| Re: [chrony-users] gpsd, pps and chrony |
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On Mon, Apr 04, 2011 at 10:02:41AM -0700, Bill Unruh wrote:
> >Oh, my freetime project at the moment is to try and get the GPIO
> >interrupts working as a kernel PPS module on the Alix board (hoping to
> >get higher accuracy than using a serial port...) This coupled with an
> >old Garmin 18 appears to allow for the possibility of a nice Stratum 1
> >time server for around the $100 ish mark... Could be cool! Will keep
> >you informed how it goes...
Cool, I'd like to see how much will the jitter improve with GPIO and
if it'll be better than the Soekris computer
(http://phk.freebsd.dk/soekris/pps/).
> You still get
> usec level wander (ie it takes about 1-2 usec for the system to service the
> interrupt and do the time stamping-- tested by having a program generate an
> interrupt by putting out a signal on a parallel pin to the parallel interrupt
> line (ACK) timing when that output went out and then having the interrupt
> service routine timestamp the incoming interrupt.) but that is pretty good. A
> Garmin 18 is not capable of better than that anyway.
Isn't that a delay in the parallel port hw rather than a sw delay? I
think with GPIO the interrupts go straight to the CPU, reducing the
delay significantly.
> For a cheaper version, get the sure GPS board. It is supposed to give about
> 20ns timing on the PPS line, rather than garmin's 500-1000ns
Someone reported on the ntp list that Garmin 18x is stable to 60 ns,
not sure about the older version.
--
Miroslav Lichvar
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