Revision as of 12:00, 24 August 2016

In this tutorial, we will look at 8051 Interrupts. Interrupts are useful in many cases wherein the process simply wants to continue doing its main job and other units(timers or external events) seek its attention when required. In other words, the microcontroller, need not monitor the timers, the serial communication or the external pins P3.2 and P3.3. Whenever an event related to these units occur, it is informed to the microcontroller with the help of interrupts.

A single microcontroller can serve several devices by two ways:

• Polling: The microcontroller continuously monitors the status of all the devices. Whenever any device needs the service,it provides the service and moves on to the next device until everyone is serviced. This will be done in an infinite loop.

• Interrupts: Whenever any device needs its service, the device notifies the microcontroller by sending it an interrupt signal Upon receiving an interrupt signal, the microcontroller interrupts whatever it is doing and serves the device. The program which is associated with the interrupt is called the interrupt service routine (ISR) or interrupt handler

What is an Interrupt?

An interrupt is an external or internal event to get the CPU's attention. Once the controller detects the interrupt, it suspends the current job and executes a special service routine know as Interrupt Service Routine(ISR).

Upon activation of an interrupt, the microcontroller goes through the following steps

1. First it finishes the instruction it is executing and saves the address of the next instruction (PC) on the stack.
2. It also saves the current status of all the interrupts internally.
3. It jumps to a fixed location in memory, called the interrupt vector table, that holds the address of the ISR. The microcontroller gets the address of the ISR from the interrupt vector table and jumps to it It starts to execute the interrupt service subroutine until it reaches the last instruction of the subroutine which is RETI (return from interrupt)
4. Upon executing the RETI instruction, the microcontroller returns to the place where it was interrupted.
5. First, it gets the program counter (PC) address from the stack by popping the top two bytes of the stack into the PC.
6. Then it starts to execute from that address.

Video Tutorial

Basics

Interrupt sources for 8051

For the 8051 Microcontroller there are six interrupt sources as shown in the table below: }}

Example 1: Timer Interrupts

#include<reg51.h>
sbit Led1 =  P3^6; // will show switch status(inverted)
sbit Led2 =  P3^7; // this will be blinking every at 50ms
sbit SW1 = P3^2;
void timer0() interrupt 1
{
  Led2 = ~Led2;
 
  //reload the timer
  TH0 = 0X4b;
  TL0 = 0Xff; 
}
void main()
{
  TMOD = 0X01; //Timer 0 mode 1; 16 bit timer
  TH0 = 0X4b;
  TL0 = 0Xff;
  IE = 0x82; //Enable Interrupts
  SW1 = 1; //make switch as input
  TR0 = 1; //turn on timer
  P3 = 0x0F;//turning off both leds
 
  while(1)
  {
    Led1 = SW1;
  }
 
}

Example 2: External Interrupts

#include<reg51.h>
sbit Led1 =  P3^6; 
sbit Led2 =  P3^7; 
sbit SW1 = P3^2;
sbit SW2 = P3^3;
void ext_int_0 () interrupt 0 
{ 
    Led1 = 1; 
} 
void ext_int_1 () interrupt 2 
{ 
   Led2 = 1;
}
void main()
{
   P3 = 0X0F; //leds output, switches input
   EA = 1;
   EX0 = 1;
   EX1 =1;
 
   while(1)
   {
     Led1 = 0;
  Led2 = 0;
 //you could add any other code here as well
   }
}

Schematic

Schematic