On Sat, 17 Apr 2021 14:14:03 -0700 (PDT), in
gmane.comp.hardware.beagleboard.user Mykolas Juraitis
<[email protected]> 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
--
For more options, visit http://beagleboard.org/discuss
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