[ntp:questions] best jitter on qnx always = 0.977
nowhere at mail.ru
Tue May 10 18:11:41 UTC 2011
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.
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
2011/5/10 Dave Hart <hart at ntp.org>
> On Tue, May 10, 2011 at 16:22 UTC, Николай Орехов <nowhere at mail.ru> wrote:
> > Why on QNX I always get the same best jitter 0.977 even on PPS ?
> I see the same thing on a laptop running Windows Vista. This likely
> indicates ntpd measured your system clock precision as 2 ** -10
> seconds, or 1/1024th of a second. ntpd uses the precision of the
> local clock as the minimum jitter, because every local timestamp
> involved in its calculations is trusted only to the level of the clock
> Reading the clock probably costs less than a millisecond on your
> system, but the value read changes only once per millisecond. If you
> could convince QNX to provide a higher-precision system clock, you can
> drive the precision down and bring your jitter figures with it.
> Dave Hart
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