Revision as of 14:11, 2 August 2014

Intro

In this tutorial we will look at 8051 Interrupts. Interrupts are useful in many cases wherein the process simply wants to continue doing it's 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 microcontroller with the help of interrupts.

Video Tutorial

Basics

Interrupt sources for 8051

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

Table. 1: Interrupt Vector Table
Interrupt	ROM Location(Hex)	Pin	Flag Clearing	Interrupt no. in C
Reset	0000	9	Auto	--
External HW Interrupt 0 (INT0)	0003	P3.2(12)	Auto	0
Timer 0 Interrupt(TF0)	000B	-	Auto	1
External HW Interrupt 1 (INT1)	0013	P3.3(13)	Auto	2
Timer 1 Interrupt(TF1)	001B	-	Auto	3
Serial Com Interrupt(RI and TI)	0023	-	Program SW	4

As seen in the above table,

the reset vector has just 3 bytes allocated to it, meaning it can hold a jump instruction to the location where the main program is stored.
The other interrupts have 8 bytes allocated to each of them, hence a small Interrupt service routine(ISR) can be placed here. However, if the ISR needs to larger in length, it has to placed else where and the allocated 8 bytes need to have the code that simple redirects the control to the ISR.
INT0 and INT1 are external interrupts on P3.2 and P3.3 respectively. These can be configured to be low level triggered or edge triggered interrupt sources.
TF0 and TF1 are timer overflow interrupts for timer 0 and 1 respectively
The Serial COM Interrupt can be configured to trigger upon transmit or receipt of a byte during serial communication.

Enabling and Disabling the Interrupts

It should be noted that when the MCU is reset, all the interrupts are disabled. Hence in order to use them, we should enable them. In 8051 Interrupt Enable(EA) Register is used to enable or disable the interrupt. The register is shown below:

EA
D7	D6	D5	D4	D3	D2	D1	D0
EA	-	ET2	ES	ET1	EX1	ET0	EX0

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EA: 0;Disables all interrupts. 1;Enables all interrupts. It is kind of master control, before enabling any of the interrupts this bit should be 1. For all the below interrupts, setting(1) the bit enables the interrupt, 0 disables it.
ET2: Timer 2 Overflow interrupt(8052)
ES:Serial Port Interrupt
ET1:Timer 1 overflow interrupt
EX1:External Interrupt 1 on P3.3
ET0:Timer 0 overflow interrupt
EX0:External Interrupt 0 on P3.2

Example 1: Timer Interrupts

To demonstrate use of timer interrupts, we will blink a LED connected to P3.7 at 50ms(using timer zero) and in the mean time we would also read the SW connected to P3.2 and display on LED connected to P3.6.
Important thing to understand here is, in the main program, the switch is continuous read and displayed on the led, the timer keeps ticking on its own and when it over follows the led connected to P3.7 is toggled.

In real time we should be able to see the LED connected to P3.7 blink continuously and also switch status shown on LED P3.6.

#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

In this example the main program does nothing, whereas when an low signal interrupt is received on P3.2(INT0) we turn on Led1(P3.6) and when interrupt occurs on P3.3(INT1) we turn on Led2(P3.7). Note that two switches are connected to the two pins as shown in the schematic.

#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

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