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Best way to implement system clock on nRF51

Hi everybody,

a while ago I used the approach proposed here devzone.nordicsemi.com/.../ to keep the date and time on a NRF51822 device while using Softdevice S110 and the Timeslot advertiser-scanner (github.com/.../nRF51-multi-role-conn-observer-advertiser).

In practice I created an app_timer timer that executes every 250 milliseconds (this is the resolution I wanted) and inside the handler I incremented some variables accordingly to keep date and time. The problem is that I noticed that the time was not accurate, it was wrong, sometimes of several seconds, already after 1 hour. Here is the first question: I think that the time was wrong mainly because the app_timer handler gets delayed by the BLE stack, am I correct?

My idea to improve this is to connect an RTC1 COMPARE event to the TIMER1 COUNT task through PPI. The RTC1 will be configured to fire the event every second. This way I could keep the time in TIMER1 as a Unix timestamp (seconds since 1970-01-01) and I could read the number of milliseconds from RTC1 (I will use the number of ticks together with the prescaler to compute the milliseconds). This way there is no software handler involved and therefore it could not get delayed, is it correct? Is this going to be more precise than the solution using the app_timer? Will this solution consume more power? Do I need to keep on the 16MHz clock?

The implication of this solution is that I cannot use the app_timer library anymore or I could modify it to connect the RTC COMPARE event with the TIMER COUNT task and this should not interfere with the normal operation of the library, right?

I could try to implement the solution but since I am not sure it will work I would like some feedback from you.

Thanks a lot. Alessandro

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  • Hi John, I didn't implement it yet, I might try over the weekend. I think I missed an important problem, the fact that the RTC counter will continue to count and after the first COMPARE event it will be greater than the CC register. In this situation, the next COMPARE event will be triggered only after the RTC will overflow and start again from zero. The first idea that comes to my mind is to use the COMPARE event to trigger the COUNT on TIMER1 and the CLEAR on the RTC using PPI. This should be possible as written on the reference manual, but I don't know if it is a problem for the app_timer module.

    It is also not very clear to me how the app_timer uses the COMPARE[0] event.

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  • Hi John, I didn't implement it yet, I might try over the weekend. I think I missed an important problem, the fact that the RTC counter will continue to count and after the first COMPARE event it will be greater than the CC register. In this situation, the next COMPARE event will be triggered only after the RTC will overflow and start again from zero. The first idea that comes to my mind is to use the COMPARE event to trigger the COUNT on TIMER1 and the CLEAR on the RTC using PPI. This should be possible as written on the reference manual, but I don't know if it is a problem for the app_timer module.

    It is also not very clear to me how the app_timer uses the COMPARE[0] event.

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