Interrupt-on-Change occurs twice on PIC16F1825

Question

I am having a mind blowing misbehaviour in my PIC16F1825. Basically I am using pin 3 (RA4) to toggle a LED using an Interrupt-on-Change. Problem is, although it toggles the led whenever I press the button (which is externally pulled up), after a second, the LED toggles again, which means that the interrupt routine was called again, and it shouldn't, even though I clear the interrput flag.

Here's the code (only function IOconfig_portA() and toggleLed() matter for the problem at hand:

#include <stdio.h>
#include <stdlib.h>
#include <PIC16F1825.h>

char temp=0;

void interrupt toggleLed() {

    INTCONbits.GIE = 0;

    if(INTCONbits.IOCIF == 1) {
        temp = PORTA;
        INTCONbits.IOCIF = 0;
        LATC3 = ~LATC3;
        for (int i = 0; i < 100000; i++);
    }

    INTCONbits.GIE = 1;
}

void CLOCKconfig() {
    OSCCON  = 0x6A; //Sets the internal oscillator fosc = 4 MHz
    OSCSTAT = 0x00;
    OSCTUNE = 0x00;
}

void IOconfig_portA() {
    ANSELA = 0x00;           // All ports set as DIGITAL
    TRISAbits.TRISA4 = 1;    // Set as input
    //OPTION_REG &= 0x7F;    // Clear bit 7, to enable the weak pull-up
    //WPUA |= (1<<2);        // Enable the WPU for RA2
    CM1CON0 = 0x00;
    CM1CON1 = 0x00;
    IOCANbits.IOCAN4  = 1;  // Generate Interrupt on Negedge
    IOCAP  = 0x00;  // Disable Interrupt on Posedge
    INTCON = 0x88;    // Enable GIE and IoC interrupts
}

int main(int argc, char** argv) {

    TRISC = 0;

    CLOCKconfig();

    ANSELC &= 0x00; // All bits on port C are set to Digital I/O's
    TRISC  &= 0x00; // All bits on Port C are set to Outputs
    APFCON0 |= (1<<5); // Don't use special features on Pin RC3
    APFCON1 |= (1<<2); // Don't use special features on Pin RC3

    IOconfig_portA();

    LATC |= (1<<3);

    while(1) {

    }

    return (EXIT_SUCCESS);
}

Thanks in advance!

Answer 1

According to Page 92 of the PIC16F1825 Datasheet:

Note 1: The IOCIF Flag bit is read-only and cleared when all the Interrupt-on-Change flags in the IOCxF register have been cleared by software

So basically when you do this in your code:

INTCONbits.IOCIF = 0;

It actually does nothing at all - the interrupt flag is not cleared because that bit is read-only.

In order to clear the interrupt flag for Interrupt-on-Change sources, you need to write to the IOCAF register to clear the flags you want. If you want to clear them all, you can do:

IOCAF = 0x0;

If you only want to clear certain bits, like IOCAF4 in your case, you can do:

IOCAFbits.IOCAF4 = 0;

Answer 2

Found the answer! It is quite obscure, I must say. Basically, the PIC has a Watchdog timer that resets the device every 4 seconds. Oddly enough, it comes enabled by default (does that even make sense?). To disable the watchdog timer, you need to add the following pre-compiler directive:

#pragma config WDTE = OFF

Answer 3

Is it possible the circuit resets due to multiple interrupts (eg. stack over flow).

On a reset you would repeat all of the code and re-enable the interrupt. (As in the other answer there could be switch bounce that causes this to.)

The counter delay of 100k may be too short, try increasing this to something humanly measurable like a total delay of 500ms if possible.

Also to test for an unwanted reset give yourself a one time power up routine to flash a unique code on the LED, that will tell you if the whole thing recycled.