Re: PPS stopped once => stopped forever?

[Gary E. Miller]

Yo Ulrich!

On Tue, 5 Nov 2024 09:26:02 +0000
"Wielant, Ulrich" <U.Wielant@leonardogermany.com> wrote:

> > > This means gpsd will never forward the PPS signal to chrony/ntp
> > > if the GPS receiver is started after gpsd.  
> >
> > Correct.  Don't do that!  
> 
> That would be good. But reality is sometimes very different.

gpsd certainly knows that reality is not as expected.

> How does gpsd handle USB devices?

Depends on your OS/Distro, and how you have configured gpsd.
The very few GBSS USB that has PPS are almost all configured
statically on the command line, like:
        gpsd -n /dev/ttyUSB0

> I haven't tried them but they are
> started and connected with hotplug capability. Is gpsd not capable to
> use them as a time source?

gpsd can use USB GNSS as time source, but the jitter with only NMEA
is a few hundred milli seconds.  If you happen to get the rare PPS unit
then you can get jitter around 1 milli second.  Barely acceptable
as a strtum 1.

> > No.  And trying to do so would be a bad idea.  If you are doing
> > timekeeping you should start gpsd with the time source on the
> > command line.  And that source had better be very reliable.  
> 
> chrony and ntpd are fine and designed for alternating time sources.

Yes, which is why you feed gpsd in chronyd or ntpd.

> So gpsd should also be fit for it: you never have a 100% reliable
> source.

And gpsd fits well in that role.

> > As a general solution no.  Hard enough to get PPS right without
> > trying to handle error conditions that should never happen.  
> 
> "Should" - but it happens - you cannot ignore that fact.

Fix your hardware problems.  Only so much we can do with your
broken hardware.  Have you looked at the things I asked you to look at?
Checking the logic voltages, checking the gpsd logs for checksum errors,
etc?

RGDS
GARY
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Gary E. Miller Rellim 109 NW Wilmington Ave., Suite E, Bend, OR 97703
        gem@rellim.com  Tel:+1 541 382 8588

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