Re: [chrony-users] Debugging PPS messages in chrony

[Rob Janssen <chrony-users@xxxxxxxxx>]

    Morten,
    
    Ok I overlooked that GPIO, I normally use RS232 ports (DCD input)
    and then ldattach is required.
    For GPIO you will need to do a different setup.
    
    It is best to use PPS only with a "lock" directive like this:
    
    # PPS via /dev/pps0
    refclock    PPS /dev/pps0 refid PPS trust prefer lock THUN
    
    # SHM clock from thunderbolt running with SHM0 parameter
    refclock    SHM 0 poll 6 refid THUN noselect
    
    (in my case the SHM segment is "connected" to a Trimble Thunderbolt
    GPSDO hence the name THUN)
    
    The "lock THUN" will indicate to chrony that it should only use the
    PPS refclock when the THUN refclock
    is locked, i.e. when it has already locked to absolute time.
    Of course when you try to debug it you issue the command "chronyc
    sources" to see what inputs are
    received and if they are locked.  You can look under the "Reach"
    header for the 377 which means the
    inputs are seen and valid, and in the second column a * shows the
    source is locked.
    
    I hate systemd so I normally work around it.  The original initd
    could run a script to start deamons and
    it could do several things in defined sequence.  So I normally use
    my old scripts and start them from
    /etc/rc.local.
    systemd people say that this is bad, so probably you will find one
    that can explain you how it is
    to be done with systemd.
    Of course when debugging you can just start the programs from the
    prompt and at least see if
    the configuration works.
    
    Rob

On 2024-07-08 18:51, Morten Nissov wrote:

Rob,

I know that an absolute source of time information is necessary, I just figured there would be some indication of PPS being received by chrony. Note, this is not actually a GPS, and the PPS line is going to a GPIO pin, as a result I do not believe ldattach is necessary following http://paul.chavent.free.fr/pps.html.

I don't entirely understand the last point, my understanding was that it is chrony who creates the SHM/socket depending on the configuration, in this case how can I start the SHM program before starting chrony? The motivation of the original question is that without some confirmation that at least the PPS is received, it becomes very difficult to debug with PPS+SHM simultaneously, if this doesn't work immediately (which it didn't). Do you have any suggestions for how to proceed?

Best regards,

Morten

On Mon, Jul 8, 2024 at 6:06 PM Rob Janssen <chrony-users@xxxxxxxxx> wrote:

Morten

You cannot have only a PPS source, because that does not give you absolute time information, only modulo-1-second information.
So when you use PPS you always need to add another source.
That can be a network source (NTP) with server or pool, or you can use a serial port (NMEA) to the GPS device.
When you have internet or the PPS source provides NTP, it is preferable to use that.
It is also possible to use SHM for that, but you need another program that puts the time info into SHM.
Furthermore, when you start things using systemd it is tricky because systemd provides no sequence of starting
daemons, and you have to start the program for SHM and the pps (ldattach) BEFORE starting chrony!

Rob

On 2024-07-08 10:07, Morten Nissov wrote:
> Hi all,
>
> I am looking at setting up a relatively custom thing for timekeeping, which attempts to function as a GPS (in terms of PPS + unix stamp) using an RTC and microcontroller.
>

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