I have a Raspberry Pi 2 model B and a RTC (precision 1 ppm) hooked up with, without Internet conneciton. When the RPi is power off is the RTC the responsible to keep the current hour but when the RPi is power on is the system clock which keep the hour.

But, how many ppm has the system clock in the RPi? If the RTC is more accurate than the system clock, is it better to set the RTC hour every x time to the system clock, or does ir reduce the performance of the device?

Thank you in advance

  • the raspberry pi's system clock definitely is much less accurate than 1ppm. Nov 12, 2018 at 19:23
  • performance of the device .... which device?
    – jsotola
    Nov 12, 2018 at 19:42
  • the device is refered to the Raspberry Pi And if the raspberry pi's system clock is much less accurate than the RTC (in this case 1 ppm), will be possible to communicate the system clock with the RTC each 'x' time to synchronize? Or does it take a lot of time to do it?
    – user940969
    Nov 13, 2018 at 7:26

1 Answer 1


To update the system time from the hardware clock you can use hwclock, for example:

rpi ~$ hwclock --hctosys

Maybe you have to use sudo. I can't test it. I don't have a hardware clock. You can use a sysstemd.timer unit or a cron job to execute it repeatedly. I think this command does not take much performance if you execute it for example every 12 hours. Depending on the precision you need you can reduce the time intervall for updates. The shorter the more performance it is consuming but it is most from the context switch of the process for executing the command. Updating every hour should not make any harm. Updating every minute may be possible but updating every second would not make sense for me. It is up to you to find a reasonable compromise. You can look what time the command is consuming in real, user and sys context with:

rpi ~$ time hwclock --hctosys

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