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

From: Mike Simpson
Subject: Re: gpsd time service: PPS-synchronised SHM samples rare, both sources gradually less and less frequent
Date: Sat, 19 Sep 2020 16:40:50 +0100


Just to let you know that if you disable GLONASS then the 4second pps offset 
will disappear. 

I kept getting that offset with my ublox-8 running protocol version ?10 and 
disabling GLONASS *after* GPSD startup using ubxtool worked a treat. 

Now I just need to remember to do it after each reboot. 
I’m sumosing it’s because my GPS firmware is very old but I don’t run any 
Windows so can’t run the proprietary ublox software to update it. 


> On 18 Sep 2020, at 22:38, 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), 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, PPS signal looks like it should both
> on the oscilloscope (one 100-ms square pulse every second) and in
> ppstest (two events in short succession every second), and while so far
> there has been fairly sizeable offset on both sources (between 3 and 4 s
> on SHM 0, solid 4002 ms on SHM 1; will keep it running to see what
> happens) which resulted in them having been marked as false tickers, the
> jitter stabilised pretty quickly at 100 ms and 440 us, respectively.
> Native TTL + pps-gpio, on two different systems: unusable. Cheap USB 1.1
> serial converter: works fine, until something starts competing with the
> device for I/O bandwidth anyway. I honestly do not know what is going on
> here.
> -- 
> MS

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