Introduction

An interrupt is an event that occurs randomly in the flow of continuity. It is just like a call you have when you are busy with some work and depending upon call priority you decide whether to attend or neglect it.  

Same thing happens in microcontrollers. 8051 architecture handles 5 interrupt sources, out of which two are internal (Timer Interrupts), two are external and one is a serial interrupt. Each of these interrupts has their interrupt vector address. Highest priority interrupt is the Reset, with vector address 0x0000.

Vector Address: This is the address where controller jumps after the interrupt to serve the ISR (interrupt service routine).

Reset

Reset is the highest priority interrupt, upon reset 8051 microcontroller start executing code from 0x0000 address.

Internal interrupt (Timer Interrupt)

8051 has two internal interrupts namely timer0 and timer1. Whenever timer overflows, timer overflow flags (TF0/TF1) are set. Then the microcontroller jumps to their vector address to serve the interrupt. For this, global and timer interrupt should be enabled.

Serial interrupt

8051 has serial communication port and have related serial interrupt flags (TI/RI). When the last bit (stop bit) of a byte is transmitted, TI serial interrupt flag is set and when last bit (stop bit) of receiving data byte is received, RI flag get set.

IE register: Interrupt Enable Register

IE register is used to enable/disable interrupt sources.

Bit 7 – EA: Enable All Bit

            1 = Enable all interrupts

            0 = Disable all interrupts

Bit 6,5 – Reserved bits

Bit 4 – ES: Enable Serial Interrupt Bit

            1 = Enable serial interrupt

            0 = Disable serial interrupt

Bit 3 – ET1: Enable Timer1 Interrupt Bit

            1 = Enable Timer1 interrupt

            0 = Disable Timer1 interrupt

Bit 2 – EX1: Enable External1 Interrupt Bit

            1 = Enable External1 interrupt

            0 = Disable External1 interrupt

Bit 1 – ET0: Enable Timer0 Interrupt Bit

            1 = Enable Timer0 interrupt

            0 = Disable Timer0 interrupt

Bit 0 – EX0: Enable External0 Interrupt Bit

            1 = Enable External0 interrupt

            0 = Disable External0 interrupt

Interrupt priority

Priority to the interrupt can be assigned by using interrupt priority register (IP)

Interrupt priority after Reset:

In the table, interrupts priorities upon reset are shown. As per 8051 interrupt priorities, lowest priority interrupts are not served until microcontroller is finished with higher priority ones. In a case when two or more interrupts arrives microcontroller queues them according to priority.

IP Register: Interrupt priority register

8051 has interrupt priority register to assign priority to interrupts.       

Bit 7,6,5 – Reserved bits.

Bit 4 – PS: Serial Interrupt Priority Bit

            1 = Assign high priority to serial interrupt.

            0 = Assign low priority to serial interrupt.

Bit 3 – PT1: Timer1 Interrupt Priority Bit         

            1 = Assign high priority to Timer1 interrupt.

            0 = Assign low priority to Timer1 interrupt.

Bit 2 – PX1: External Interrupt 1 Priority Bit    

            1 = Assign high priority to External1 interrupt.

            0 = Assign low priority to External1 interrupt.

Bit 1 – PT0: Timer0 Interrupt Priority Bit         

            1 = Assign high priority to Timer0 interrupt.

            0 = Assign low priority to Timer0 interrupt.

Bit 0 – PX0: External0 Interrupt Priority Bit     

            1 = Assign high priority to External0 interrupt.

            0 = Assign low priority to External0 interrupt.

External interrupts in 8051

• 8051 has two external interrupt INT0 and INT1.
• 8051 controller can be interrupted by external Interrupt, by providing level or edge on external interrupt pins PORT3.2, PORT3.3.
• External peripherals can interrupt the microcontroller through these external interrupts if global and external interrupts are enabled.
• Then the microcontroller will execute current instruction and jump to the Interrupt Service Routine (ISR) to serve to interrupt.
• In polling method microcontroller has to continuously check for a pulse by monitoring pin, whereas, in interrupt method, the microcontroller does not need to poll. Whenever an interrupt occurs microcontroller serves the interrupt request.

External interrupt has two types of activation level

1. Edge triggered (Interrupt occur on rising/falling edge detection)
2. Level triggered (Interrupt occur on high/low-level detection)

In 8051, two types of activation level are used. These are,

Low level triggered

            Whenever a low level is detected on the INT0/INT1 pin while global and external interrupts are enabled, the controller jumps to interrupt service routine (ISR) to serve interrupt.

Falling edge triggered

            Whenever falling edge is detected on the INT0/INT1 pin while global and ext. interrupts are enabled, the controller jumps to interrupt service routine (ISR) to serve interrupt.

There are lower four flag bits in TCON register required to select and monitor the external interrupt type and ISR status.

TCON: Timer/ counter Register

Bit 3- IE1:      

       External Interrupt 1 edge flag, set by hardware when interrupt on INT1 pin occurred and cleared by hardware when interrupt get processed.

Bit 2- IT1:

 This bit selects external interrupt event type on INT1 pin,

       1= sets interrupt on falling edge

       0= sets interrupt on low level

Bit 1- IE0:      

       Interrupt0 edge flag, set by hardware when interrupt on INT0 pin occurred and cleared by hardware when an interrupt is processed.

Bit 0 - IT0:

This bit selects external interrupt event type on INT0 pin.

1= sets interrupt on falling edge

0= sets interrupt on low level

Example

Let’s program the external interrupt of AT89C51 such that, when falling edge is detected on INT0 pin then the microcontroller will toggle the P1.0 pin.

Programming steps

1. Enable global interrupt i.e. EA = 1
2. Enable external interrupt i.e. EX0 = 1
3. Enable interrupt trigger mode i.e. whether interrupt is edge triggered or level triggered, here we will use falling edge trigger interrupt, so make IT0 = 1.

Program

/*
 * 8051_External_Interrupt
 
 */

#include <reg51.h>	/* Include x51 header file */
sbit LED = P1^0;	/* set LED on port1 */ 

void Ext_int_Init()				
{
	EA  = 1;	/* Enable global interrupt */
	EX0 = 1;      	/* Enable Ext. interrupt0 */
	IT0 = 1;      	/* Select Ext. interrupt0 on falling edge */
}
											
void External0_ISR() interrupt 0
{
	LED = ~LED;	/* Toggle pin on falling edge on INT0 pin */
} 

void main()
{
	Ext_int_Init(); /* Call Ext. interrupt initialize */
	while(1);
}