I'm programming a PIC microcontroller to service two events via interrupts on a very time constrained environment.

PIC microcontrollers allow a SLEEP mode which wakes the PCU on any external interrupt (INTn) but has a wake delay depending on the selected oscillator mode.

I want to know which option would start servicing the interrupts faster. A main loop like this:

//CPU never goes to sleep, keeps executing a jump
void main(){

or a main loop like this:

//CPU goes to sleep (idle), waits for INTn interrupts.
void main(){
     asm SLEEP;

Waking from idle sleep seems to take 2 cycles according to the PIC18F4XK22 whereas a jump instruction (of an infinite while loop may take longer and may or may not be interruptible halfway into the execution of the jump.


1 Answer 1


You said it yourself, waking up from sleep incurs some delay, depending on the oscillator. The delay will be relatively small for R-C oscillators, and much longer for crystal oscillators.

The second method uses less power, assuming you can sleep for long enough on average for the reduced current to be meaningful.

  • \$\begingroup\$ Idle sleep seems to wake in ~2 clock cycles. If jump instructions cannot be interrupted halfway (I don't know) idle sleep might be a better option. Power consumption, not an issue. \$\endgroup\$
    – NeonMan
    Commented Sep 3, 2014 at 15:02
  • \$\begingroup\$ @NeonMan - add that information about 2-clock-wake to your question. Maybe include sub-questin about un-interuptable jump. \$\endgroup\$
    – gbulmer
    Commented Sep 3, 2014 at 16:14
  • \$\begingroup\$ Also add that part about "Power consumption, not an issue". I predict that you will get nearly universal agreement to keep the thing awake. Put in 10 NOP instructions (or a hundred) if you want a 90 (or 99) percent chance of catching the interrupt without a JMP instruction annoying you. \$\endgroup\$
    – User.1
    Commented Sep 3, 2014 at 22:43
  • \$\begingroup\$ @OlinLathrop interesting. Can you add details on why a crystal would take longer? I imagine the RC only takes the time it takes to charge the cap. Ignoring the chip cycles to get out of sleep. \$\endgroup\$
    – kenny
    Commented Sep 3, 2014 at 23:10
  • \$\begingroup\$ Interrupts can be serviced in between instructions. If an interrupt request happens while an instruction is executing, the instruction must run to completion before the interrupt can be serviced. I think the question you're actually worrying about is, what is the min/max interrupt latency for PIC18F. \$\endgroup\$
    – MarkU
    Commented Sep 4, 2014 at 4:33

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