[ntp:questions] TXCO derived PPS and Atom PPS Clock Discipline driver

Terje Mathisen "terje.mathisen at tmsw.no" at ntp.org
Mon Oct 25 06:58:46 UTC 2010


Maarten Wiltink wrote:
> "Maarten Wiltink"<maarten at kittensandcats.net>  wrote in message
> news:4cc43626$0$81474$e4fe514c at news.xs4all.nl...
>> "Nero Imhard"<nim at pipe.nl>  wrote in message
>> news:4CC40319.7060302 at pipe.nl...
>
>>> [...] A TXCO is a frequency source, and not
>>> a time source. ntp's PPS drivers expect a time source. Each pulse
>>> should be precisely aligned with the start of a UTC-second.
>>>
>>> If your PPS pulses are not aligned with UTC, you cannot use the signal
>>> as a time source!
>>
>> Not quite true. If you where in the second the pulse falls, you can.
>
> Curses. If you *know* where in the second the pulse falls, you can.

Indeed.

Really high-perf clock sources, like the Motorola Oncore UT+, have 
configuration commands that allows you to offset the PPS from the top of 
the second, on the assumption that many other programs will tend to wait 
for that moment in time in order to do som processing:

By moving the PPS pulses a few hundred ms to the side, the host cpu is 
more likely to be idle at the time of the interrupt.

(The Oncore driver will of course adjust the received timestamps by the 
same amount, so NTPD will never notice the offset.)

Terje
-- 
- <Terje.Mathisen at tmsw.no>
"almost all programming can be viewed as an exercise in caching"




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