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Cortex-M4: guaranteed wakeup from WFI?

Markus Grabner
Note: This was originally posted on 8th August 2013 at http://forums.arm.com

Hi all!

I'm wondering how to ensure that the Cortex-M4 is always woken up on an interrupt from the WFI instruction when executing code like this:

__disable_irq();
// do something ...
__enable_irq();
__WFI();

If an interrupt is triggered between __enable_irq() and __WFI(), the interrupt handler is processed [font="Arial"]immediately and completed before [/font][font="Courier New"]__WFI() is executed, i.e., [/font][font="Arial"][font="Courier New"]__WFI()[/font] will wait for the next interrupt such that the first one is effectively lost. The ATmega* processors have a very simple solution to this problem: enabling interrupts is always delayed by one clock cycle such that the processor can safely enter sleep mode before interrupts can occur and wake up the processor. Thereby it is guaranteed that the processor is always woken up on an interrupt. How is this done on the Cortex-M4? As far as I know, there is no such enable interrupt delay, or is there a totally different mechanism to ensure wakeup on interrupts?

Thanks & kind regards,
Markus


P.S.: A lengthy discussion on a similar topic is here, though in German language.
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  • 0
    Note: This was originally posted on 9th August 2013 at http://forums.arm.com


    Swapping the order of the __WFI() and the __enable_irq() should work.

    Interesting. Is it guaranteed that pending interrupts are immediately executed as soon as interrupts are enabled, i.e., that after the code sequence

    __WFI();
    __enable_irq();
    __disable_irq();

    at least one interrupt handler was completed?

    Thanks & kind regards,
    Markus
Reply
  • 0
    Note: This was originally posted on 9th August 2013 at http://forums.arm.com


    Swapping the order of the __WFI() and the __enable_irq() should work.

    Interesting. Is it guaranteed that pending interrupts are immediately executed as soon as interrupts are enabled, i.e., that after the code sequence

    __WFI();
    __enable_irq();
    __disable_irq();

    at least one interrupt handler was completed?

    Thanks & kind regards,
    Markus
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