RE: [chrony-dev] pps source is still marked as synchronised when no absolute clock sources are available

["Hattink, Tjalling (FINT)" <T.Hattink@xxxxxxxx>]

>On Wed, Jan 20, 2010 at 04:36:17PM +0100, Hattink, Tjalling (FINT)
wrote:
>> I agree with you that chrony will be able to keep time between the
pps 
>> pulses. But in my situation I cannot blindly trust the incoming PPS 
>> signal being the correct time. When the GPS card has no lock it keeps

>> generating PPS pulses from its own oscillator, but it will be free 
>> running and drifting fast as no GPS corrections come in. So somehow I

>> have to tell chrony not to trust the PPS source anymore when GPS lock

>> is gone. I used to do this for ntpd by making the SHM clock 
>> unreachable, but it works differenty in chrony.
>
>I agree that sometimes it may be dangerous to keep using PPS signal
without
>any reference, but sometimes it's useful. When local stratum is
enabled,
>you can even use PPS signal without any reference and correct the
seconds
>by wristwatch if needed.
>
>Ideally, this would be configurable. Perhaps a refclock option to bind
it
>with another one. This would also allowed to use PPS signal immediately
>without waiting for first sync.

This would be a really nice solution. It gives the user full control in
how the PPS should be treated. I can bind the PPS signal to the SHM
reference clock, which makes it clear to chrony that the PPS signal
should only be trusted when the SHM clock is trusted. If no binding is
made chrony should always trust the PPS signal.

>
>> If you would like to always trust any incoming PPS signals, even if
no 
>> absolute reference source is available, maybe I can solve it by 
>> implementing a chronyc command to disable specific reference clocks.
>
>Suspend/resume refclock commands would be certainly useful also in
other
>cases.
>

I was already wondering why there were no commands in chronyc for
configuring the reference clocks, as there are available for ntp
sources.

--
Tjalling Hattink

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