RE: RTC_COMPARE_OFFSET_MIN value

Stefan Birnir Sverrisson — Fri, 27 Feb 2015 10:23:55 GMT

Hi Manju

I recommend to set the app_timer prescaler with the following #define, as done in the examples in the SDK

#define APP_TIMER_PRESCALER

Setting RTC_COMPARE_OFFSET_MIN to 1 will contain the risk of app_timer not triggering, so that is not recommended.

There seem to be two limiting factors to the setting of the prescaler:

#define RTC_COMPARE_OFFSET_MIN  3                 //in app_timer.c
#define APP_TIMER_MIN_TIMEOUT_TICKS  5            //in app_timer.h

The RTC_COMPARE_OFFSET_MIN is set to ensure that the RTC CC register is not set too close to the current RTC1 counter value, which will risk that no interrupt will be triggered on CC compare match.
The APP_TIMER_MIN_TIMEOUT_TICKS which is set to ensure adequate timing accuracy of the app_timer.

So if you set the prescaler to 0x2FF. With that, the tick frequency is 15.625ms and you can set the RTC CC register to N+8 with

APP_TIMER_TICKS(125, APP_TIMER_PRESCALER)

That should be both safe and accurate