Sync RTC to GNSS time without large time step

[WestCoastDaz]

Hi Nordic Dev team,

I'm using the older nRF SDK in a project that is built using a nRF52840.  I have an RTC and a GNSS connected to the nRF52840.  I use one of the LP timers internally to keep track of time as an internal RTC that can quickly be accessed in memory for time stamping my sensor values.  I'm looking for ways of keeping the RTC from drifting from GNSS time that doesn't create large time discontinuities (steps) in my sensor logs.  Do you have any examples of this that I can refer too?

Thanks,

[Sigurd Hellesvik]

Hi,

For me to better understand the issue here:

What is the thing that does not work because of the drift?
Im checking for an XY problem.

Regards,
Sigurd Hellesvik

[WestCoastDaz]

Hi Sigurd, 

I am using the RTC to provide a UTC timestamp for each sensor reading.  This stored in RAM and then sent via serial to be downloaded by the customer.  When correcting a slow RTC with GNSS time we get discontinuity in the timestamps as time appears to jump backwards.  The requirements are asking for very accurate timestamps as they may be used in failure analysis. 

[Sigurd Hellesvik]

Can I interest you in Falsehoods programmers believe about time.

Related: https://xkcd.com/2867/

I suggest that you adapt your error analyses to allow for time to flow backwards occasionally.

I guess in the end it is a question of "how acurate do you need?" and "how often does it happen"? To decide if it is can be improved or if is "just something you have to deal with when working with time"

[WestCoastDaz]

Sorry for the delay,  we need time to be quite accurate in our system.  

[hmolesworth]

If using an interrupt to increment the timestamp you can do the same kind of adjustment as (say) temperature calibration. The trick is very slow adjustments as you note in the post.

    else if (NRF_RTC1->EVENTS_TICK == 1)
    {
        NRF_RTC1->EVENTS_TICK = 0;
#if defined(V_AND_V_TEST_CODE)
        nrf_gpio_pin_toggle(TICK_EVENT_OUTPUT);
#endif
        // 8Hz tick gives us 17 years operation using 32-bit unsigned timers
        // Coin cells are unlikely to last that long, maybe a year or more
        if (mAdjustTickCount < 0)
        {
           // Slow down, don't increment the 125mSec tick
           mAdjustTickCount++;
        }
        else if (mAdjustTickCount == 0)
        {
           // Spot on, usual situation
           mRTC_PacketSampleTimer++;
        }
        else
        {
           // Speed up
           mRTC_PacketSampleTimer+=2;
           mAdjustTickCount--;
        }
     }