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 implemented something along this tonight.

I used TIMER1 in counter mode and PPI to trigger TASKS_COUNT from EVENTS_OVRFLW.

The one thing that took be a little bit to figure out is the only way to read the counter value is to trigger a capture, not sure why TIMERn doesn't have a COUNTER field like the RTC.

So far it seems to work, will see how it is doing tomorrow.

The one thing I'm not sure about is what to do with app_timer when no timers are pending. The code wants to stop the timer. Once I'm sure things are working with the overflow counter I'll try clearing the CC interrupt.

Here is my access function:

void app_timer_ticks(uint32_t *p_overflow, uint32_t *p_ticks)
{
    uint32_t overflow0, overflow1, counter;
    
    APP_TIMER_OVERFLOW_COUNTER->TASKS_CAPTURE[0] = 1; // trigger capture 
    overflow0 = APP_TIMER_OVERFLOW_COUNTER->CC[0]; // before 
    counter   = NRF_RTC1->COUNTER;
    APP_TIMER_OVERFLOW_COUNTER->TASKS_CAPTURE[1] = 1; // trigger capture 
    overflow1 = APP_TIMER_OVERFLOW_COUNTER->CC[1]; // after 
   
    if (overflow0 != overflow1)
    {
        /* overflow occurred (rare) 
         * we don't know if overflow1 changed before or after 
         * we sampled counter1, so we just sample again 
         */
        counter = NRF_RTC1->COUNTER;
    }

    *p_overflow = overflow1;
    *p_ticks    = counter;
}

When I fetch the time with CTS, I sample this value and that becomes my epoch offset.

Now if Android bothered to implement CTS this would be so much easier.