How to achieve low interrupt latency from sleep ?

[Nicolas Brunner]

Hello, i'm using a nRF52840 on a PCA10056 and want to wake up from __WFE() with a minimum delay. When i'm using the debugger, i achieve a low latency but without the debugger, the latency is increased by ~14us. What should i do for achieving the same low latency with or without debugger ?

[ovrebekk]

Hi Nicolas

You can use the constant latency sub power mode to keep the clock running during sleep, which will ensure a quicker wakeup.

To enable this mode simply activate the CONSTLAT task like this, before entering sleep:

NRF_POWER->TASKS_CONSTLAT = 1;

Please be aware that this will increase the sleep current significantly, so if you need low sleep currents you might want to reconsider.

For more information about the sub power modes, please refer to chapter 16.4.1 on page 73 of the nRF52840 datasheet.

Best regards
Torbjørn

[Nicolas Brunner]

I have tested with pin_change_int example. With the debugger i always get 5.7us latency but without debugger i got:

• loop: 5.7us
• __WFE: 19.4us
• __WFE + CONSTLAT: 15.9us

With CONSTLAT, i still lose ~10us. Is there another thing i can do to lower latency ?

[ovrebekk]

Sorry for the slow reply.
I tested this myself and got 15us without CONSTLAT and 11us with CONSTLAT.
I think this is about the best you can get with the nRF52832, but I am trying to find someone that can explain in more detail what is happening during this startup sequence, and why the debug mode is so much quicker.
I will get back to you once I have some more information on this.

Best regards