[ntp:questions] PPS without a peer

Ulrich Windl wiu09524 at pc5234.rz.uni-regensburg.de
Mon Aug 18 14:40:05 UTC 2003

Jeff Mock <jeff-ml at mock.com> writes:

> I am working on an application using NTP where a  machine sits in
> an environment without access to the Internet (cable TV head end).
> The machine does not need to be synchronized to absolute time, but
> it does need to be synchronized an external PPS source so that the 
> passing of time happens at the precision of the PPS source.
> The PPS source is good to better than 1ppm, the internal clock on the 
> machine might be 100ppm or worse.
> I would like to use NTP to synchronize the computer clock to the PPS
> signal, but I'm having some problems.  I'm using a Linux machine
> with a 2.4.20 kernel patched with the ppskit.  I have the PPS signal
> working properly on the DCD pin of a serial port.
> The problem seems to be that the PPS driver needs a "prefer" driver to 
> lock before enabling the PPS driver.  I was initially hoping the 
> the guts of my ntp.conf file might by as simple as:
>     # PPS on DCD serial port pin
>     server minpoll 4 maxpoll 4
> This doesn't work, ntpd never uses the PPS information because there
> isn't a peer driver to lock to first
> My second clever idea is to use the local clock discipline as the 
> peer.  ntpd will lock to itself and then switch to the PPS driver.
> Something like:
>     # local clock
>     server prefer minpoll 4   
>     # PPS on DCD serial port pin
>     server minpoll 4 maxpoll 4
> This doesn't really work either.  It seems there is some prioritization
> of clock sources going on inside ntpd and it always seems to prefer
> the local clock and doesn't switch over to the pps clock.
> This situation might be complicated since the epoch of a second for
> the local clock is probably unrelated to the epoch of a second of the 
> PPS signal.  The PPS signal might be +/- 0.5s relative to the computer
> clock when ntpd starts.

You can "fudge" the offset of the ATOM peer, or you can set an offset
via PPS API (or ppsctl supplied with PPSkit).


> So, I'm looking for some help.  Maybe some clever configuration wisdom
> or maybe some insight on how to hack ntpd so that it will lock up to
> a PPS source without requiring a peer.
> thanks,
> jeff

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