PIC16F877A External Interrupt Programming in XC8

 

Introduction

External interrupt is an outside event that notify the controller. Controller may response or ignore to this event by its routine. Using this feature main program loop doesn’t need to poll for this event, reducing latency in program.

Pin diagram – RB0 can trigger external interrupt to controller.

This interrupt caused by input logic change on RB0 of Port B. It’s programmable by user. Logic change could be,

• Rising edge (from low to high logic)
• Falling edge (from high to low logic)

With the advantage of internal weak pull-up resistor of Port B, it’s suitable to select interrupt on falling edge when using with push button input.

External Interrupt Programming

Interrupt programming in C is simple. Programmer doesn’t need to save context before program jumping to Interrupt Service Routine (ISR), and restoring context after ISR executing is completed.

Programmer just needed to select and prepare targeted interrupt source. In ISR it’s required to write any C code to handle interrupt source.

void interrupt extISR(void){
    if(INTE&&INTF){
        toggleLed^=1;
        INTF=0;
    }
}

 

Schematic Design

We configure RB0 as digital input with internal weak pull-up resistor enabled. It trigger interrupt signal whenever connected input button is pressed, changing input logic from high to low. Each time interrupt occurs, an LED connects to RB7 toggles. Crystal oscillator is 20MHz supplied by a +5V DC voltage source.

Schematic Diagram

 

Programming in C

Main program loop in C blinks output LED connects to RB4. External interrupt is handled in ISR.

/*
 * PIC16F877A External Interrupt Programming
 * in MPLABX XC8 Example
 */
#include <xc.h>
 
// PIC16F877A Configuration Bit Settings
// CONFIG
#pragma config FOSC = HS        // Oscillator Selection bits (HS oscillator)
#pragma config WDTE = OFF       // Watchdog Timer Enable bit (WDT disabled)
#pragma config PWRTE = OFF      // Power-up Timer Enable bit (PWRT disabled)
#pragma config BOREN = ON       // Brown-out Reset Enable bit (BOR enabled)
#pragma config LVP = OFF        // Low-Voltage (Single-Supply) In-Circuit Serial Programming Enable bit (RB3 is digital I/O, HV on MCLR must be used for programming)
#pragma config CPD = OFF        // Data EEPROM Memory Code Protection bit (Data EEPROM code protection off)
#pragma config WRT = OFF        // Flash Program Memory Write Enable bits (Write protection off; all program memory may be written to by EECON control)
#pragma config CP = OFF         // Flash Program Memory Code Protection bit (Code protection off)
 
#define  _XTAL_FREQ  20000000
#define  blinkLed    RB4
#define  toggleLed   RB7
 
void main(void){
    /*Clear Port B*/
    PORTB=0x00;
    /*RB0 input*/
    TRISB=0x01;
    /*Turn on pull up resistor*/
    nRBPU=0;
    /*Enable external interrupt*/
    INTE=1;
    /*External interrupt on falling edge*/
    INTEDG=0;
    /*Turn on global interrupt*/
    GIE=1;
    /*Clear external interrupt flag*/
    INTF=0;
    /*Loop of main program*/
    while(1){
        blinkLed^=1;
        __delay_ms(500);
    }
}
 
/*Interrupt Service Routine*/
void interrupt extISR(void){
    /*Check interrupt flag*/
    if(INTE&&INTF){
        toggleLed^=1;
        INTF=0;
    }
}

Click here to download source file.