How to Interface Interrupts with 8051

External Interrupt Interfacing With 8051 Primer

Tags: 8051 interrupts, interrupts in 8051, External Interrupts Handling in 8051,8051 interrupts programming,8051 interrupts tutorials,
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8051 Primer Board

The 8051 Primer board is specifically designed to help students to master the required skills in the area of embedded systems. The kit is designed in such way that all the possible features of the microcontroller will be easily used by the students. The kit supports in system programming (ISP) which is done through serial port.

NXP’s 8051 (AT89V51RD2), 8051 Primer Kit is proposed to smooth the progress of developing and debugging of various designs encompassing of speed 8-bit Microcontrollers.

External Interrupts

An interrupt caused by an external source such as the computer operator, external sensor or monitoring device, or another computer. Interrupts are special events that require immediate attention.

Interfacing External Interrupts

Fig. 1 shows how to interface the External Interrupt to microcontroller. Interrupts cause the processor to cease the running task to serve a special task for which the interrupt event had occurred. After the special task is over, the processor resumes performing the original task.

The processor can also serve these events by polling method. But polling is an inefficient technique as compared to interrupts. In the polling method, the processor has to continuously wait for the event signal. Thus it always remains busy using extra resources.


Fig. 1 Interfacing External Interrupts to Microcontroller

Interfacing External Interrupts with 8051

We now want to display some messages in PC when occur an external interrupt signal in 8051 Primer Board. The Interrupt signal is occurred by using switches. When the switch is pressed to LOW, then the external interrupt is occurred.

The Vectored Interrupt Controller (VIC) takes 5 interrupt request inputs and programmable assigns them into 3 categories, timer, external and serial interrupts.

The 8051 Primer board has two numbers of External Interrupts, connected with I/O Port lines (P0.14 & P0.15) as switches.

Pin Assignment with 8051



8051 Lines


Tactile Switch



SW29– (INT0)



SW31 – (INT1)



Circuit Diagram to Interface Ext-Interrupt with 8051


Source Code

The Interfacing External Interrupt with 8051 program is very simple and straight forward, that accessed by using switches. When the two external interrupts occurring, some messages are displayed in PC through serial port at 9600 baud rate. The C programs are written in Keil software.


C Program to interface Ext-Interrupt using 8051


Title : Program to interface an external Interrupt





#include<LPC214x.h>     // Define LPC2148 Header File

int volatile EINT2   =   0;


void ExtInt_Serve2(void)__irq;

void ExtInt_Serve1(void)__irq



   EXTINT |= 2;

   VICVectAddr = 1;


void ExtInt_Serve2(void)__irq



   EXTINT |= 4;

   VICVectAddr = 0;


void main(void)



   ExtInt_Init1(); //Initialize Interrupt 1

   ExtInt_Init2(); //Initialize Interrupt 2


   printf ("***************** External Interrupt

Demo ***************************\n\n\r");



 printf (">>> Press Interrupt Keys SW2(INT1) and

SW3(INT2) for Output... \n\n\n\r");





      printf("INT1 Generated : %d  |  INT2 Generated

 : %d \r", EINT1, EINT2);





void ExtInt_Init2(void)


   EXTMODE  |= 4;          //Edge sensitive mode on EINT2

   EXTPOLAR = 0;           //Falling Edge Sensitive

   PINSEL0  |= 0x80000000; //Enable EINT2 on P0.15

   VICVectCntl1 = 0x20 | 16; // 16 is index of EINT2

   VICVectAddr1 = (unsigned long) ExtInt_Serve2;

   VICIntEnable |= 1<<16;    //Enable EINT2



void ExtInt_Init1(void)


   EXTMODE  |= 2;         //Edge sensitive mode on EINT1

   EXTPOLAR = 0;          //Falling Edge Sensitive

   PINSEL0  |= 0x20000000;   //Enable EINT2 on P0.14

   VICVectCntl0 = 0x20 | 15; //15 is index of EINT1

   VICVectAddr0 = (unsigned long) ExtInt_Serve1;

   VICIntEnable |= 1<<15;    //Enable EINT1



void Serial_Init(void)



   PINSEL0 |=   0X00000005; //Enable Txd0 and Rxd0

   U0LCR    =   0x00000083; //8-bit data,1-stop bit

   U0DLL   =   0x00000061; //XTAL = 12MHz

   U0LCR   =   0x00000003; //DLAB = 0;



void DelayMs(unsigned int count)



   unsigned int i,j;      








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