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Cortex-M4: guaranteed wakeup from WFI?

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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Top replies

Martin Weidmann over 12 years ago +1 verified

Note: This was originally posted on 9th August 2013 at http://forums.arm.com I don't tend to use the M class, but... Could you use WFE instead of WFI? WFE (Wake for Event) will cause the processor to...

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0 Markus Grabner over 12 years ago

Note: This was originally posted on 9th August 2013 at http://forums.arm.com

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

I have just been pointed to the following page:

http://infocenter.ar...a/BIHBFEIB.html

The Cortex-M doesn't require the __ISB(), though ARM recommends to use it to ensure compatibility with other implementations of the ARMv7-M architecture.So these four lines of code seem to be the answer to my initial question!

Thanks & kind regards,
Markus
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0 Markus Grabner over 12 years ago

Note: This was originally posted on 9th August 2013 at http://forums.arm.com

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

I have just been pointed to the following page:

http://infocenter.ar...a/BIHBFEIB.html

The Cortex-M doesn't require the __ISB(), though ARM recommends to use it to ensure compatibility with other implementations of the ARMv7-M architecture.So these four lines of code seem to be the answer to my initial question!

Thanks & kind regards,
Markus
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