Thank you for a comprehensive answer. I will look into external clocks.
On Sun, Apr 18, 2021 at 5:05 PM Dennis Lee Bieber
wrote:
> On Sat, 17 Apr 2021 14:14:03 -0700 (PDT), in
> gmane.comp.hardware.beagleboard.user Mykolas Juraitis
> wrote:
>
> >Hello,
> >
> >I am experimenting with rtcwake on BeagleBone Blue. Running command:
> >
>
> Note that there is no REAL RTC on most BeagleBone variants (I don't
> know if the Blue has an add-on RTC). cf:
>
> https://learn.adafruit.com/adding-a-real-time-clock-to-beaglebone-black/overview
>
>
> >I tried to check time before and after standby by running:
> >
> >sudo hwclock && date && sudo rtcwake -d /dev/rtc0 -m standby -s 20 &&
> date
> >&& sudo hwclock
> >2021-04-17 21:42:40.123908+00:00
> >Sat 17 Apr 2021 09:42:40 PM UTC
> >rtcwake: wakeup from "standby" using /dev/rtc0 at Sat Apr 17 21:43:02 2021
> >Sat 17 Apr 2021 09:43:06 PM UTC
> >2021-04-17 21:43:07.802256+00:00
> >
> >Last 2 time lines are printed after manual wake up by pressing power
> >button. They seem to be showing that only twenty some seconds passed but
> >actually much more time passed until I pressed power button. Which is
> weird
> >because if RTC doesn't work then I would expect time to be reset. If it
> >does work it should be correct time.
> >
>
> Beaglebones periodically save the system clock value, and use that
> to
> reload the system clock on startup. The value you are seeing is likely that
> of the last time this fake hwclock was written... cf:
> https://manpages.debian.org/jessie/fake-hwclock/fake-hwclock.8.en.html
>
> Also...
>
> https://unix.stackexchange.com/questions/187261/automatically-update-hwclock-at-boot
> https://groups.google.com/g/beagleboard/c/n1noGnM30sQ
>
>
>
> --
> Dennis L Bieber
>
> --
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