Best way to implement system clock on nRF51

[Alessandro]

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

[Clem Taylor]

I like the idea of mixing a RTC timer and a counter to get seconds and ticks. Especially if can be done both low power and without an interrupt. But it is not clear how you would do this while sharing the RTC with app_timer. On the nrf52 you have an extra RTC to use, but on the nrf51 only the one RTC is available for app use and it is used by app timer.

Using a spare timer to count RTC overflows seems pretty straight forward, I'll have to switch over to that. The race window on incrementing my overflow count is a quite a bit bigger then I originally thought. I was only thinking of it when the timer is read from the main app and not thinking that I mostly read it from other interrupt handlers. I really wish the nrf51 had more priority levels available to to the application when using the softdevice. Luckily the nrf52 adds more priority levels which will be useful for stuff like this.