The 16 bit address bus can address a 64K( 216) byte code memory space and
a separate 64K byte of data memory space.
● The 8051 has 4K on-chip read only code memory and 128 bytes of internal
Random Access Memory(RAM)

8051 Features
● Besides internal RAM, the 8051 has various Special Function Registers (SFR)
such as the Accumulator, the B register, and many other control registers.
● 34 8-bit general purpose registers in total.
● The ALU performs one 8-bit operation at a time.
● Two 16 bit /Counter timers
● 3 internal interrupts (one serial), 2 external interrupts.
● Four 8-bit I/O ports
● Some 8051 chips come with UART for serial communication and ADC for analog
to digital conversion.

Pin Diagram

● Pin 1 to Pin 8 (Port – 1) – Pin 1 to Pin 8 is assigned to Port 1 for simple I/O
operations. These ports are work as a bidirectional port. It means all the pins of port 1 work
as a input pin or output pins. If Logic 1 (one) is applied to the I/O port it will act as a input pin
and if logic 0 (zero) is applied to the I/O port it will act as a output pin.
● Pin 9 (RST) – It is a reset input Pin, which is used to reset the 8051 microcontrollers to its
initial values when logic 1 is applied to this pin. It is active high pin.
● Pin 10 to Pin 17 (Port-3) – Pin 10 to Pin 17 are assigned to Port 3. This port is also a
bidirectional I/O port like port 1. This port performs some special functions like interrupts,
control signals, timer input, serial communication etc. These are explained in detail as below.

● P 10 (RXD) – Pin 10 is used as a RXD (serial data receive pin) which is for serial input pin.
By using this input signal microcontroller receives data for serial communication.
● P 11 (TXD) – Pin 11 is used as a TXD (serial data transmit pin) which is serial output pin. By
using this output signal microcontroller transmits data for serial communication.
● P 12 and P 13 (INT0′, INT1′ ) – Pins12 and 13 are used for External Hardware Interrupt 0
and Interrupt 1 respectively. When this interrupt is activated (i.e. when it is low), 8051 gets
interrupted means it stopped whatever it is doing and jumps to the vector table where ISR’s
(Interrupt Service Routine) are stored and starts performing Interrupt Service Routine (ISR)
from that vector location.

● P 14 and P 15 (T0 and T1) – Pin 14 and 15 are used for Timer 0 and Timer 1
external input. They can be connected with 16-bit timer/counter.
● P 16 (WR’) – Pin 16 is used for external memory write i.e. writing data to the
external memory.
● P 17 (RD’) – Pin 17 is used for external memory read i.e. reading data from
external memory.
● Pin 18 and Pin 19 (XTAL2 And XTAL1) –Pins 18 and 19 i.e. XTAL 2 and XTAL 1
are the pins for interfacing external oscillator. Mostly, a Quartz Crystal Oscillator is
connected here to get the system clock.

● Pin 20 (GND) – Pin 20 is the Ground Pin. It is connected to the 0V (negative terminal) of the
Power Supply.
● Pin 21 to Pin28 (Port 2) – Pin 21 to pin 28 are Port 2 pins. Port 2 is also a bidirectional Input
/Output port i.e. all pins of port 2 work as a input pin or as a output pins. But, this is only
possible when we are not using any external memory. If we use external memory, then these
pins will work as high order address bus (A8 to A15).
● Pin 29 (PSEN) – The Pin 29 is the Program Strobe Enable Pin (PSEN). It is used to enable
external program memory and read a signal from the external program memory.
● Pin 30 (ALE) – Pin 30 is the Address Latch Enable Pin. This pin is used to enable or disable
the external memory interfacing.

● Pin 31 (EA) – Pin 31 is the External Access Enable (EA) Pin. This pin allows external
Program Memory. It is an input pin and connected from VCC or GND. If we want to access
the program from external program memory, it must be connected with GND. If we want to
use on-chip memory, it must be high (connected with VCC).
● Pin 32 to Pin 39 (Port 0) – Pin 32 to Pin 39 are Port 0 pins. When we don’t use any external
memory, these pins are used as a bidirectional pin like port 2 and port 3. But, when ALE or
Pin 30 is at 1, then this port is used as data bus. And when the ALE pin is at 0, then this port
is used as a lower order address bus (A0 to A7).
● Pin 40 (VCC) – This pin is used to provide (+5v ) power supply to the 8051 microcontroller
circuit.

8051 Microcontroller Basic Circuit

Architecture of 8051

Functional Units:
● Central Processing Unit (CPU)
● Memory – 128bytes RAM & 4kB ROM
● I/O ports
● Special Function Registers
● Serial Port
Central processing Unit:
● ALU
● Registers – A,B, PSW, temporary registers
● 16 bit program counter (PC)
● Data pointer (DPTR) & Stack pointer (SP)

Arithmetic Logic Unit (ALU):
● It can perform 8-bit arithmetic operations
● It can also manipulate one bit and 8bit logical operations.
Accumulator (Register A):
● It is an 8-bit register
● It is used to hold the source operand and also receives the result of the function.
Register B:
● It is an 8bit register used as general purpose register
Special Function Register (SFR):
● In 8051, access to the I/O ports, CPU registers, Timer/counter, Logic control
registers can be done through the SFRs (located from 80H to FFH).

Program Status Word Register (PSW):
CY AC F0 RS1 RS0 OV - P
B7 B6 B5 B4 B3 B2 B1 B0
Carry Flag Auxiliary
Carry
flag
Flag 0 Register Bank
Select Signals
Overflow Flag
Parity Flag

Data Pointer (DPTR):
● It is a 16bit register used to hold a 16 bit address.
● It is used as a base register in JUMP/Lookup table instructions and external
data transmission.
Program Counter (PC):
● It is a 16 bit register used to hold the address of memory location from which
the next instruction is to be fetched.
DPH DPL
16 bit DTPR
8 bit 8 bit

System Bus:
● The system bus consists of an 8-bit data bus, 16-bit address bus and control
signals.
● It is used to connect all the internal devices to the CPU.
I/O ports:
● 8051 has 32 bidirectional I/O pins which are organized as four parallel 8-bit
I/O ports.
Port 0 : I/O ports & multiplexed AD0 – AD7
Port 1 : Only for I/O
Port 2 : I/O ports & Higher – order Address A8- A15
Port 3 : I/O ports & multifunctional

Serial Port:
● The serial port of 8051 is full-duplex i.e. it can transmit and receive from/to an
external device.
● Used for serial communication
Memory:
● For operation Micro-controller required a program. This program guides the
microcontroller to perform the specific tasks. This program installed in
microcontroller required some on chip memory for the storage of the program.
● Microcontroller also required memory for storage of data and operands for the
short duration.
● In microcontroller 8051 there is code or program memory of 4 KB that is it has
4 KB ROM and it also comprise of data memory (RAM) of 128 bytes.

Interrupt:
● 8051 provides 5 interrupt sources
● External hardware interrupts : INT0, INT1
● External Timer Interrupts : TF0, TF1
● Serial Communication Interrupt : RI/ TI
Oscillator:
● A clock signal allows the operations inside the microcontroller and other parts
to be synchronous.
● A Clock Generator is an integral part of the Microcontroller’s Architecture and
the user has to provide an additional Timing Circuit in the form of a Crystal.

8051 Memory Organization

● The 8051 microcontroller's memory is divided into Program Memory and Data
Memory. Program Memory (ROM) is used for permanent saving program being
executed, while Data Memory (RAM) is used for temporarily storing and keeping
intermediate results and variables.
1.Program memory: (4kB ROM)
● Program memory accessed through EA pin. In program memory two categories
takes place:
a)If EA is high, internal program memory is accessed up to 0FFFH memory
location and external program memory accessed from 1000H to FFFFH memory
locations.
b)If EA is low, only external program memory accessed from 0000H to FFFFH
memory locations.

2.Data memory:
● Data memory is used to store the memory in the registers each of 64k bytes size,
to access the data memory instruction MOVX is used. Data memory is of two
types Internal and external.
i) Internal data memory:
● The internal data memory consists of 256 bytes, these are divided into two parts:
a) 00H-7FH for internal data RAM (lower 128 bytes)
b) 80H-FFH for special function registers (upper 128 bytes)
ii) External data memory:
● The 8051 gives the facility to interface external RAM and ROM. External RAM is
accessed by DPTR and up to 64KB of RAM can be interfaced. External data
memory interfacing is of two types i.e. RAM and ROM interfacing.

Data Memory (RAM) of 8051 Microcontroller
● In 8051 we have 128bytes of RAM.
● Address locations = 00H to 7FH
● These 128 bytes are divided into 3 different groups
128B i.e., memory addresses from 00H to 7FH is divided into Working
Registers (organized as Register Banks), Bit – Addressable Area and
General Purpose RAM (also known as Scratchpad area).

● In the 128B of RAM (from 00H to 7FH), the first 32B i.e., memory from
addresses 00H to 1FH consists of 32 Working Registers that are organized
as four banks with 8 Registers in each Bank.
● The 4 banks are named as Bank0, Bank1, Bank2 and Bank3. Each Bank
consists of 8 registers named as R0 – R7. Each Register can be addressed in
two ways: either by name or by address.
● To address the register by name, first the corresponding Bank must be
selected. In order to select the bank, we have to use the RS0 and RS1 bits of
the Program Status Word (PSW) Register (RS0 and RS1 are 3rd and 4th bits
in the PSW Register).

Question: If R7 = 5 i.e 5 is written or stored in R7. In that case where 5 will
get stored when all the banks have registers named R0- R7.
The answer is only one register bank will be active at a time. Lets say
Register Bank 1 is active and if any value is written onto R7 register then it
will get stored in the location 0FH of Bank 1

● The next 16B of the RAM i.e., from 20H to 2FH are Bit – Addressable
memory locations.
● These locations can be bit and byte addressable
● There are totally 128 bits that can be addressed individually using 00H to 7FH
or the entire byte can be addressed as 20H to 2FH.
● For example 32H represents the bit 2 of the internal RAM location 26H.
Ex: MOV A, #03H copy byte 03 to register A
MOV 26H, A copy data byte from register A to memory location 26H
SETB 32H  set to 1
CLR 32H  set to 0

● The final 80B of the internal RAM i.e., addresses from 30H to 7FH, is the
general purpose RAM area which are byte addressable.
● It is known as Read/Write Storage or scratch pad
● Used for read and write storage of data
● If we need more registers we simply use RAM locations 30H to 7FH

Program Memory (ROM) of 8051 Microcontroller
● In 8051 Microcontroller, the code or instructions to be executed are stored in the Program
Memory, which is also called as the ROM of the Microcontroller. The original 8051
Microcontroller by Intel has 4KB of internal ROM.
● In case of 4KB of Internal ROM, the address space is 0000H to 0FFFH. If the address space
i.e., the program addresses exceed this value, then the CPU will automatically fetch the code
from the external Program Memory.
● For this, the External Access Pin (EA Pin) must be pulled HIGH i.e., when the EA Pin is high,
the CPU first fetches instructions from the Internal Program Memory in the address range of
0000H to 0FFFFH.
● And if the memory addresses exceed the limit, then the instructions are fetched from the
external ROM in the address range of 1000H to FFFFH.

Register Set of 8051

Accumulator (Register A):
● It is an 8-bit register
● It is used to hold the source operand and also receives the result of the
function.
Register B:
● It is an 8bit register used as general purpose register

Stack pointer register:
● The stack refers to an area of internal RAM that is used in conjunction with certain
opcode data to store and retrieve data quickly.
● The stack pointer register is used by the 8051 to hold the internal RAM address
that is called the top of the stack.
● The stack pointer register is 8-bit wide.
● The stack array can reside anywhere in on-chip RAM
Special Function registers:
● 8051 uses memory mapped I/O through a set of SFRs that are implemented in
the address space immediately above the 128bytes of RAM.
● All access to four I/O ports, CPU registers, interrupt-control registers,
timer/counter, UART and power control are performed through registers between
80H and FFH.

Timers & Counters

● 8051 has two timers/counters circuits which can be used either as timers or
counters.
Timer:
● To generate the time delay.
● To maintain the timing of an operation in sync with the clock.
Counter:
● To count the number of events happening outside the controller.
● Each counter may be programmed to count internal clock pulses i.e. acting as
timer or programmed to count external pulses as counter.

8051 Timers
● 8051 has two 16 bit timer registers
a) Timer 0
b) Timer 1
TL – low byte register
TH – high byte register
TH0 TL0 TH1 TL1
16 bit register 16 bit register
8bit 8bit 8bit 8bit

Registers Used in Timers/Counters
TMOD [Timer Mode] Register:
TMOD is the 8-bit special function register which is responsible for following operations.
● Select Timer 0 as a timer/counter
● Select Timer 1 as a timer/counter
● Select the mode of the timer
GATE C/T1 M1 M0 GATE C/T1 M1 M0
MSB 7 LSB 0
B6 B5 B4 B3 B2 B1
Timer 1
Timer 0

● M0 and M1 bits are used to select timer mode.
M1 M0 Operating Mode
0 0 Mode 0: 8-bit timer /counter with 5-bit prescalar
0 1
Mode1: 16–bit timer /counter
THx & TLx are cascaded
1 0 Mode 2: 8-bit auto reload timer or counter
1 1
Split timer mode: The function depends on Timer 0 and
Timer 1

C/T1
● C/T1 = 0 for selecting timer operation
● C/T1 = 1 for selecting counter operation
GATE (G): starting and stopping of timers/counters
● If G = 1 , Timer0 or Timer1 will be in Run Mode when TRX bit of TCON
register is high.
● If G= 0, Timer0 or Timer1 will be in Run Mode when TRX bit of TCON register
is high and INT0 or INT1 is high.

Timer Modes
Mode 0 : 0000 – 1FFF
● It is a 13-bit timer/ counter (8 bit timer/counter + 5 bit prescalar)
● It uses 5 bits of TL0 or TL1 and all of the 8-bits of TH0 or TH1.

Mode 1: 0000- FFFFH
● It is a 16-bit timer/counter.
● In this case every event for counter operations or machine cycles for timer
operation, the TH0– TL0 register-pair will be incremented by 1. When the
register pair overflows from FFFFH to 0000H, then the TF0 of TCON register
will be high, and it stops the timer/counter.

Mode 2: 00 – FFH
● This a 8 bit auto reload timer/counter
● In this case every event for counter operations or machine cycles for timer operation,
the TL1register will be incremented by 1. When the register pair overflows from FFH to
00H, then the TF1 of TCON register will be high, also theTL1 will be reloaded with the
content of TH1 and starts the operation again.

Mode 3: Split Timer mode
● Mode 3 is different for Timer0 and Timer1.
1) When the Timer0 is working in mode 3, the TL0 will be used as an 8-bit timer/counter. It will be
controlled by the standard Timer0 control bits, T0 and INT0 inputs.
The TH0 is used as an 8-bit timer but not the counter. This is controlled by Timer1 Control bit TR1.
When the TH0 overflows from FFH to 00H, then TF1 is set to 1. In the following diagram, we can
Timer0 in Mode 3.
2) When the Timer1 is working in Mode 3, it simply holds the count but does not run.

TCON [Timer Control] Register:
It is an 8-bit special function register used to control the following operations:
● Start and stop the Timer x (Timer 0 and Timer 1)
● To provide the status of external interrupts: INT0 & INT1
● To provide the status of overflows TF0 TF1
● To configure the interrupts either as level- triggered or edge- triggered
TF1 TR1 TF0 TR0 IE1 IT1 IE0 IT0
MSB7 B6 B5 B4 B3 B2 B1 LSB0

● TF1 – Timer 1 overflow flag
● TR1 – Timer 1 Run control bit
● TF0 – Timer 0 overflow flag
● TR0 – Timer 0 Run control bit
● IE1 – Interrupt 1 edge flag
● IT1 – Interrupt 1 type control bit
● IE0 – Interrupt 0 edge flag
● IT 0 – Interrupt 0 type control bit
Start and Stop of timers
Control external h/w interrupts

Timer/Counter Control Logic
Run Control
1= Run
0= stop

● Crystal oscillator is divided by 12 to get the operating frequency for the timer.
● If C/T = 0, it act as timer and C/T = 1 act as counter .
● We can control the timer operations by using the signals from AND gate,
where one input bit is from TR bit of TCON.
● The other input is given by the OR gate, for this OR gate there are again two
inputs given : One is from GATE bit of TMOD and the other is external
hardware interrupt INT x from TCON register.

Serial communication
Microcontroller
Peripheral
devices
Send
Receive
8051
8-bit parallel
device
Parallel to serial
Serial
Serial to parallel
Communication

Types of Serial Communication
Synchronous Asynchronous
Transmitter and receiver are
synchronized
Not synchronized
Share a common clock Separate clock
First sync character is sent and then
data
Start bit, Data , Stop bit
Block of data is transmitted
/received at a time.
Single bit is transmitted/received at
a time.

Serial Port of 8051
● The serial port of 8051 is full duplex i.e. it can transmit and receive the data
simultaneously.
● Pins used for serial communication : TxD (transmit data) & RxD (receive data)
in port 3
● Registers used in serial communication:
1) SBUF Register (serial data buffer)
2) SCON Register (serial control)
3) PCON Register (power control)

SBUF Register [Serial Data Buffer]
● SBUF is an 8-bit register is used to hold data during serial communication.
● Separate SBUF registers for transmit and receive of data.
During Transmission, SBUF register is used to
● Store the data to be transmitted via TxD pin (P3.1)
● Frame the data with start and stop bits
● Transmit the data serially through transmitted pin
During Reception
● When 8051 receives data serially via RxD pin, the start and stop bits are
separated from data in SBUF register.

SCON Register [Serial control Register]
● SCON register is an 8-bit register used to select the programmable mode of
8051.
● SM0 and SM1 are used for mode selection of SCON
SM0 SM1 SM2 REN TB8 RB8 TI RI
MSB7 LSB0
B6 B5 B4 B3 B2 B1

SM0 SM1 Programmable Mode
0 0 Mode 0: Shift register – Baude rate f/12
0 1 Mode 1: 8-bit UART – variable data rate
1 0 Mode 2: 9-bit UART – fixed data rate
1 1 Mode 3: 9-bit UART – variable data rate

SM2: Serial mode control bit
● Multiprocessor communication bit
● It is set by software to disable reception of frame
● In mode 2 & 3, if SM2 is set, will enable multiprocessor communication.
REN: Receiver enable control bit
● To enable/ disable serial data reception
TB8: Transmit bit 8
● It is set/cleared by hardware to determine state of 9th bit transmitted in 9bit UART mode.
RB8: Receive bit 8
● It is set/cleared by hardware to indicate the state of 9th bit received in mode 2 & 3
TI : Transmit interrupt flag RI: Receive interrupt flag

PCON Register [Power control Register]
● It is used for power control in 8051 microcontroller.
SMOD: Serial Bauderate Modify Bit
● To select baude rate
● SMOD = 0 at reset, baude rate equal to timer 1 rate
● SMOD = 1 by the program to double the bauderate using timer 1 in mode 1,2 &3
● Bit 7 is only used for serial communication
SMOD - - - GF1 GF0 PD IDL
MSB7 B6 B5 B4 B3 B2 B1 LSB0

● Bits 6,4,5 are reserved for future use
GF1: General purpose user flag bit 1
GF0: General purpose user flag bit 0
PD0: Power down bit
● To enter Power Down Mode, set to 1
● In the Power Down Mode, the oscillator will be stopped and the power will be reduced to 2V.
● To terminate the Power Down Mode, you have to use the Hardware Reset.
IDL: Idle mode bit
● It is set to 1 by program to enter idle mode configuration.
● To enter Idle Down Mode, set to 1
● During Idle Mode, the Microcontroller will stop the Clock Signal to the ALU (CPU) but it is
given to other peripherals like Timer, Serial, Interrupts, etc.

Operating Modes of Serial port
Mode 0: synchronous serial communication
● It uses 8-bit data
● In this mode serial data enters and exits through RxD and TxD is used for
clock signals.
● The Bauderate is fixed at 1/12th the oscillator frequency
Mode1: 8-bit UART
● In this mode 10bits are transmitted through TxD and received through RxD.
● SBUF is used to store the data
● TI/RI are set once transmission/reception is completed

● Baude rate is variable = [2SMOD /32] * Timer 1 overflow flag
Mode 2:
● In this mode 11 bits are transmitted through TxD and received through RxD
● The Bauderate is programmable to either 1/32 or 1/64 of the oscillator
frequency.
Start bit (1) 8-bit data Stop bit (1)
Start bit (1) 8-bit data Programmable
9th data bit
Stop bit (1)

Mode 3:
● It is similar to mode 2
● 11 bits are transmitted/received
● The Bauderate is variable.

Interrupts in 8051

● Interrupt is an event or signal that stops the main program temporarily and transfers the
control to an external task.
● After executing the task, the control returns to its main program.
Interrupt Sources in8051:
8051 has 5 interrupt sources
● Two external hardware interrupts
1) INT0 – External hardware interrupt 0
2) INT1 – External hardware interrupt 1
● Two timer overflow interrupts
1) TF0 – Timer 0 overflow interrupt
2) TF1 - Timer 1 overflow interrupt
● Serial Communication interrupt
1) RI/TI – Receiver/Transmit Interrupt

● The Timer and Serial interrupts are internally generated by the microcontroller, whereas
the external interrupts are generated by additional interfacing devices or switches that
are externally connected to the microcontroller.
● When an interrupt occurs, the microcontroller executes the interrupt service routine so
that memory location corresponds to the interrupt that enables it. The Interrupt
corresponding to the memory location is given in the interrupt vector table below.

Interrupt Structure of 8051 Micro controller
Registers used in Interrupt process:
1) IE (Interrupt Enable) Register
2) IP (Interrupt Priority) Register
3) TCON (Timer Control) Register
● Upon ‘RESET’ all the interrupts get disabled, and therefore, all these interrupts
must be enabled by a software. In all these five interrupts, if anyone or all are
activated, this sets the corresponding interrupt flags.
● All these interrupts can be set or cleared by bit in some special function register
that is Interrupt Enabled (IE), and this in turn depends on the priority, which is
executed by IP interrupt priority register.

Interrupt Enable Register:
● It is a bit addressable register which is responsible for enabling or disabling
the interrupts.
EA - - ES ET1 EX1 ET0 EX0
MSB7 LSB0
B6 B5 B4 B3 B2 B1

Interrupt Priority Register:
● The priority levels of the interrupts can be changed by changing the
corresponding bit in the IP register.

Timer Control Register:
● TCON register is used to specify the type of external interrupt to 8051
microcontroller.

MPI UNIT 5 - (INTERRUPTS OF 8086, INTRODUCTION TO 8051).pptx

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