Re: Issue with RTC and PPS

I know that RTC is not ideal and it is not GPS, but it is necessary. I know
for sure that it will take some time to lock the GPS signal since the
booting of the RPi (few minutes). Not only this, I know that I can lose GPS
connection for a long period of time, and that is why the *rtcsync *directive.
In my mind the RTC should be used by chrony during this blackout periods

Also the RTC that I am using is 3.5 ppm, so it should be 1s over few days

----------------------------------------------------------------------------------
Dr. Gabriele Coppi
Marie Curie Skłodowska Fellow,
Department of Physics, Università degli Studi Milano-Bicocca
https://gabrielecoppi.github.io/
Phone Number: +39 0264482356



On Mon, Nov 14, 2022 at 5:27 PM Bill Unruh <[email protected]> wrote:

> I am confused. RTC is NOT GPS. RTC is an onboard (or for the RPI I guess a
> supplimentary board) which chrony uses ONLY at startup to set the intial
> system clock to approximately the correct time. It tends to drift badly --
> 1000PPM might be good (That is about a second per hour).
>
> PPSis pulse per second. It carries zero time information except that when
> the
> pulse is sent, it is exactly (towithin nanoseconds) on the second.It needs
> some other clock to tel it what the second is. That other clock is either
> the
> from your GPS receiver or some other clock. Otherwise it assumes that the
> time
> from the system clock is correct for the labeling of which second the
> pulse is
> assocated with. (This may well be fine if the GPS went down for a while,
> obviously not so good if the computer switched off. ) Many people use gpsd
> to
> get the seconds label from the GPS sattelite. (that may be what your GPS
> source is delivering).
>
> You will have to find out if your pps is actually delivering pulses. Look
> at
> /sys/devices/virtual/pps/pps0/assert to see if it is increasing its pulse
> count (you will also have to determine whether your second pulse occurs on
> assert or clear-- probably assert)
>
>
>
>
>
>
> William G. Unruh __| Canadian Institute for|____ Tel: +1(604)822-3273
> Physics&Astronomy _|___ Advanced Research _|____ Fax: +1(604)822-5324
> UBC, Vancouver,BC _|_ Program in Cosmology |____ [email protected]
> Canada V6T 1Z1 ____|____ and Gravity ______|_ www.theory.physics.ubc.ca/
>
> On Mon, 14 Nov 2022, Gabriele Coppi wrote:
>
> > [CAUTION: Non-UBC Email]Hello,
> > I am currently using chrony (v4.0.8) on a RPi4. Due to my application, I
> need to use only GPS, PPS and RTC
> > for time keeping. The RTC is necessary to preserve the time when GPS
> coverage drops or at boot when the GPS
> > lock is not immediate.
> >
> > I have this current chrony.conf directives (other that standard ones)
> >
> > refclock PPS /dev/pps0 refid PPS trust lock GPS
> > refclock SHM 0 refid GPS precision 1e-1 offset 0.3 delay 0.2 noselect
> > rtcsync
> > rtconutc
> >
> > where  the first two are defining my sources and the second should take
> care of the RTC. I would like to use
> > the rtcfile directive to have chrony to take care of everything but it
> seems that Rasbian Kernel is built
> > without support for RTC_UEI_ON.
> >
> > Now I have the following problems, when I shutdown/reboot the RPi:
> >
> >  1. it seems that chrony never selects the PPS source, and given that
> GPS is marked as noselect, I don't have
> >     any GPS timing
> >  2. any RTC support is outside chrony, but as said I would like to track
> it with chrony
> > I specify that chrony is run as daemon at startup with the -s flag
> >
> > Any ideas?
> >
> >