Re: Linux system time with NTP/Chrony but no PPS

[Gary E. Miller]

Yo Mike!

On Thu, 8 Sep 2022 15:57:53 +0100
Mike Tubby <mike@tubby.org> wrote:

> Perhaps I wasn't quite clear enough in my previous post... but
> firstly some background - I run two NTP Stratum 0 clocks over at:
> 
>      ntp0.thorcom.net
>      ntp1.thorcom.net
> 
> which are in the UK and also run many clocks on Emergency Services 
> customer sites.

I assume they are PPS with multiple chimers configured.

> We use to use Trimble GPS and the 'parse' driver 127.128.8.0 in
> Trimble TSIP mode:
> 
> # Local Trimble TSIP Reference Clock
> server 127.127.8.0 mode 10

If you are useing that driver, why do you need gpsd?

> (which incidently I had to debug and fix because it was originally
> Big Endian only and written for Sun Workstations or some such similar
> and was buggy on Intel).

Yup, not well maintained, another reason to avoid it.

> We switched to using GPSD several years ago. My Trimble GPS receivers 
> have PPS on the RS232 DCD pin and this was a good fit with GPSD as it 
> "just works"(tm) for our Reference Clocks. So for our big systems we 
> have Trimble GPS with PPS, GPSD with PPS, NTPD and NTPD syncs with 
> SHM(1) refid".PPS."

Good.

> This is our current setup on our Reference Clocks:

> # GPS Serial data reference (NTP0)
> server 127.127.28.0 minpoll 4 maxpoll 4
> fudge 127.127.28.0 time1 0.0 flag1 1 refid GPS

For faster startup, this should be last in the file.

> # GPS PPS reference (NTP1)
> server 127.127.28.1 minpoll 4 maxpoll 4 prefer
> fudge 127.127.28.1 flag1 1 refid PPS

For faster startups, this should be first in the file.

> *SHM(1)          .PPS.            0 l   14   16  377    0.000 +0.004 0.017

Could be a tad better, I assume that server has other loads interferring.

> However that is all well and good, but our new requirement is
> different:
> 
>   * mobile device running Yocto Dunfell on an NXP iMX6 @ 1000MHz
>   * internet over 4G (could could be unreliable)
>   * uBlox MAX-M8W connected via serial but without PPS
>   * GPSD 3.24
>   * NTPD or Chrony - to be decided

The no PPS is not so good...

> I have never used GPSD without PPS.  The documentation over at:
> 
> https://gpsd.gitlab.io/gpsd/gpsd-time-service-howto.html
> 
> implies that PPS is part of the solution

Then it is not worded strongly enough.  Without PPS, the time from the
GPS is not good.  Maybe +/- 300 ms.


> and we don't have PPS on
> this mobile platform.

So fix that.

> 
> So I'm looking to confirm that I can use:
> 
>      uBlox MAX-M8W (no PPS) -> GPSD (SHM0, no PPS on SHM1)

Just uses your existing config.  ntpd will see no PPS, then select one
of your servers.

-> NTPD or
> Chrony

Depends on your use case.

> initial tests did not work as expected and Chrony responded with all 
> zeros rather than working "out of the box", but we're changing
> multiple things on an embedded system under Yocto - which is hard
> work - hence the request for hints or config snippets.

Chrony should work fine, but without seeing your config hard to say what
is wrong.

RGDS
GARY
---------------------------------------------------------------------------
Gary E. Miller Rellim 109 NW Wilmington Ave., Suite E, Bend, OR 97703
        gem@rellim.com  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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