The document provides an in-depth explanation of the interrupt handling architecture in the 8051 microcontroller, detailing interrupt types, processing sequences, and program design for using interrupts effectively. It discusses how various interrupts can be enabled, prioritized, and processed, as well as illustrating practical examples for implementing timer and serial port interrupts. Additionally, the document touches on designing applications with real-time control capabilities through efficient interrupt-driven mechanisms.

1. INTERRUPTS OF 8051
Introduction
8051 Interrupt organization
Processing Interrupts
Program Design Using Interrupts
Timer Interrupts
Serial Port Interrupts
External Interrupts
Interrupt Timings
1

2. INTERRUPTS
An interrupt is the occurrence of a condition--an
event -- that cause a temporary suspension of a
program while the event is serviced by another
program (Interrupt Service Routine ISR or
Interrupt Handler).
Interrupt-Driven System-- gives the illusion of doing
many things simultaneously, quick response to
events, suitable for real-time control application.
Base level--interrupted program, foreground.
Interrupt level--ISR, background.
2

3. 3
Main program (base level, foreground)
Main Main Main Main
ISR
ISR
ISR
Time
Interrupt
level execution
Base-level
execution
Interrupt (occurs asynchronously)
Return from interrupt instruction
Program execution without interrupts

4. 8051 INTERRUPT ORGANIZATION
5 interrupt sources: 2 external, 2 timer, a serial port
2 programmable interrupt priority levels
fixed interrupt polling sequence
can be enabled or disabled
IE (A8H), IP (B8H) for controlling interrupts
4

5. ENABLING AND DISABLING INTERRUPTS
IE (INTERRUPT ENABLE REGISTER A8H)
Two bits must be set to enable any interrupt: the individual enable bit
and global enable bit
SETB ET1
SETBEA
MOV IE,#10001000B
5
Bit Symbol Bit Address Description (1=enable, 0=disable)
IE.7 EA AFH Global enable/disable
IE.6 - AEH Undefined
IE.5 ET2 ADH Enable timer 2 interrupt (8052)
IE.4 ES ACH Enable serial port interrupt
IE.3 ET1 ABH Enable timer 1 interrupt
IE.2 EX1 AAH Enable external 1 interrupt
IE.1 ET0 A9H Enable timer 0 interrupt
IE.0 EX0 A8H Enable external 0 interrupt

6. INTERRUPT PRIORITY (IP, B8H)
0= lower priority, 1= higher priority, reset IP=00H
Lower priority ISR can be interrupted by a high priority
interrupt.
A high priority ISR can not be interrupted.
6
Bit Symbol Bit Address Description (1=high, 0=low priority)
IP.7 - - Undefined
IP.6 - - Undefined
IP.5 PT2 BDH Priority for timer 2 interrupt (8052)
IP.4 PS BCH Priority for serial port interrupt
IP.3 PT1 BBH Priority for timer 1 interrupt
IP.2 PX1 BAH Priority for external 1 interrupt
IP.1 PT0 B9H Priority for timer 0 interrupt
IP.0 PX0 B8H Priority for external 0 interrupt

7. INTERRUPT FLAG BITS
 The state of all interrupt sources is available through the respective
flag bits in the SFRs.
 If any interrupt is disabled, an interrupt does not occur, but software
can still test the interrupt flag.
7
Interrupt Flag SFR Register & Bit Position
------------------------------------------------------------------------------
External 0 IE0 TCON.1
External 1 IE1 TCON.3
Timer 1 TF1 TCON.7
Timer 0 TF0 TCON.5
Serial port TI SCON.1
Serial Port RI SCON.0
Timer 2 TF2 T2CON.7 (8052)
Timer 2 EXF2 T2CON.6 (8052)

8. POLLING SEQUENCE
If two interrupts of the same priority occur simultaneously, a fixed polling sequence
determines which is serviced first.
The polling sequence is External 0 > Timer 0 > External 1 > Timer 1 > Serial Port > Timer
2
8

9. 9
IE1
IT1
1
0
IE0
IT0
1
0
INT0
INT1
TF0
TF1
RI
TI
EXF2
TF2
Interrupt
enables
Global Enable
Accent
interrupt
High priority
interrupt
Low
priority
interrupt
Interrupt
polling
sequence
IP register
IE register

10. PROCESSING INTERRUPTS
When an interrupt occurs and is accepted by the CPU, the
main program is interrupted. The following actions occur:
 The current instruction completes execution.
 The PC is saved on the stack.
 The current interrupt status is saved internally.
 Interrupts are blocked at the level of the interrupt.
 The PC is loaded with the vector address of the ISR
 The ISR executes.
The ISR finishes with an RETI instruction, which retrieves
the old value of PC from the stack and restores the old
interrupt status. Execution of the main program
continues where it left off.
10

11. INTERRUPT VECTORS
Interrupt vector = the address of the start of the ISR.
When vectoring to an interrupt, the flag causing the
interrupt is automatically cleared by hardware. The
exception is RI/TI and TF2/EXF2 which should be
determined and cleared by software.
11
Interrupt Flag Vector Address
System Reset RST 0000H (LJMP 0030H)
External 0 IE0 0003H
Timer 0 TF0 000BH
External 1 IE1 0013H
Timer 1 TF1 001BH
Serial Port RI or TI 0023H
Timer 2 TF2 or EXF2 002BH

12. PROGRAM DESIGN USING INTERRUPTS
I/O event handling:
 Polling: main program keeps checking the flag, waiting for the occurrence of the event.
Inefficient in some cases.
 Interrupt-driven: CPU can handle other things without wasting time waiting for the
event. Efficient, prompt if ISR is not so complex. Suitable for control application.
 I/O processor: dedicated processor to handle most of the I/O job without CPU
intervention. Best but most expensive.
12

13. 8051 PROGRAM DESIGN USING INTERRUPT
13
ORG 0000H ;reset entry point
LJMP Main ;takes up 3 bytes
ORG 0003H ;/INT0 ISR entry point
. ;8 bytes for IE0 ISR or
. ; jump out to larger IE0 ISR
ORG 000BH ;Timer 0 ISR entry point
.
.
ORG 0030H ;main program entry point
Main: .
.
.

14. SMALL INTERRUPT SERVICE ROUTINE
8 bytes for each interrupt vector. Small ISR utilizes
the space.
For example: (assume only T0ISR is needed in the
case)
ORG 0000H
LJMP MAIN
ORG 000BH
T0ISR: .
.
RETI
MAIN: . ;only T0ISR
14

15. LARGE INTERRUPT SERVICE ROUTINE
8 bytes not enough. Use LJMP to large ISR
ORG 0000H
LJMP MAIN
ORG 000BH ; T0 ISR entry point
LJMP T0ISR
ORG 0030H ;above int vectors
MAIN: .
.
T0ISR: . ; Timer 0 ISR
.
RETI ;return to main
15

16. A 10-KHZ SQUARE WAVE ON P1.0 USING TIMER
INTERRUPTS
16
ORG 0 ;reset entry point
LJMP Main
ORG 000BH ;T0 interrupt vector
T0ISR: CPL P1.0 ;toggle port bit
RETI
ORG 0030H
Main: MOV TMOD,#02H ;T0 MODE 2
MOV TH0,#-50 ;50 mS DELAY
SETB TR0 ;START TIMER 0
MOV IE,#82H ;ENABLE T0 INT
SJMP $ ;DO NOTHING

17. TWO 7-KHZ & 500-HZ SQUARE WAVES USING
INTERRUPTS
17
ORG 0 ;reset entry point
LJMP Main
ORG 000BH ;T0 interrupt vector
LJMP T0ISR
ORG 001BH ;T1 vector
LJMP T1ISR
ORG 0030H ;main starts here
Main: MOV TMOD,#12H ;T0 mode 2, T1 mode 1
MOV TH0,#-71 ;7-kHz using T0
SETB TR0 ;START TIMER 0
SETB TF1 ;force T1 int
MOV IE,#8AH ;ENABLE T0,T1 INT
SJMP $ ;DO NOTHING

18. TWO 7-KHZ & 500-HZ SQUARE WAVES USING
INTERRUPTS (CONT.)
18
T0ISR: CPL P1.7 ;7-kHz on P1.7
RETI
T1ISR: CLR TR1 ;stop T1
MOV TH1,#HIGH(-1000)
MOV TL1,#LOW(-1000)
;1 ms for T1
SETB TR1 ;START TIMER 1
CPL P1.6 ;500-Hz on P1.6
RETI

19. SERIAL PORT INTERRUPTS
SPISR must check RI or TI and clears it.
TI occurs at the end of the 8th bit time in mode 0 or
at the beginning of stop bit in the other modes.
Must be cleared by software.
RI occurs at the end of the 8th bit time in mode 0 or
half way through the stop bit in other modes when
SM2 =0. If SM2 = 1, RI = RB8 in mode 2,3 and RI
= 1 only if valid stop bit is received in mode 1.
19

20. OUTPUT ACSII CODE SET USING INTERRUPT (1)
20
;dump ASCII codes to serial port
ORG 0
LJMP Main
ORG 0023H ;serial port vector
LJMP SPISR
ORG 0030H ;main entry point
Main: MOV TMOD,#20H ;Timer 1 mode 2
MOV TH1,#-26 ;use 1200 baud
SETB TR1 ;start T1
MOV SCON,#42H ;mode1, set TI to force first
;interrupt; send 1st char.
MOV A,#20H ;send ASCII space first
MOV IE,#90H ;enable SP interrupt
SJMP $ ;do nothing

21. OUTPUT ACSII CODE SET USING INTERRUPT (2)
21
SPISR: CJNE A,#7FH,Skip ;if ASCII code = 7FH
MOV A,#20H ;wrap back to 20H
Skip: MOV SBUF,A;start to transmit
INC A
CLR TI ;clear TI flag
RETI
The CPU speed is much higher than 1200 baud serial transmission.
Therefore, SJMP executes a very large percentage of the time.
Time for one char = (1/1200 baud)(8+1+1) = 8333.3 mS compared to
1 mS machine cycle!
We could replace SJMP instruction with other useful instructions
doing other things.

22. 22
#include “io51.h”
char *ptr;
void InitialUART(int BaudRate) /*Max baudrate = 9600*/
{
SCON = 0x52;
TMOD = 0x21;
TH1 = 256-(28800/BaudRate); /*11.059M/384=28800*/
TR1 = 1;
}
static const char msg1[]=“UART interrupt message!!”;
void main(void)
{
InitialUART(9600);
EA = 1;
ptr = msg1;
ES = 1;
while(1); /*wait for SP interrupt*/
}

23. 23
interrupt [0x23] void SCON_int(void) /*Serial port ISR*/
{
if(RI==1)RI=0; /* we did nothing in this program for RxD */
if(TI==1)
{
TI=0;
if(*ptr!=„0‟) /*string ends with „0‟ character*/
{
SBUF=*ptr;
++ptr;
}
else
{
ES=0; /*complete a string tx, clear ES and let*/
TI=1; /*main program decide next move */
}
}
}

24. EXTERNAL INTERRUPTS
/INT0 (P3.2 or pin12) and /INT1 (P3.3 or pin 13) produce
external interrupt in flag IE0 and IE1 (in TCON) in S5P2.
/INT0 and /INT1 are sampled once each machine cycle
(S5P2) and polled in the next machine cycle. An input
should be held for at least 12 clock cycles to ensure
proper sampling.
Low-level trigger (IT0 or IT1 =0): interrupt when /INT0 or
/INT1 = 0
Negative edge trigger (IT0 or IT1 = 1): interrupt if sense
high on /INT0 or /INT1 in one machine cycle and low in
next machine cycle.
IE0 and IE1 are automatically cleared when CPU is
vectored to the ISR.
24

25. EXTERNAL INTERRUPTS
If the external interrupt is low-level triggered (IT0
or IT1 =0), the external source must hold the
request active until the requested interrupt is
actually generated.
Then it must deactivate the request before the ISR
is completed, or another interrupt will be
generated.
Usually, an action taken in the ISR causes the
requesting source to return the interrupting
signal to the inactive state.
25

26. FURNACE CONTROLLER
26
INT0
INT1
P1.7
8051
1 = solenoid engaged (furnace on) if T < 19C
0 = solenoid disengaged (furnace off) if T > 21C
HOT = 0 if T > 21C
COLD = 0 if T < 19C
T = 21C
T = 19C
T = 20C
HOT
COLD
P1.7

27. FURNACE CONTROLLER
27
ORG 0
LJMP Main
EX0ISR: CLR P1.7 ;turn furnace off
RETI
ORG 13H
EX1ISR: SETB P1.7 ;turn furnace on
RETI
ORG 0030H
Main: MOV IE,#85H ;enable ext int
SETB IT0 ;negative edge trigger
SETB IT1
SETB P1.7 ;turn furnace on first
JB P3.2,Skip ;if T>21?
CLR P1.7 ;yes, turn furnace off
Skip: SJMP $ ;do nothing

28. INTRUSION WARNING SYSTEM
28
INT0 P1.7
8051
74LS04
Door opens
P1.7
P1.7
2.5 ms
1.25 ms
400Hz
1 sec

29. INTRUSION WARNING SYSTEM
29
ORG 0
LJMP Main
LJMP EX0ISR
ORG 0BH
LJMP T0ISR ;use T0 and R7=20 time 1 s
ORG 1BH
LJMP T1ISR ;use T1 to sound alarm
Main: SETB IT0 ;negative edge trigger
MOV TMOD,#11H ;16-bit timer
MOV IE,#81H ;enable EX0 only
Skip: SJMP $ ;relax & wait for intrusion

30. INTRUSION WARNING SYSTEM
30
EX0ISR: MOV R7,#20 ;20x50000 ms = 1 s
SETB TF0 ;force T0 ISR
SETB TF1 ;force T1 ISR
SETB ET0 ;enable T0 int
SETB ET1 ;enable T1 int
RETI
T0ISR: CLR TR0 ;stop T0
DJNZ R7,SKIP ;if not 20th time, exit
CLR ET0 ;if 20th, disable itself
CLR ET1 ;and tone
LJMP EXIT
SKIP: MOV TH0,#HIGH(-50000) ;reload 0.05 s
MOV TL0,#LOW(-50000) ;delay
SETB TR0 ;turn on T0 again
EXIT: RETI

31. INTRUSION WARNING SYSTEM
31
;
T1ISR: CLR TR1 ;stop T1
MOV TH1,#HIGH(-1250) ;count for 400 Hz
MOV TL1,#LOW(-1250) ;tone
CPL P1.7 ;music maestro
SETB TR1 ;
RETI

32. INTERRUPT TIMING 1
Since the external interrupt pins are sampled once each
machine cycle, an input high or low should hold for at
least 12 oscillator periods to ensure sampling.
If the external interrupt is transition-activated, the external
source has to hold the request pin high for at least one
cycle, and then hold it low for at least one cycle. This is
done to ensure that the transition is seen so that
interrupt request flag IEx will be set.
IEx will be automatically cleared by the CPU when the
service routine is called.
32

33. INTERRUPT TIMING 2
If the external interrupt is level-activated, the
external source has to hold the request active
until the requested interrupt is actually
generated. Then it has to deactivate the request
before the interrupt service routine is completed,
or else another interrupt will be generated.
33

34. INTERRUPT TIMING 3
Response Time
The /INT0 and /INT1 levels are inverted and latched into
IE0 and IE1 at S5P2 of every machine cycle. The values
are not actually polled by the circuitry until the next
machine cycle.
If a request is active and conditions are right for it to be
acknowledged, a hardware subroutine call to the
requested service routine will be the next instruction to
be executed. The call itself takes two cycles. Thus, a
minimum of three complete machine cycles elapse
between activation of an external interrupt request and
the beginning of execution of the first instruction of the
service routine.
34

35. INTERRUPT TIMING 4
A longer response time would result if the request is
blocked by one of the 3 previously listed conditions.
1. An interrupt of equal or higher priority level is already in
progress.
2. The current (polling) cycle is not the final cycle in the
execution of the instruction in progress. (ensure
completion)
3. The instruction in progress is RETI or any write to the IE
or IP registers. (one more instruction will be executed)
If an interrupt of equal or higher priority level is already in
progress, the additional wait time obviously depends on
the nature of the other interrupt’s service routine.
35

36. INTERRUPT TIMING 5
If the instruction in progress is not in its final cycle, the
additional wait time cannot be more than 3 cycles, since
the longest instructions (MUL and DIV) are only 4 cycles
long, and if the instruction in progress is RETI or an
access to IE or IP, the additional wait time cannot be
more than 5 cycles (a maximum of one more cycle to
complete the instruction in progress, plus 4 cycles to
complete the next instruction if the instruction is MUL or
DIV).
Thus, in a single-interrupt system, the response time is
always more than 3 cycles and less than 9 cycles.
36

37. INTERRUPT RESPONSE TIMING DIAGRAM:
FASTEST
37

38. INTERRUPT RESPONSE TIMING DIAGRAM:
LONGEST
38
RETI MUL AB Save PC ISR
Level 0 ISR Main program Level 0 ISR
9 cycles
Level 1 interrupt occurs
here (missed last chance
before RETI instruction)

39. INTERRUPT TIMING 6
Single-Step Operation
The 80C51 interrupt structure allows single-step execution
with very little software overhead. As previously noted,
an interrupt request will not be responded to while an
interrupt of equal priority level is still in progress, nor will
it be responded to after RETI until at least one other
instruction has been executed. Thus, once an interrupt
routine has been entered, it cannot be re-entered until
at least one instruction of the interrupted program is
executed.
One way to use this feature for single-step operation is to
program one of the external interrupts (e.g., /INT0) to be
level-activated.
39

40. INTERRUPT TIMING 7
The service routine for the interrupt will terminate with the
following code:
JNB P3.2,$ ;Wait Till /INT0 Goes High
JB P3.2,$ ;Wait Till /INT0 Goes Low
RETI ;Go Back and Execute One Instruction
Now if the /INT0 pin, which is also the P3.2 pin, is held
normally low, the CPU will go right into the External
Interrupt 0 routine and stay there until /INT0 is pulsed
(from low to high to low). Then it will execute RETI, go
back to the task program, execute one instruction, and
immediately re-enter the External Interrupt 0 routine to
await the next pulsing of P3.2. One step of the task
program is executed each time P3.2 is pulsed.
40

41. INTERRUPT TIMING 8
Reset
The reset input is the RST pin, which is the input to a
Schmitt Trigger.
A reset is accomplished by holding the RST pin high for at
least two machine cycles (24 oscillator periods), while
the oscillator is running. The CPU responds by
generating an internal reset.
The external reset signal is asynchronous to the internal
clock. The RST pin is sampled during State 5 Phase 2 of
every machine cycle. The port pins will maintain their
current activities for 19 oscillator periods after a logic 1
has been sampled at the RST pin; that is, for 19 to 31
oscillator periods after the external reset signal has
been applied to the RST pin.
41

42. 42
Reset Timing

43. INTERRUPT TIMING 9
The internal reset algorithm writes 0s to all the
SFRs except the port latches, the Stack Pointer,
and SBUF.
The port latches are initialized to FFH (input
mode), the Stack Pointer to 07H, and SBUF is
indeterminate.
The internal RAM is not affected by reset. On
power up the RAM content is indeterminate.
43