RTC accuracy calculation for Tick event

[Vlad Mokhnachov]

Hi, Nordic team!

I want use RTC with Prescaler 32765 for generating Tick event with 1s interval.
Could you help with calculation of jitter for Tick event in microseconds. I will use external XTAL and PPI for triggering of other Timer.

Also, could you explain about "TASK and EVENT jitter/delay". Does it affect Tick event, routed through PPI?

[Hung Bui]

Hi Vladislav, 

The "TASK and EVENT jitter/delay" is the un-synchronize  between the LFCLK which the RTC run on and the HFCLK in which the CPU and the bus running on. 
So as it's explained in the specification, you may have a delay/jitter from the time you trigger the START task until you receive the first COUNTER/TICK event. But this wouldn't affect the distance between the TICK events. 
The accuracy of the 1s interval is affected by the accuracy of the clock source for the LFCLK. If you are using the internal RC the tolerance can be ±250 ppm

[Vlad Mokhnachov]

Ok, thanks. I understand.
I notice, that maximum value for Prescaler is 4095. So, Tick event will have interval 125 ms.
For example, XTAL have ±20ppm. Does it mean, that 125 ms Tick event may have jitter ±2.5 us?

[Hung Bui]

Hi Vladislav, 

Yes you are right. The maximum value for the Prescaler is 4096. And if you have the XTAL with 20ppm tolerance you will have ±2.5 us jitter on each TICK event. So it will accumulate and you will have a larger drift over time if you are syncing it to other devices.