PPI with WFE/WFI

[eric ding]

Hi team,

I understand PPI's main feature is to connect the peripherals automatically and save CPU power.

Let's say peripheral A's event is connected to the peripheral B's task using PPI and CPU is halted by WFE/WFI.

Will the CPU be woke up if the event is becoming active?

Because to my understanding, WFE is wait for event, if there is a event present, the CPU should be woke up.

Could you help to elaborate on this?

Thanks.

Eric

[Einar Thorsrud]

Hi Eric,

PPI is independent of the CPU and will not wake up the CPU. However, the same event that can be connected to a PPI channel can also be configured to generate an interrupt if you want to wake the CPU. The figure in the beginning of the Peripheral interface chapter in the Product specification shows how this is related.

Cheers,

Einar

[eric ding]

Hi Einar,

Either the CPU is halted by WFI or WFE, as long as the event is not enabled and connected to NVIC, the CPU will not be woke up, is this the correct understanding?

Thanks.

Eric.

[Einar Thorsrud]

Yes, that is correct. The CPU will only we waken up if there is an event and the event is configured to generate an interrupt in the INTENSET register.

[eric ding]

okay, understood, thanks.

Then , one more question, what's the difference between WFI and WFE in the nRF serial platform?

or in which case should the WFI be used and in which case using WFE?

Thanks 

Eric

[Einar Thorsrud]

The difference between WFI and WFE is described in the ARM Cortex-M4 Devices Generic User Guide. Interrupts also cause wake-up from WFE, so WFE and WFI can be used interchangeably in most cases. Note that SEV can be used together with WFE as shown below to ensure that the device goes to sleep also when the event register is already set.

Most SDK examples and the SoftDevice use code similar to this to go to sleep:

// Make sure the event register is cleared
__SEV();
__WFE();
// Enter System ON sleep mode
__WFE();

[eric ding]

Got it. Big thanks.

[pungent1188]

Why is __WFE needed twice? Wouldn't that mean losing first event?

[Einar Thorsrud]

See this post.