[LinuxPPS] LinuxPPS comparable to FreeBSD here

[Hal V. Engel]

On Monday 21 June 2010 04:54:07 am Miroslav Lichvar wrote:
> On Fri, Jun 18, 2010 at 11:01:41AM -0700, Hal V. Engel wrote:
> > This was running on my machine for about 12 hours starting yesterday
> > afternoon.  I am using the Oncore driver with a UT+.  It took it awhile
> > (perhaps 10 minutes) to not show errors for ntp_adjtime() and
> > ntp_gettime() when running ntptime.    After about 12 hours the clock
> > still had offsets of 2 to 3 microseconds and it never got below 2
> > microseconds at any time when running ntpq -p.
> >
> > # ntptime
> > ntp_gettime() returns code 0 (OK)
> >   time cfc61292.26fac5f4  Fri, Jun 18 2010  8:43:14.152, (.152264848),
> >   maximum error 3236 us, estimated error 2 us
> > ntp_adjtime() returns code 0 (OK)
> >   modes 0x0 (),
> >   offset 0.000 us, frequency -38.732 ppm, interval 256 s,
> >   maximum error 3236 us, estimated error 2 us,
> >   status 0x2107 (PLL,PPSFREQ,PPSTIME,PPSSIGNAL,NANO),
> >   time constant 4, precision 0.001 us, tolerance 500 ppm,
> >   pps frequency -38.741 ppm, stability 0.021 ppm, jitter 2.606 us,
> >   intervals 272, jitter exceeded 9, stability exceeded 0, errors 1.
> >
> > But there appears to be some issues with hardpps.  When I run
> > without hardpps this system will have offsets of less than 1
> > microsecond most of the time once things have stabilized (IE. after
> > about 1 hour) if it is lightly loaded.
> 
> If the PPS signal really has 2.6us jitter, I think it's working as
> expected. FLL doesn't filter noise, so the result will be only as good
> as is the input signal.
> 

The Oncore itself should have a jitter that is on the order of +-50 
nanoseconds and after correcting for sawtooth this should be less than +-10 
nanoseconds.   I suspect that the 2.6us jitter is related to the variations in 
the latency of the interrupt handler rather than the refclock itself.

Hal