FreeRTOS tickless idle not supported by SysTick port for NRF52

[Stefan Johansson]

The SysTick port for NRF52 does not support tickless idle on freeRTOS, what is the reason for this limitation?
Are there any known problems behind this limitation?

[jakbird]

SysTick runs off the core instruction clock.  When entering IDLE mode the clock stops, so SysTick stops as well.

FreeRTOS needs to know the elapsed number of ticks while the core sleeps.  If SysTick is used as the time base there is no way to determine the elapsed time while the core is asleep.  Nor is there a mechanism to wake up after the maximum tick delay for tickless mode.

The solution is to use RTC1 for the timebase.  Since it uses LFCLK it continues to run, and a compare register can be set to force an interrupt when the maximum tick delay is reached.

This works in System On, IDLE mode but not System Off.  If you have to track time in System Off mode you'll need an external RTC.  LFCLK is stopped so RTC1 won't work.

  Jack Peacock