clock_gettime sometimes returns the wrong time (seconds in the past)

[inge2]

Hello,

I've got an issue with the clock_gettime function.

When I set the RTC time to a known hour (00h11h00s in my case), I don't read the same value immediatly after, I get 00h10m59s.

Does someone has already encounter this issue? Is there a better way to update the RTC value, avoiding this drift (in the past)?

To set the RTC value:

I'm using timeutil_timegm64() to convert POSIX date time into a timestamp and clock_settime with the CLOCK_REALTIME to set my new value.

To get the RTC value:

I'm using clock_gettime with the CLOCK_REALTIME to get the timestamp.

Then I convert it into POSIX date time with gmtime_r().

I'm using a nFR52833 devkit, with Zephyr toolchain nRF Connect SDK V1.9.1

[hmolesworth]

Outside of Zephyr, I use repeated sampling to ensure no rollover issues; one would expect Zephyr does something similar, but perhaps not.

// 64-bit unsigned RTC time: typically number of 125 millisecond ticks from Jan 1st 2024
static volatile uint64_t mRTCTimer;

uint64_t getRTCTime(void)
{
    // Ensure no rollover issues as we cannot be sure this instruction is atomic
    uint64_t timeLatch;
    do {
        timeLatch = mRTCTimer;
    } while (timeLatch != mRTCTimer);
    return timeLatch;
}