12F683 output not behaving as expected

Question

I have a 12F683 micro and this is my first program using the new MPLAB with XC8 compiler.

I have configured Timer1 to trigger about every ms. For now I would need an LED to flash on and off but need to do this with an IF statement because later I will add other actions in there. Here is the interrupt code:

#define PWM GPIObits.GP5

void interrupt High_Priority_Interrupt(void) {
  if (CCP1IF) {
      if (PWM) {
          PWM = 0; //NEVER excuted
      }
      else {
          PWM = 1; //ALWAYS executed
      }
      CCP1IF = 0;
  }
}

For some reason the output is never set to 1. If I replace the whole code with PWM^=1; it works like a charm. But I need to do it with the if statement. What am I doing wrong?

Answer 1

This is a problem with input/output registers on baseline and mid-range cores. Both input and output registers of ports are read from input buffer and if you configured pin as analog input or output, digital input is disconnected and you can't read output port register. In your case, pin is configured as output and every time you try to read it to check it's value, you get zero (from input buffer).

To solve this, either configure pin as input and add button to control input signal and toggle LED on another pin or change PWM to variable and it will work.

Also, in same document there's another issue regarding IO ports and read-modify-write instruction answered here

You can read more here (Page 2, READ-MODIFY-WRITE INSTRUCTIONS ON PORT REGISTERS)