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Pi configuration for Real time clock

colporteur

What is the optimum/ideal Pi configuration setup to trigger a refresh of system clock from the RTC on reboot?

 

The RTC post raised in the Pi 4 discussion provided motivation to ask the question.

 

I just discovered a time sync loss between the RTC and the system clock after the Pi was left powered off for three weeks. On boot up the system clock had a time based on or around the time the unit was powered down. The RTC closely reflected the current time.

 

What is the link between system clock and RTC on bootup? I placed the following command hwclock --hctosys in /etc/rc.local to seed the system clock with the hwclock time on boot. This seems like a kludge. I assumed an RTC installation would provide a mechanism to ensure this happens. I haven't found anything that closely resembles that I am looking for.

 

This adafruit setup https://learn.adafruit.com/adding-a-real-time-clock-to-raspberry-pi/set-rtc-time  doesn't provide any reboot configuration details for Jessie but Wheezy it does make reference to /etc/rc.local

 

The RTC is working satisfactorily. The system clock is working satisfactorily. I only notice the issue of time sync loss after the Pi was powered down for an extended time.

 

What are your solutions to sync'ing the clocks?

 

BoM

  • Pi3B+
  • Raspbian 2019-04-08
  • RTC DS3231

 

Sean

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  • 0

    Hello Sean,

     

    In the past as long as the time zone was set in raspi-config the RPi would automatically update the date/time on boot after an Ethernet interface is brought up -if connected to the internet.

     

    The ntp clock update could be forced-sync by doing:

    sudo /etc/init.d/ntp stop
sudo ntpd -q -g
sudo /etc/init.d/ntp start

     

    Few years have passed and my understanding is that is now done differently. Now ntp update is handled by the systemd-timesyncd daemon.

    In summary, sudo timedatectl set-ntp True activates the ntp update. To check the status timedatectl status

     

    If a manual force-update doesn't work, maybe a firewall is blocking the NTP. Is also worth checking the timezone configuration

     

    Luis

  • 0 in reply to luislabmo

    Thanks for the response.

     

    If I use the Adafruit posting as a reference, the Wheezy section /etc/rc.local is used to sync system time by drawing from the RTC. That would have been with NTP in play.

     

    I recall in my NTP days, NTP daemon was not capable of syncing time if the system time was >1000sec off from the NTP sourced clock. An ntpdate was required to get the clock close and then NTP could kick system time in the pants when needed.

     

    systemd-timesyncd.service uses NTP to draw time on boot of the Pi. A last system clock value is stored on shutdown that is also used.

     

    I speculating, if the system clock is to far a skew on boot up, does the same limit apply to system clock that  NTP can't correct it.

     

    The scenario of the time being out of sync when the Pi has been turned off for an extended period of time has happened more than once. It took me a while to twig to the condition. The last time it happened I noticed because I had just worked on the unit a short time ago.

     

    Sean

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  • 0 in reply to luislabmo

    Thanks for the response.

     

    If I use the Adafruit posting as a reference, the Wheezy section /etc/rc.local is used to sync system time by drawing from the RTC. That would have been with NTP in play.

     

    I recall in my NTP days, NTP daemon was not capable of syncing time if the system time was >1000sec off from the NTP sourced clock. An ntpdate was required to get the clock close and then NTP could kick system time in the pants when needed.

     

    systemd-timesyncd.service uses NTP to draw time on boot of the Pi. A last system clock value is stored on shutdown that is also used.

     

    I speculating, if the system clock is to far a skew on boot up, does the same limit apply to system clock that  NTP can't correct it.

     

    The scenario of the time being out of sync when the Pi has been turned off for an extended period of time has happened more than once. It took me a while to twig to the condition. The last time it happened I noticed because I had just worked on the unit a short time ago.

     

    Sean

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