8051 Interrupts

Interrupt- Facility provided in microprocessor using which attention of microprocessor may be drawn for some specific purpose.Microprocessor suspends its current job- saves the status. Microprocessor attends to the device/system/event causing interrupt- ISR is executed. Microprocessor goes back to suspended job and starts executing from the point where it was suspended.

One to one correspondance between Interrupt and ISR i.e. for each interrupt- ISR is specified and stored in memory.

Interrupt Requested

Execution

p normal Program

Normal Program

execution

Interrupt service Routine executed

For a microprocessor having n interrupts. XX1, XX2---------XXn address of ISR in memory.

ISR-n

ISR-2

ISR-1

XXn

XX2

XX1

When Interrupt K occurs ISR-K for interrupt is executed by loading address XXK in PC.XX1 - ------XXn starting address of ISR for interrupts.

vector addresses of Interrupts.

Other issues in Interrupts

Hardware/Software Interrupts

Interrupt Priority

Enabling / Disabling of Interrupts.

Masking of Interrupts.

Edge/Level Triggered Hardware Interrupts

8051 has five interrupt sources.Each interrupt can be programmed to two priority levels.

INT0 External Request from P3.2 pin

Timer 0 Overflow from Timer 0 activates interrupt request flag TF0.

INT1 External Request from P3.3

Timer1- Overflow from Timer1 activates interrupt request flag TF1

Serial Port completion of transmission or reception of a serial frame activates the flags TI or RI.

The complete interrupt system may be disabled or enabled by storing 0 or 1 in EA bit of IE SFR

- IE.7.

SETB IE.7, - Enable Interrupts

CLR IE.7, - Disable Interrupts

When Interrupt system is enabled i.e. IE.7=1, then each of the five interrupts can be enabled/disabled individually by making a specified bit in IE register as 1 or 0.

- Interrupt masking

bit = 0 Interrupt is disabled i.e. masked

bit = 1 Interrupt is enabled

IE Register Interrupt Enable Register

76543210

EX0- Enable/Disable External Interrupt 0

ET0- Enable/Disable Timer 0 overflow Interrupt

EX1- Enable/Disable External Interrupt 1

ET1- Enable/Disable Timer 1 overflow Interrupt

ES -Enable/Disable Serial port Interrupt

EA - Enable/Disable All interrupts

Examples

-Disable all interrupts

CLR IE.7

EAXXESET1EX1ET0EX0

Enable external interrupt 1 and Timer 0 overflow interrupt

IE = 1 0 0 0 0 1 1 0 = 86H

MOV A, # 86H

MOV IE, A

Interrupt Priority

There are two priority levels for interrupts in 8051- High and Low.Each interrupt can be programmed to a low or high priority level, by making bits of IP(Interrupt Priority) SFR as 0 or 1.

bit = 0 Interrupt at Low priority level.

bit = 1 Interrupt at High priority level.

or

MOV IE, # 86H

Interrupt Priority Register

76543210

PX0 Priority External Interrupt 0

PT0 Priority Timer 0 overflow

PX1 Priority External Interrupt 1

PT1 Priority Timer 1 overflow

PS - Priority Serial port Interrupt

Example To place Timer 0, external interrupt 1 at high priority and other interrupts at low priority.

IP = 0000 0110 = 06H

MOV IP, # 06H

IP -

SETB IP.1

SETB IP.2

or

XXXPSPT1PX1PT0PX0

A low priority interrupt cant interrupt a high priority interrupt but a high priority interrupt can interrupt low priority interrupt.In the above example

(i) 8051 is executing ISR for serial port interrupt i.e. it is servicing serial port interrupt.

- If interrupt request at external interrupt 01 occurs since it is at high level, the 8051 will be interrupted.

(ii) 8051 is executing ISR for timer 0.

- An interrupt request at external interrupt 0 will not interrupt 8051 since EX0 is at low level

- For interrupts which are programmed at same priority level, following rule is applied.

SourcePriority Level

- External Interrupt 0 -Highest

Timer 0 Overflow -

External Interrupt 1-

Timer 1 Overflow -

Serial Port - Lowest

When two or more interrupt requests come at the same time and all the interrupts at the same priority level the 8051 will select an interrupt for servicing based on above.All the interrupts are separately scanned during each m/c cycle.

An interrupt request will be serviced i.e. corresponding ISR will be executed unless-An interrupt of equal or higher level is already in progress.Current m/c cycle is not the final m/c cycle in the execution of instruction in progress i.e. no interrupt request will be responded to until the instruction in progress in completed.

Edge/Level Triggered Interrupts :-

In 8051, external interrupts i.e. INT0 and INT1 can be programmed to be low level triggered or high to low edge triggered.

Level triggered When signal at INT0 or INT1 is low-interrupt is caused.Edge Triggered When signal at INT0 or INT1 becomes low from high-interrupt is caused. Bits 0 to 3 of TCON register are used for programming level or edge triggered interrupts.

TCON :-

76543210

IT0 (TCON.0) and IT1 (TCON.2) are used for Level/Edge triggered programming.TF1TR1TF0TR0IE1IT1IE0IT0

When IT0 = 0INT0is Low Level triggered

IT0 = 1INT0 is High to Low edge triggered

IT1 = 0INT1 Low level triggered

IT1 = 1INT1High to Low edge triggered

When edge triggered external interrupt occurs Hardware will set the flag IE0 (TCON.1) for INT0 and IE1(TCON.3) for INT1.In case of edge triggered interrupts, occurrence of interrupt needs to be saved.

The ISR location for each interrupt is predefined in 8051.

SourceISR Location

External Interrupt 0-0003H

Timer 0 Overflow-000BH

Ext. Interrupt 1-0013H

Timer 1 Overflow-001BH

Serial Port-0023H

The ISR must start at these locations and jump to another memory location if it is bigger.

For Example:- ORG0013H

SJMPNIMT1.

NIMT1 :-

Programming

External Interrupts :- In a plant when there is gas leakage, the gas detector sends a high to low interrupt on INT0. The 8051 (12MHz clock) starts water spray by sending control signal (5V) to P0.1 and sends alarm signal by blinking LED connected to P2.0(Common Anode)

Main Program :-

ORG00

SJMP030H

ORG030H

; Enable Interrupt INT0

IE

= 1000 0001 = 81H

MOV IE, # 81H

; Program INT0 to edge triggered by making IT0=1 i.e. (TCON.0=1)

SETB TCON.0[ or SETB IT0]

; Set INT0 to high priority

IP =

MOVIP, # 01H [or SETB IP.0]

SJMP $

EAXXESET1EX1ET0EX0XXXPSPT1PX1PT0PX0

ISR for INT0

ORG0003H

SJMPNXINT0

NXINT0: SETB P0.1

MOV R3, # 100

REPET:CLRP2.0

ACALLDEL1S

SETBP2.0

DJNZ R3, REPET

RETI

DEL1S:- Delay of 1second

RET

END

Timer Interrupt:- In a Rly station when a train passes a signal point, P0.2 line of 8051 becomes 1. 8051 waits for 30 seconds and watches P1.2 line which may be activated by station master. If not activated in 30 sec., 8051 sends all clear signal by lighting LED connected to P1.7, to enable other trains to come.

Note:- Station master will activated P1.2 if any coach of train is left out.

8051 is having 12MHz clock 12 oscillator m/c cycle.Delay calculation

12 MHz clock.

1 m/c cycle = 12/12 sec. = 1sec.

Use Timer 0 as Timer for delayTimer 0 ISR will light LED connected to P1.7 if P1.2=0

Mode = 01 16 bit timer

count = 0000 to FFFF i.e. 216 sec= 1 Overflow

1 Overflow = 65536 count

30 x 106 sec = 65536 x n

n = 457.76 = 458 times overflow

= 2 x 229 times overflow

Main Program:-

ORG0000H

SJMP0030H

ORG0030H

TMOD = = 01H

MOVTMOD, # 01H

MOVR3, # 229

MOVR4,# 2

JNBP0.2, $

MOVTL0, # 00

MOVTH0, # 00

; Enable Interrupt

SETBIE.7

SETBIE.1

Timer 0

[ MOV IE, # 82H]

GATEC/ TM1M0

; Set priority -

SETBIP.1

; Clear Interrupt flag

CLRTF0 [TCON.5]

; Initiate Timer 0

SETBTR0 [TCON.4]

SJMP$

Timer 0 ISR

ORG000BH

; Timer interrupt has to cause repeated interrupts 458 i.e. 2 x 229 times.

; Check if R3= 0

DECR3

CJNER3, #00, KK1

DECR4

CJNER4,# 00 , KK2

JNBP1.2 , KK3

; Station master has activated P1.2

SJMPKK4

KK2:MOV R3, # 229

SJMP KK1

KK3:SETB P1.7

SJMP KK4

KK1:MOV TL0, # 00

MOV TH0, # 00

CLR TF0 ; clear flag

SETB TR0 initiate timer 0

MOV IE, # 82H; Enable Interrupt

KK4:RETI

Timer Interrupt:- Generate a pulse train of 1KHz at P1.0 using timer 0, (12 MHz clock frequency.)

Delay calculation:-

1 m/c cycle = 1 sec.

1KHz = 103Hz =103 cycles per sec.

1 clock period = 10-3 sec = 1 msec.

UP time= 0.5 milli sec.

DN time= 0.5 milli sec.

1 m/c cycle= 1 sec

0.5 milli sec.= 0.5 x103 m/c cycle

= 500 m/c cycle

For mode = 01 16 bit counter

Preset value 65535-500=65035 = FE0BH

Considering additional m/c cycle for roll over

Preset value = FE0CH

Main Program:-

; Program timer mode.

Gate C/T M1M0

TMOD = = 01H

0 0 0 00 0 01

MOV TMOD , # 01H

; Load preset value

MOV TL0, # 0CH

MOVTH0, # FEH

; Clear Timer 0 control interrupt bit TF0

CLR TF0 [ CLR TCON.5]

; Enable Timer 0 interrupt

EA ET0 EX0

IE = = 82H

MOV IE, # 82H

; Set priority for Timer 0 interrupt

PTO PXO

IP = = 02H

10 0100 010

Note : - Timer 0, Timer 1 and serial port interrupts are generated internally. Thus level / Edge trigger programming is not required.

;Set P1.0 to low level

CLR P1.0

;Initiate Timer 0

SETB TR0[SETB TCON.4]

SJMP $

Timer 0 ISR

ORG 000BH

SJMP TMR0

TMR0:CPLP1.0

;Enable Timer 0 interrupt

MOVIE, #82H

; Clear TF0

CLR TF0[TCON.5]

;Reload value

MOV TL0, # 0CH

MOV TH0,# FEH

Initiate Timer 0

SETB TR0[TCON.4]

RETI

Serial Port Interrupt

Note :- Two flags TI (Transmission) and (RI) Reception are there.

-Both cause only one interrupt

- Serial Port Interrupt

-Serial Port Interrupt

-Thus in ISR , You have to check the status of RI and TI to know whether Transmit or Receive Interrupt has occurred.

T1

R1

Example

Send 10 bytes of data stored at location 0030H onwards serially in mode1 (8 bits start- stop) at baud rate 4800.8051 has clock frequency of 11.0592MHz

Main Program

ORG 00H

SJMP 030H

ORG 030H

MOV R2, # 00H for count

;Set mode, =01, SM0, SM1 =01

REN = 1

SCON =

=50H

MOV SCON, #50H

76543210SM0SM1SM2RENTB8RB8T1RI01010000

;Enable Serial Interrupt.

IE =

= 90H

MOV IE, # 90H

;Set Priority as High

IP =

=10H

MOV IP, # 10H

76543210EAXXESET1EX1ET0EX0100100007654321000010000XXXPSPT1Px1PT0PX0

;Set Timer 1 to mode 2 Auto reload

TMOD =

MOV TMOD, #20H

;Load Preset value, in TH1

;Preset value for 4800 = -6 decimal or FAH

MOV TH1, # FAH [MOV TH1 , # -6]

;Load byte to be transmitted to SBUF

Loop1:CJNE R2, #10, XY

SJMP XY1

XY: MOV A, R2

MOV R0, A

0010 Timer 1Timer 0GateC/TM1M0

ADD A, #30H

MOV A, @R0

INC R2

MOV SBUF, A

;Clear TI and RI flags

CLR TI [CLR SCON.1]

CLR RI [CLR SCON.0]

;Initiate Timer 1

CLR TF1 [CLR TCON.7]

SETBTR1 [SETB TCON.6]

;At 4800 baud the 10 bit transmission

;will take 10/4800 sec = 1000/480 2 msecond

; We need to give delay of 2ms . and then

;Check TI bit to find if the

;transmission is complete

[ ACALL DEL 2MS ]

JNB TI, $

CLR TI

;Load next byte.

INC R2

CJNE R2, # 9, Loop1

SJMP $

SJMP LOOP1

XY1: NOP

END

ISR Serial Interrupt

ORG 0023 H

SJMP SRINT

;Enable serial Interrupt

SRINT: MOV IE, #90H

CLR T1

;Initiate Timer 1

CLR TF1

SETB TR1

RETI

; Similar steps to receive 20 byte of data and store in memory.