Question

Can a interrupt flag be set by the code as in the example below or is that line just an error of thinking? This is just the main function. Below this code snipet are the interrupt it self, is it correct and nessasary to clear the interrupt flag in the end of the code?

if(duty != (uint8_t) (SOFT_PWM_PERIOD - 1))

    {
        // Request an immediate interrupt if the timer counter has
        // already the initial period. This helps minimize glitches
        // when changing duty cycles
        if(duty < TMR4)
            PIR3bits.TMR4IF = 1;

        // Finally (re-)start the timer
        T4CON =
            0    << 3 |   // 1x post-scaler
            1    << 2 |   // Active
            2 /* << 0 */; // 16x pre-scaler

        IPR3bits.TMR4IP = 1;    // TMR4 Overflow Interrupt Priority bit High
        PIE3bits.TMR4IE = 1;    // TMR4 Overflow Interrupt Enable bit
    }

The Intrrupt code ->

      // Deal with PWM timer interrupts. Add this to the high-priority interrupt handler.
     void SoftPWM_Interrupt(void)
     {
volatile uint8_t _SoftPWM_Toggle; // Is this variable really accessed by both the ISR and       mainline functions? (C.G)

/* Has a flank been reached yet? */
if(PIR3bits.TMR4IF)
{
    /* Alternate between the low and high periods */
    PR4 ^= _SoftPWM_Toggle;

    /* Try to deal gracefully with the new period already having been reached. */

    /* The hardware timer works by checking if TMR4 = PR4 when it is time to increase */
    /* counter, in which case TMR4 is reset instead. Thus if is already TMR4 > PR4 due to */
    /* interrupt latency then we've missed the period and an extra interrupt is needed. */
    /* First acknowledging the flag and then conditionally setting it is necessary to */
    /* avoid a race between reading TMR4 and changing the flag. */
    /* Finally the the TMR4 > PR4 test is actually implemented as skip if TMR4 < PR4 + 1 */
    /* but the increment cannot overflow since the interrupt won't be used for 0% or 100% */
    /* duty cycles */
    PIR3bits.TMR4IF = 0;

    _asm
        INCF PR4,0,ACCESS
        CPFSLT TMR4,ACCESS
    _endasm

    /* if(TMR4 > PR4) */
        PIR3bits.TMR4IF = 1; // Cant only the harware set this flag? (C.G)

    /* Finally toggle the output pin */
    SOFT_PWM_PIN ^= 1;

    /*Important?*/
    PIR3bits.TMR4IF = 0;
}
     }

Answer 1

Yes you can set an interrupt flag by soft. But IMO it is not really a good practice to do it…

If you really want the behavior of your ISR to be executed in the normal context, why don’t you externalize your ISR code in a function that you can call in your main function?

About the interrupt flag, if you don’t clear it, the ISR will be executed in loop and you will never go back to your main program.