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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 15:53:57 -0700

Yo Marek!

On Fri, 18 Sep 2020 23:34:41 +0200
Marek Szuba <> wrote:

> On 2020-09-18 21:36, Marek Szuba wrote:
> > 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 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?  
> Yet another update: two hours ago I connected the same receiver (no
> factory reset, no reconfiguration) to the same system using a USB-TTL
> converter (with the PPS line connected to DCD),

You realize that the NEO-M8L does not output TTL levels?  Trying to
use TTL levels will cause weird issues.

> fired up gpsd and
> configured chrony to listen to SHM 0 and SHM 1. Result: ever since the
> received got the 3D fix chrony has been getting samples for both NTP0
> and NTP1 every 4 seconds at most,

Typical of what you see with slight voltage mismatch.

Until you fix that, no point running any program that uses the gpio
or serial lines.

> Native TTL + pps-gpio, on two different systems: unusable.

As expected.  No Raspberry Pi is TTL.  In fact, TTL can burn out gpio
pins.  I have done so.

You keep looking too far downstream, you problem is on your hardware inputs.

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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