Interrupts of microcontroller 8051

Interrupts of 8051 Microcontroller
Dr. Nilesh Bhaskarrao Bahadure
https://www.sites.google.com/site/nileshbbahadure/home
July 25, 2021
Dr. Nilesh Bhaskarrao Bahadure () Unit - II (Part II) July 25, 2021 1 / 54

Overview I
1 What is Interrupt
Introduction to 8051 Microcontroller Interrupts
2 Interrupts of 8051 Microcontroller
3 Interrupt Vs Polling
4 IE register
5 IP register
6 What happens when an interrupt occurs?
7 What happens when an interrupt Ends?
8 Programming Timer Interrupt
9 Serial Interrupt
10 External Hardware Interrupt
Level Triggered Interrupt
Edge Triggered Interrupt
TCON Register
11 Examples
Example - 1: Example on IE

Overview II
Example - 2: Example on IP
Example - 3: Example on IP
Example - 4: Generation of Square Wave Using Timer 0 Mode 2
Example - 5: Generation of Square Wave Using Timer 0 Mode 1
Example - 6: Generation of Square Wave of 50 Hz Using Timer 0
Example - 7: Transmission of bits serially using Interrupt
Example - 8: Reception of bits serially using Interrupt
Example - 9: LED ON using External Hardware Interrupt
Example - 10: LED ON using External Hardware Interrupt and Pulse
generator
Example - 11: Transfer - Receive & Square Wave using Interrupt

What is Interrupt
Interrupt is the routine program to break or stop the current execution and
branch to another program. Interrupt of microcontroller 8051 consists two
hardware interrupts as well as generated interrupts using timers and serial
communication.
Main Slide

Introduction to 8051 Microcontroller Interrupts
Interrupts are signals that cause the Microcontroller to suspend what is doing
and transfer to a special program called an interrupt handler. The interrupt
handler is responsible for determining the cause of interrupt, servicing the
interrupt and returning the control to the point from where the interrupt
was caused.
Main Slide

Introduction to 8051 Microcontroller Interrupts...
When Microcontroller executes any program then it is called Main pro-
gram. In between the main program, Microcontroller is interrupted then
Microcontroller will branch from main program to subprogram this subpro-
gram is called Interrupt service routine (ISR). After executing this ISR then
RETI/RET instruction comes then Microcontroller returns back from ISR
to the main program from where Microcontroller has left.
Main Slide

Interrupts of 8051 Microcontroller
Question
What events can trigger interrupts, and where do they go?
We can configure the 8051 so that any of the following events will cause
an interrupt:
1 Timer 0 Overflow.
2 Timer 1 Overflow.
3 Reception/Transmission of Serial Character.
4 External Event 0.
5 External Event 1.
Main Slide

Interrupts of 8051 Microcontroller...
The six interrupts in the 8051 are allocated as follows:
1 RESET: When the reset pin is activated, the Microcontroller 8051
jumps to the address location 0000h. This is also referred as power
on reset.
2 Timers: Two interrupts are set aside for the timers: Timer 0 and
Timer 1. Memory location addresses 000Bh and 001Bh in the
interrupt vector table belongs to timer 0 and timer 1, respectively.
3 Hardware external interrupts: Two interrupts are set aside for
hardware external interrupts. Pin number 12 (P3.2) and 13 (P3.3)
are for the external hardware interrupts also referred as EX 1 and EX
2. Memory location addresses 0003h and 0013h in the interrupt
vector table are assigned to INT0 and INT1 external hardware
interrupts respectively.
4 Serial communication: serial communication has a single interrupt
that belongs to both transmit and receive. The interrupt vector table
location 0023h belongs to the serial communication interrupt.
Main Slide

Interrupts of 8051 Microcontroller...
Table : Interrupts with their handler addresses
Interrupt Flag Interrupt handler address/Vector location
External 0 IE0 0003h
Timer 0 TF0 000Bh
External 1 IE1 0013h
Timer 1 TF1 001Bh
Serial RI/TI 0023h
Main Slide

Interrupt Vs Polling
A single Microcontroller can serve several devices. There are two ways to
do that: Interrupts and polling.
Interrupts:
In the interrupt method, whenever any device needs its service, the device
notifies the Microcontroller by sending an interrupt signal, upon receiving
an interrupt signal, the Microcontroller interrupts whatever it is doing and
serve the device. The program associated with the interrupt is called the
interrupt service routine (ISR) or interrupt handler.
Polling:
In polling, the Microcontroller continuously monitors the status of a several
devices, when the status condition is met, it perform the service, after that,
it moves on to monitor the next device until each one is serviced. Although
polling can monitors the status of several devices and serve each of them as
certain conditions are met, it is not an efficient use of Microcontroller.
Main Slide

Interrupt Vs Polling...
Figure : Polling method to access several devices
Main Slide

Interrupt Vs Polling...
The advantage of the interrupts is that the Microcontroller can serve many
devices, not all at the same time; each device can get the attention of the
Microcontroller, based on the priority assigned to it. The polling method
cannot assign priority since it checks all the devices in a round robin fashion
as shown in the figure 1. Also in the interrupt method, the Microcontroller
can also ignore or mask or reset a device request for service. This is not
possible in polling.
The most important reason that the interrupt method is preferable over
the polling is that the polling method waste the time of Microcontroller in
order to search the devices one by one, so in order to avoid tying down the
Microcontroller, interrupts are used.
For example, the instruction LABEL: JNB TFx, LABEL, waited until the
timer rollover and while we were waiting we could not do anything else
i.e. a waste of this Microcontroller’s time that could have been used to
perform some other useful tasks. In this case, the Microcontroller can go
about doing other tasks, and when the timer flag bit is raised, the timer will
interrupt the Microcontroller in whatever it is doing.

IE register
D7 D6 D5 D4 D3 D2 D1 D0
EA – – ES ET1 EX1 ET0 EX0
Main Slide

IE register...
Table : Explanation of function of IE register
Bit Name Bit address Function
7 EA AFh Global Interrupt Enable / Disable
6 – AEh Undefined (don’t care)
5 – ADh Undefined (don’t care)
4 ES ACh Enable serial communication interrupt
3 ET1 ABh Enable timer 1 interrupt
2 EX1 AAh Enable external 1 hardware interrupt
1 ET0 A9h Enable timer 0 interrupt
0 EX0 A8h Enable external 0 hardware interrupt
Main Slide

IE register...
The schematic representations of the interrupt are as follows:
Figure : Interrupt structure of 8051 Microcontroller

IP register
D7 D6 D5 D4 D3 D2 D1 D0
– – PT2 PS PT1 PX1 PT0 PX0
Main Slide

IP Register...
Table : Explanation of function of IP register
Bit Name Bit address Function
7 – BFh Undefined (don’t care)
6 – BEh Undefined (don’t care)
5 PT2 BDh Timer 2 interrupt priority (8052 only)
4 PS BCh Serial communication interrupt priority
3 PT1 BBh External 1 hardware interrupt priority
2 PX1 BAh Timer 1 interrupt priority
1 PT0 B9h External 0 hardware interrupt priority
0 PX0 B8h Timer 0 interrupt priority
Main Slide

What happens when an interrupt occurs?
When an interrupt is triggered, the following actions are taken
automatically by the Microcontroller:
1 The current Program Counter is saved on the stack, low-byte first.
2 Interrupts of the same and lower priority are blocked.
3 In the case of Timer and External interrupts, the corresponding
interrupt flag is set.
4 Program execution transfers to the corresponding interrupt handler
vector address.
5 The Interrupt Handler Routine executes.
Take special note of the third step: If the interrupt being handled is a
Timer or External interrupt, the Microcontroller automatically clears the
interrupt flag before passing control to your interrupt handler routine.
Main Slide

What happens when an interrupt Ends?
An interrupt ends when your program executes the RETI instruction.
When the RETI instruction is executed the following actions are taken by
the Microcontroller::
1 Two bytes are popped off the stack into the Program Counter to
restore normal program execution.
2 Interrupt status is restored to its pre-interrupt status.
Main Slide

Programming Timer Interrupt
Figure : Timer 0 and Timer 1 interrupt invoked by TF0 and TF1

Programming Timer Interrupt...
We know that timer flag is raised when the timer rolls over i.e. overflows
and the overflow condition are checked by monitoring TF bit using the in-
struction “JNB TFx, Label”. In polling method, we have to wait until the
timer flag bit TF is raised. The problem with this method is that the Micro-
controller is tied down while waiting for TF flag bit to be raised and cannot
do anything else. Using interrupts this problem getting solve and avoids
tied down the Microcontroller. If the timer interrupt in IE (Interrupt
enable) register is enabled, whenever the timer rolls over, timer flag TF
bit is raised automatically and Microcontroller is interrupted in whatever
it is doing, and jump to the interrupt vector table to service the interrupt
service routine (ISR) of the timer. In this way the Microcontroller can do
other things until it is notified that the timer has rolled over. Whenever the
timer flag bit is raised and if the corresponding bit of the timer interrupt flag
bit is set in the IE register, Microcontroller automatically switch or jump
to the vector location of timers.
Main Slide

Serial Interrupt of Microcontroller 8051
The working of Serial Interrupts is slightly different than the rest of the
interrupts. This is due to the fact that there are two interrupt flags: RI
and TI and only one interrupt flag bit is associated for both. If either flag
is set, a serial interrupt is triggered. As you will recall from the previous
section on the serial port, the RI bit is set when a byte is received by the
serial port and the TI bit is set when a byte has been sent. This means that
when your serial interrupt is executed, it may have been triggered because
the RI flag was set or because the TI flag was set or because both flags were
set. Thus, your routine must check the status of these flags to determine
what action is appropriate. Also, since the 8051 does not automatically
clear the RI and TI flags you must clear these bits in your interrupt handler.
Main Slide

Serial Interrupt of Microcontroller 8051...
INT SERIAL: JNB RI, CHECK TI ; If the RI flag is not set, we jump to check TI
MOV A, SBUF ; If we got to this line, it’s because the RI bit was set
CLR RI ; Clear the RI bit after we’ve processed it
CHECK TI: JNB TI, EXIT INT ; If the TI flag is not set, we jump to the exit point
CLR TI ; Clear the TI bit before we send another character
MOV SBUF, #’A’ ; Send another character to the serial port
EXIT INT: RETI
Main Slide

Programming External Hardware Interrupt
There are only two external hardware interrupts in the 8051 Microcontroller:
INT0 and INT1. They are located on pin P3.2 and P3.3 of port 3
respectively. The interrupt vector table locations 0003h and 0013h are set
aside for INT0 and INT1 respectively. They are enabled and disabled using
IE register. There are two types of activation for the external hardware
interrupts, they are
1. Level triggered
2. Edge triggered
Main Slide

Programming External Hardware Interrupt...
Figure : Level & Edge triggered for INT0 and INT1
Main Slide

Level Triggered Interrupt
In the level triggered mode, INT0 and INT1 pins are normally high and if
a low level signal (as they are low level active pins) is applied to them, it
triggers the interrupt. Upon receiving the interrupt signal, Microcontroller
stops whatever it is doing and jumps to the interrupt vector table to service
the interrupt. This is called level triggered or level activated interrupt and is
the default mode of activation upon reset of the 8051. The low level signal
at the 8051 pin must be removed before the execution of the last instruction
of interrupt service routine i.e. RETI, otherwise, another interrupt will be
generated.
Main Slide

Edge Triggered Interrupt
Upon reset the 8051 makes INT0 and INT1 low level triggered interrupts.
To make them edge triggered interrupts, we must program the bits of the
TCON register. The TCON register consists of IT0 and IT1 flag bits that
determine level or edge triggered mode of the hardware interrupts. IT0 and
IT1 are bits D0 and D2 of the TCON register respectively. Since TCON
is a bit addressable register, these bits are also referred as TCON.0 and
TCON.2. Upon reset TCON.0 and TCON.2 are both 0s i.e. external hard-
ware interrupts of INT0 and INT1 pins are low level triggered. By making
TCON.0 and TCON.2 bits high with instruction such as SETB TCON.0 and
SETB TCON.2 the external hardware interrupts INT0 and INT1 becomes
edge triggered interrupts. For example by applying High to Low pulse on
INT1 pin, then INT1 hardware interrupt becomes edge triggered hardware
interrupts.
Main Slide

TCON SFR
D7 D6 D5 D4 D3 D2 D1 D0
TF1 TR1 TF0 TR0 IE1 IT1 IE0 IT0
Main Slide

TCON SFR...
Bits Name Function
3 IE1 External hardware interrupts 1 edge flag. Set by CPU when the ex-
ternal interrupt edge (High to Low transition) is detected. Cleared
by CPU when the interrupt is processed
2 IT1 Interrupt 1 type control bit. Set/Cleared by software to specify
falling edge (Edge triggered)/Level triggered external interrupts.
1 IE0 External hardware interrupts 0 edge flag. Set by CPU when the ex-
ternal interrupt edge (High to Low transition) is detected. Cleared
by CPU when the interrupt is processed
0 IT0 Interrupt 0 type control bit. Set/Cleared by software to specify
falling edge (Edge triggered) / Level triggered external interrupts.
(IT0 = 0, then low level triggered mode is set
IT0 = 1, then edge triggered mode is set)
Main Slide

Example - 1
Show the instruction to
(a) Enable serial interrupt, timer 0 interrupt and hardware external
interrupt 0.
(b) Disable timer 0 interrupt
(c) Show how to disable all the interrupts with a single instruction.
Solution
EA – ET2 ES ET1 EX1 ET0 EX0
1 0 0 1 0 0 0 1 93h
IE = 93h
Main Slide

Assembly Language Program
(a)
MOV IE, #93H
Since IE register is a bit addressable register, we can use the following
instructions to access individual bits of register
SETB IE.7 ; EA = 1, global enable
SETB IE.4 ; enable serial interrupt
SETB IE.1 ; enable timer 0 interrupt
SETB IE.0 ; enable external hardware interrupt 0
(b)
CLR IE.1 ; disable timer 0 interrupt only
(c)
CLR IE.7 ; disable all interrupts
Main Slide

Example - 2
Program the interrupt priority (IP) register to perform the following
(a) Assign the highest priority to timer 0 and show the new priority order
(b) Discuss what happen if external hardware interrupts 0 and timer 0
interrupt are set at the same time.
Solution
0 0 0 0 0 0 1 0 02h
IP = 02h
(a) MOV IP, 02H
Or
SETB IP.2
Main Slide

Solution
(a)
Old priority order New priority order
External hardware interrupt 0 (INT0) Timer 0 interrupt (TF0)
Timer 0 interrupt (TF0) External hardware interrupt 0 (INT0)
External hardware interrupt 1 (INT1) External hardware interrupt 1 (INT1)
Timer 1 interrupt (TF1) Timer 1 interrupt (TF1)
Serial communication interrupt (RI/TI) Serial communication interrupt (RI/TI)
(b) Timer 0 is assigned the highest priority so the order of the interrupts
are
New priority order
Timer 0 interrupt (TF0)
External hardware interrupt 0 (INT0)
External hardware interrupt 1 (INT1)
Timer 1 interrupt (TF1)
Serial communication interrupt (RI/TI)
Main Slide

Example - 3
Assume that after reset, the IP register is program with the instruction
MOV IP, #0Bh. Discuss the sequence in which the interrupts are serviced.
Solution
0 0 0 0 1 0 1 1 0Bh
IP = 0Bh
This instruction sets external hardware interrupt 0 (INT0), timer 0 (TF0)
and timer 1 (TF1) to the higher priority level compared with rest of the
interrupts. So according to program bits old and new priority level of the
interrupts are shown below
Main Slide

Solution
Old priority order New priority order
External hardware interrupt 0 (INT0) External hardware interrupt 0 (INT0)
Timer 0 interrupt (TF0) Timer 0 interrupt (TF0)
External hardware interrupt 1 (INT1) Timer 1 interrupt (TF1)
Timer 1 interrupt (TF1) External hardware interrupt 1 (INT1)
Serial communication interrupt (RI/TI) Serial communication interrupt (RI/TI)
Main Slide

Example - 4
Write an assembly language program that continuously gets 8 - bit data
from port P0 and send to port P1 while simultaneously creating a square
wave of 250 µS period on port pin P2.1. Use timer 0 in mode 2 to create
the square wave, assume that crystal frequency of 8051 is 11.0592 MHz.
Solution
Step - I: Calculation of count value in TH0 Register
TON = TOFF = 125µs
TH0 = 8Dh
Step - II: Calculation of TMOD Register
TMOD = 02h
Step - III: Calculation of IE Register
IE = 82h
Main Slide

ORG 0000H
SJMP MAIN ; bypass vector location
ORG 0030H
MAIN: MOV P0, #0FFH ; make port P0 as an input port
MOV TMOD, #02H ; Timer 0 in mode 2
MOV TH0, #8DH ; load TH0 with auto - reload value
MOV IE, #82H ; enable timer 0
SETB TR0 ; start timer 0
BACK: MOV A, P0 ; get data from port P0 until TF0 raised
MOV P1, A ; send data from P0 to port P1
SJMP BACK ; repeat the process
ORG 000BH
CPL P2.1 ; interrupt routine of timer 0
CLR TF0
RETI ; return from interrupt
Main Slide

Example - 5
Repeat example 4 using timer 0 in mode 1.
Solution
Step - I: Calculation of count value in TH0 - TL0 Register
TON = TOFF = 125µs
TH0 = 8Dh
TL0 = FFh
TMOD = 01h
IE = 82h
Main Slide

ORG 0000H
ORG 0030H
MOV TMOD, #01H ; Timer 0 in mode 1
MOV TH0, #0FFH ; Load timer 0 higher byte
MOV TL0, #8DH ; load timer 0 lower byte
ORG 000BH
MOV TH0, #0FFH
MOV TL0, #0A4H
CLR TR0
CLR TF0
Main Slide

Example - 6
Write an assembly language program to generate square wave of 50 Hz
frequency on port pin P1.2 using interrupt for timer 0. Assume crystal
frequency of 8051 is 11.0592 MHz.
Solution
Step - I: Calculation of count value in TH0 - TL0 Register
TON = TOFF = 0.01s
TH0 = DCh
TL0 = 00h
TMOD = 01h
IE = 82h
Main Slide

ORG 0000H
ORG 0030H
MAIN: MOV TMOD, #01H ; Timer 0 in mode 1
MOV TH0, #0DCH ; Load timer 0 higher byte
MOV TL0, #00H ; load timer 0 lower byte
BACK: SJMP BACK ; repeat the process
ORG 000BH
MOV TH0, #0DCH
MOV TL0, #00H
CLR TR0
CLR TF0
Main Slide

Example - 7
Write an assembly language program to get data from port P1 and sends
to port P2 continuously, while giving a copy to the serial port to be
transferred serially. Assume that crystal frequency of 8051 is 11.0592 MHz
and baud rate for the serial communication is 9600.
Solution
Step - I: Calculation of SCON Register
SCON = 50h
Step - II: Calculation of TMOD Register (Timer 1 in Mode 2
for Baud Rate)
TMOD = 20h
Step - III: Calculation of TH1 Register for Baud Rate
TH1 = FDh
Step - IV: Calculation of IE Register
IE = 90h
Main Slide

ORG 0000H
SJMP MAIN
ORG 0030H
MOV TMOD, #20H ; timer 1 in mode 2 for baud rate
MOV TH1, #0FDH ; to set baud rate 9600
MOV SCON, #50H ; serial mode 1
MOV IE, #90H ; enable serial communication interrupt
SETB TR1 ; start timer 1 for baud rate
L1: MOV A, P1 ; get data from port P1
MOV SBUF, A ; send data serially
MOV P2, A ; send data to the port P2
SJMP L1 ; repeat process
ORG 0023H
JB TI, TRANS ; check for TI flag bit
CLR RI
RETI
TRANS: CLR TI
RETI
END

Example - 8
Write an assembly language program to get data from port P1 and send it
to the port P2 continuously, while sending; receive data from serial port
and transfer to port P0. Assume that crystal frequency of 8051 is 11.0592
MHz; baud rate for the serial communication is 9600.
Solution
Step - I: Calculation of SCON Register
SCON = 50h
Step - II: Calculation of TMOD Register (Timer 1 in Mode 2
for Baud Rate
TMOD = 20h
Step - III: Calculation of TH1 Register for Baud Rate
TH1 = FDh
Step - IV: Calculation of IE Register
IE = 90h
Main Slide

ORG 0000H
SJMP MAIN
ORG 0030H
MOV TMOD, #20H ; timer 1 in mode 2 for baud rate
MOV TH1, #0FDH ; to set baud rate 9600
MOV SCON, #50H ; serial mode 1
MOV IE, #90H ; enable serial communication interrupt
L1: MOV A, P1 ; get data from port P1
MOV P2, A ; send data to the port P2
SJMP L1 ; repeat process
ORG 0023H
JB RI, RECEV
CLR TI
RETI
RECEV: MOV A, SBUF
MOV P0, A
CLR RI
RETI
END

Example - 9
Assume that INT0 pin is connected to a switch that is normally high. When-
ever the switch goes low, it should turn ON LED. The LED is connected to
port pin P1.4 and is normally OFF. When it is turned ON it should stay ON
for a fraction of seconds. As long as switch is pressed low, the LED should
stay ON.
Solution
Step - I: Calculation of IE Register
IE = 81h
Main Slide

ORG 0000H
SJMP MAIN
ORG 0030H
MAIN: MOV IE, #81H
HERE: SJMP HERE
END
ORG 0003H
SETB P1.4
MOV R2, #0FFH
REP: DJNZ R2, REP
CLR P1.4
RETI
Main Slide

Example - 10
Assume that INT1 pin is connected to a pulse generator; write a program in
which the falling edge of the pulse will send a high to port pin P1.4, which
is connected to an LED, i.e. LED is turned ON and OFF at the same rate
as the pulses are applied to the INT1 pin.
Solution
Step - I: Calculation of IE Register
IE = 84h
Main Slide

ORG 0000H
SJMP MAIN
ORG 0030H
MAIN: SETB TCON.2 ; Initialize INT1 edge triggered interrupt
MOV IE, #84H
HERE: SJMP HERE
END
Interrupt service routine (ISR) for hardware interrupt INT1 to turn ON LED
ORG 0013H
SETB P1.4
MOV R2, #0FFH
REP: DJNZ R2, REP
CLR P1.4
RETI
Main Slide

Example - 11
Using interrupt, write an assembly language program to do the following
(a) Receive data serially and send it to port P0
(b) Get data from port P1 and transferred serially and give a copy to port
P2.
(c) Make timer 0 in auto - reload mode to generate a square wave of 250 µs
frequency on the port pin P0.7
Assume crystal frequency 11.0592 MHz and set baud rate for the serial
communication is 4800.
Main Slide

Solution
Step - I: Calculation of TH1 Register for Baud Rate
TH1 = FAh
Step - II: Calculation of TH0 Register for 250 micro - sec
time
TON = TOFF = 125µs TH0 = 8Dh
Step - III: Calculation of TMOD Register
TMOD = 22h
Step - IV: Calculation of SCON Register
SCON = 50h
Step - V: Calculation of IE Register
IE = 92h
Main Slide

ORG 0000H
ORG 0030H
MOV TMOD, #22H ; Timer 0 in mode 2 for square wave
;and timer 1 in mode 2 for baud rate generation
MOV TH0, #8DH ; load TH0 with auto - reload value
MOV TH1, #0FAH ; load TH1 value for 4800 baud rate
MOV SCON, #50H ; set serial mode 1
MOV IE, #92H ; enable serial communication and
; timer 0 interrupts
SETB TR0 ; start timer 0 for square wave
MOV SBUF, A ; transfers data serially
Interrupt service routine (ISR) for timer 0 to generate square wave
Main Slide

ORG 000BH
CLR TF0
Interrupt service routine (ISR) for serial port to check whether data is transmitted or received completely o
ORG 0023H
JB TI, TRANS
MOV A, SBUF
MOV P0, A
CLR RI
RETI
TRANS: CLR TI
RETI
END
Main Slide

Thank you
Please send your feedback at nbahadure@gmail.com
For more details and updates kindly visit
https://sites.google.com/site/nileshbbahadure/home
Main Slide

Interrupts of microcontroller 8051

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