[ntp:questions] best jitter on qnx always = 0.977

Dave Hart hart at ntp.org
Tue May 10 19:02:16 UTC 2011


2011/5/10 Николай Орехов <nowhere at mail.ru>:
> Well, I'm using PPS driver which measures ticks with presision more than one
> millisecond using ClockCycles and system interrupt.
> Even with ref clock jitter less than 1ms I got 0.977 jitter.
> I'm agree that system jitter is closely connected to presision of system
> clock. But reference clock jitter may be even lower.

If your PPS timestamps in fact are more precise than ~1 msec, the
excess precision is wasted.  ntpd uses system timestamps extensively,
and has no provision for using higher-precision PPS timestamps to
discipline a lower-precision system clock any better than if the PPS
precision were the same as the system clock precision.

> Can i somehow convince ntpd that this refclock jitter can be less than 1ms?
> Will it influence sync presision? For now i get offsets ranging between
> +-0.5ms

I don't believe so.  The behavior you see is as good as I would expect
given the clock precision.

Cheers,
Dave Hart



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