[ntp:questions] problem with ntpd refclock and pps via parallel port

unruh unruh at wormhole.physics.ubc.ca
Thu Feb 4 00:39:38 UTC 2010

On 2010-02-03, David Lord <snews at lordynet.org> wrote:
> Hal Murray wrote:
> There's also another problem I have to resolve and that is

ntpd has about a 1 hour half life (it takes an hour to correct the 
error level by 1/2) for short polling intervals, longer for longer.
If you hav3 a refclock you are polling at about poll level 4 ( which has
the 1 hr time scale). the only way ntp knows something has happened is
that the drift changes the time slowly, and ntp then slowly changes the
drift rate to try to bring things back into line. that all takes a long

> ntpd possibly not keeping up with temperature changes, giving
> a square wave variation of about +/- 1ms in offset, lately
> during very cold spell, with main period of about 24 hr.
> Otherwise with lesser temperature variation, the offsets ramp
> up/down to around +/- 300us corresponding roughly to heating
> system switching on/off. That's going to require experimenting
> on a spare system with addition of heater to crystal, or if I
> bring one of old 486dx back into life, possibly by swapping
> jumpers to use an external clock source.

Three possibilities. 
a) if you run linux/bsd, run chrony. It corrects for temp drifts much
much faster. 
b) put the computer into a temp controlled box or install a temp
controlled crystal.
c) get the "temp" patc hfor ntp, which uses an onboard temp sensor ( eg
the motherboard temp sensor) to correct for the temp variations ( does a
fit to the temp vs rate and then uses that to correct for the changes in

> David

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