[ntp:questions] Odd (mis)behavior when reference clock fails

Unruh unruh-spam at physics.ubc.ca
Wed Sep 24 18:10:24 UTC 2008

oberman at es.net (Kevin Oberman) writes:

>> From: Martin Burnicki <martin.burnicki at meinberg.de>
>> Date: Wed, 24 Sep 2008 09:24:43 +0200
>> Sender: questions-bounces+oberman=es.net at lists.ntp.org
>> Kevin,
>> Kevin Oberman wrote:
>> [...]
>> > Another thought...could it be PPS that is causing it? After all, the pin
>> > on the bulkhead connector is still getting the PPS signal. I am using the
>> > kernel PPS implementation, so could that be training the kernel even
>> > though ntp is not using it?
>> That's also what I've got in mind when I read you latest posts.
>> Can you disconnect the PPS signal and see what's happening?


>We have a winner! It is the PPS. If I take that out, it syncs correctly
>to all of the other systems.

>Looks like PPS will train whatever sync source is selected, not just the
>reference clock. So it was reference clock drifting off time with no
>input signal, marking the time as inaccurate so that ntpd was ignoring
>it, but still sending out the PPS such which the system was still
>listing to via the kernel NTP_SYNC, but was training the clock without
>paying any attention to the validity or presence of time from the
>reference clock.

I at least am confused. What is generating the pps signal. I would have
thought it was the hardware clock that you say is misbehaving. If so it
should not send out a PPS signal at all. Or is it your computer itself that
is sending out a PPS based on its own clock? In that case you certainly
should NOT be using it as a source of time. 

>It looks like ntpd should be disabling PPS_SYNC when the reference clock
>is bad, but is not doing so. Note: I am referring ONLY to the kernel

If the reference clock is bad it should not be sending out a PPS. Why is it
doing so?

>using PPS_SYNC. ntpd, itself seems to not pay attention to PPS unless
>the reference clock is selected for sync.

>If I get some time, I'm going to look at the PPS code in ntpd and see it
>this can be done easily.

If that pps is really not a good pps source coming from an idependent
harware time source, it should not be enabled at all. 

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