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Re: gpsd time service: PPS-synchronised SHM samples rare, both sources g

From: Gary E. Miller
Subject: Re: gpsd time service: PPS-synchronised SHM samples rare, both sources gradually less and less frequent
Date: Fri, 18 Sep 2020 15:49:22 -0700

Yo Marek!

On Fri, 18 Sep 2020 21:36:04 +0200
Marek Szuba <> wrote:

> On 2020-09-18 14:39, Marek Szuba wrote:
> > 1. 'chronyc sources' shows no data AT ALL has been received from the
> > 'PPS' refclock. Moreover, the last time the 'GPS' clock sent
> > anything was just under two hours earlier. The refclocks log
> > confirms this - no PPS lines, and GPS lines ceased abruptly about
> > 40 minutes after start-up.  
> Following up on this, I switched from chrony back to ntpsec

Which will not help since you know that ppstest fails.  Always stop
at the first failure in the chain.  In this case that is ppstest.

> I wonder
> what causes gpsd to suddenly try and switch to 4800 bps.

gpsd will only try 4,800 if it loses input signal and goes to autobaud.
Which again points to input problems.

> Is there any
> way of forcing it to stay in specific serial mode?

The "-s" option.  But that only masks your hardware issue.

> Regarding the PPS signal, I have decided to bring out the big guns and
> connected the receiver to an oscilloscope. Result - unless I have
> wired something wrong, the PPS waveform has exactly the same shape as
> what the receiver sends out on the TX line... That can't be right,
> can it?

The voltage levels and rise/fall times will be identical.  Obviously the
data is different.

> The odd thing is, I have checked both the computer-side
> connectors and (just to be extra sure) the pins of the chip (whose
> label by the way says NEO-M8N after all) for short circuits and there
> seem to be none. Something wrong with the configuration of the
> TIMEPULSE signal?

TIMEPULSE can only ever be a pulse.  You can change the duty cycle, but
little more.

You have never saaid where your u-blox came from?  Chinese parts are often
fakes or factory rejects.  Maybe spend the $15 and get a second one to test?

Gary E. Miller Rellim 109 NW Wilmington Ave., Suite E, Bend, OR 97703  Tel:+1 541 382 8588

            Veritas liberabit vos. -- Quid est veritas?
    "If you can't measure it, you can't improve it." - Lord Kelvin

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