What are External Interrupts

AT89C2051 has two external interrupts. These external interrupt inputs are located at pins P3.2 and P3.3. Pins P3.2 and P3.3 are also designated as INT0 and INT1, respectively.

The external interrupts are either edge-activated or level-activated. If an interrupt is level-activated, the interrupt is triggered when a low signal is applied to the input of the external interrupt. If an interrupt is edge-activated, the interrupt is triggered when a high-to-low signal is applied to the input of the external interrupt.

TCON Register

The TCON is already introduced in 8051 Tutorial 4: 8051 Timer/Counter Programming in C. Aside from controlling the Timer/Counter of AT89C2051, it is also responsible to the operational mode of AT89C2051′s external interrupts.

Programming External Interrupts

1. Initialize the external interrupts to be used. Select if the external interrupt is edge-activated/level-activated by setting/clearing the IT0 for external interrupt 0 or IT1 for external interrupt 1.

2. Enable which external interrupt is to be used by setting EX0 or EX1 of IE register.

3. Enable global interrupts by setting EA of IE register.

Example:

Given the circuit below, write the program that will turn the LED on when S1 is pressed once and turn the LED off when S2 is pressed once.

Solution:

#include<reg51.h>

sbit LED = P1^7;

void turn_on(void) interrupt 0
{
 LED=1;
}

void turn_off(void) interrupt 2
{
 LED=0;
}

void main(void)
{
 LED=0;           //Turn LED off
 IT0=1;           //set external interrupt 0 edge-triggered
 IT1=1;           //set external interrupt 1 edge-triggered
 EX0=1;          //enable external interrupt 0 interrupt
 EX1=1;          //enable external interrupt 1 interrupt
 EA=1;           //enable global interrupt
 while(1)        //loop forever
 {;}
}

The ISR for external interrupt 0 points to interrupt number 0 and the ISR for external interrupt 1 points to interrupt number 2. Notice that the flags of each external interrupts do not need to be cleared by software. The flags are automatically cleared by hardware after the ISRs are serviced.

The format of the ISRs of external interrupts are the following

void your_ISR(void) interrupt 0 //ISR for external interrupt 0
{
 //your routine here
}

void your_ISR(void) interrupt 2 //ISR for external interrupt 1
{
 //your routine here
}

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