On Mon, Feb 28, 2022 at 8:46 AM Miroslav Lichvar <mlichvar@xxxxxxxxxx> wrote:
On Fri, Feb 25, 2022 at 12:10:40PM +0100, Gabriele Coppi wrote:
> I am having some issues running Chrony with only GPS+PPS (coming from the
> GPS) on RPi 4. At the startup gpsd starts running with no particular issue
> and I got GPS and PPS solution with no problem checking gpsmon. I have
> installed chrony to use the GPS and PPS as a time sources to keep the clock
> in sync with those. Compared to the default chrony config file I commented
> all the debian pool server and the rtcsync since the RPi does not have a
> RTC. I also added the following lines:
>
> refclock SHM 0 precision 1e-1 delay 0.5 refid GPS
> refclock SHM 1 precision 1e-7 refid PPS
You should remove the first source and just rely on the PPS-based
SHM 1, or alternatively use SHM 0 + PPS /dev/pps0. SHM 1 from gpsd is
sufficient.
I am using the SHM1 because the /dev/pps0 it is always reported as a falseticker. I checked with ppstest and I got out the signal, so I don't know why
> The chronyc sources command shows that after a settling time the PPS source
> is chosen as the best and the GPS is discarded. This situation is pretty
> stable for 17min and then gets out of sync for 17min and then gets in sync
> again for 17min. During the out of sync, so when both sources are
> discarded, the sources still have a reach of 377.
If you see both sources marked as falsetickers (x symbol) in the
chronyc sources output, that means they don't agree with each other.
If the offset of the SHM 0 is around 0.25s, small changes in the delay
would cause the behaviour you see.
Yes, the offset of SHM0 is always between 250 and 500ms.
--
Miroslav Lichvar
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