App Timer (RTC1) drift

RE: App Timer (RTC1) drift

Samuele Forconi — Fri, 14 Oct 2016 13:41:57 GMT

The clock of the nRF was always slower than the real clock, but we don't know if it is linear with time (we can try to run the FW for some days and check what happens).

Now I've set the PRESCALER to 0 and added the 30us delay as in the previous post (to see if it improves the counting accuracy).

We tried to figure out why that delay has been added to the timer handler, but in the end we haven't found any real / explicable reason... we were wondering if it was a value that was measured empirically (ie using an oscilloscope) and set in the FW to improve the time keeping accuracy.

RE: App Timer (RTC1) drift

Ole Bauck — Fri, 14 Oct 2016 13:14:49 GMT

I don't know why the delay is added, but I can try to figure it out (I can't get hold of the person that wrote the code).

Do you have an answer to if the clock is slower or faster than the real clock?

Also is the error increasing linearly with the time? (fex one minute after one day, two minutes after two days and so on).

RE: App Timer (RTC1) drift

Samuele Forconi — Thu, 13 Oct 2016 15:25:38 GMT

Sorry, in the meanwhile I have also coded a version of the firmware that uses a 1s timer (using the app_timer library), and it seems that the accuracy is better than the firmware that reads the RTC counter directly (but I can't explain why)...

In the firmware there are other timers (always in the range of seconds, not us nor ms timers), so I believe the RTC is kept active from the SoftDevice.

Yes, with a PRESCALER set to 0 the tick interval is around 30.5us, but I can't explain why the timer loose 1 tick every second... it is documented somewhere in the docs? If I change the PRESCALER to 4095 (125ms tick interval), will the timer loose 125ms every second?

RE: App Timer (RTC1) drift

Ole Bauck — Thu, 13 Oct 2016 14:49:22 GMT

30 us is about the resolution when prescaler is 0 (1/32768Hz = 30.5us). I don't know why this is added, but you don't have to worry about this. You are reading the value of the RTC (app_timer_cnt_get(&current_tick)) directly, not assuming that a callback was called at exactly one second interval.

RE: App Timer (RTC1) drift

Ole Bauck — Thu, 13 Oct 2016 14:39:33 GMT

5 minutes over 3 days is about 5/(3 * 24 * 60) = 1157ppm, which is way too high. Is the clock slower or faster than the real time, and is it always like this?

Do you always have a timer running? The app_timer module will stop the RTC if no timers are running.

RE: App Timer (RTC1) drift

Samuele Forconi — Thu, 13 Oct 2016 14:29:51 GMT

Yes, we have checked the circuit but it seems everything ok... In the Nordic Eddystone firmware, I have found this piece of code inside a timer handler function:

...
// For every 1 second interrupt, there is 30 us delay with timer prescaler set at 0.
us_delay += 30;

if (APP_TIMER_PRESCALER != 0)
{
    //If the prescaler is not 0, then a new us_delay increment needs to be calculated...
    //Trigger a run time error here to prevent developers from blindly changing the prescaler
    APP_ERROR_CHECK(NRF_ERROR_INVALID_PARAM);
}

From what I understand from the code, it seems that the timer handler looses 30us each time is called (with a PRESCALER set to 0), but I'm not able to find on the documentation anything regarding this...

RE: App Timer (RTC1) drift

Dave_couling — Tue, 11 Oct 2016 17:38:17 GMT

My first thought would be that there is some inaccuracy in your Xtal circuit design. Have you measures your PCB.s parasitic capacitance and factored it into the tuning capacitor values?