Question

I am using the Microchip C18 compiler and on occurrence of an interrupt I am experiencing a quite long delay before the ISR code starts running.

As an experiment, this is in my main function:

while(1)
{
    LATAbits.LATA4 = 1;
    LATAbits.LATA4 = 0;
}

As interrupt handler I am using this code I copied from some example (I don’t know why it’s done this way):

#pragma interrupt high_isr
void high_isr(void)
{
    LATAbits.LATA4 = 1;
    LATAbits.LATA4 = 1;
    LATAbits.LATA4 = 0;
    LATAbits.LATA4 = 1;
    LATAbits.LATA4 = 1;
    LATAbits.LATA4 = 0;
}

#pragma code high_vector=0x08
void interrupt_at_high_vector(void)
{
_asm GOTO high_isr _endasm
}

I am receiving bytes via SPI and shortly after a byte is received, the main loop stops. Then a delay of 16.5 µs follows before the ISR code starts running. That is 165 instruction cycles!

enlarge image

I know there is some context saving associated with interrupts and that it’s even worse with low priority interrupts. I have disabled IPEN and I am using only the high priority vector. Is 165 instructions a normal duration for context saving?

Answer 1

Under some circumstances interrupt overhead can be great as yours!
Take a look at this.