[ntp:questions] best jitter on qnx always = 0.977
nowhere at mail.ru
Wed May 11 04:46:44 UTC 2011
My BSP gives right nanoseconds from ClockCycles since last 1ms interrupt. So
It's far more precise than 1ms.
How can I convince ntpd that I have higher presision ? There's a define in
#define ADJUST_PERIOD 0.97 /* [s] */
May be I should lower it? Or is there another way?
2011/5/11 Dave Hart <hart at ntp.org>
> 2011/5/10 Николай Орехов <nowhere at mail.ru>:
> > Well, I'm using PPS driver which measures ticks with presision more than
> > 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
> > 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.
> Dave Hart
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