[ntp:questions] Pi 4 and Ultimate hat weirdness

David Taylor davidtaylor at writeme.com
Tue Jan 26 11:06:41 UTC 2021

On 25/01/2021 16:33, Jim Pennino wrote:
> But the PPS has a lot of jitter. Note the xPPS(0) in the ntpq line.
[]> The default for NTP is 4800 baud, but I am running at 9600 and there is
> no loss of data.
> It doesn't make any difference if I run the 20 driver in mode 1 (lower
> bits) or 13, it still has the uncorrectable offset problem.
> Right now, having been running for a little over 12 hours, ntpq shows:
> x127.127.20.0    .GPS.            0 l   14   16  335    0.000  -761.21 307.138
> *    .SHM.            0 l   12   16  377    0.000  -30.575  35.029
> o127.127.22.0    .PPS.            0 l    3   16  377    0.000   -0.378   0.034
> I'm going to let it run for several days without disturbing anything.
> I doubt it will keep the 28 clock selected with that high jitter.

If the PPS has a lot of jitter I would try to check it with an
oscilloscope.  Possibly there is a poor connection, or if you are using
a very long cable (unlikely) there could be capacitive coupling between
signals.  How did you correct the problem for your "over 12 hours" ntpq

I would suggest a much higher baud rate, and disabling sentences other
than $GPRMC.  You don't then need any lower bits set.

Are you sure that the type 20 and type 28 driver can co-exist?  I note
that minicom can't access ttyAMA0 when gpsd is using it.

 From data I'm recording from another GPS I see offsets between -100 and
+100 milliseconds from a nominal, with an older Skylab SKG16B device
running at 9600 baud.

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