[time-nuts] PPS offset between GPS receivers

[Azelio Boriani]

Yes, this is a good test: to evaluate how your preferred uP can perform as
a time interval counter, you can hook two GPDSOs' PPS to it and see the
result. The best would be to have at hand also a real TIC (HP53132A,
PM6681, SR620 or similar) and compare.

On Sat, Dec 8, 2012 at 4:28 AM, Bob Camp <lists at rtty.us> wrote:

> Hi
>
> A lot depends on exactly what the interrupt structure is. It may also
> depend on the phase of the cpu clock relative to the pps signal. What's
> reasonably sure is that there is indeed some offset between the two where
> the answer is indeed "ft's" random. Another thing to check - how wide is
> the random region?
>
> Bob
>
> On Dec 7, 2012, at 9:19 PM, Chris Albertson <albertson.chris at gmail.com>
> wrote:
>
> > One more test to try.  Connect one PPS signal to both GPIO ports and see
> > how close to zero offset you get.   It would likely be random which gets
> > read first.
> >
> >
> > On Wed, Dec 5, 2012 at 5:37 AM, Gabs Ricalde <gsricalde at gmail.com>
> wrote:
> >
> >> Hi everyone,
> >>
> >> As Tom suggested, I redid the test with less than 1 ft. of wire from the
> >> PPS output to the GPIO without any logic gates or line receivers. Same
> >> result,
> >> the SKG25A1 was 2 microseconds ahead of the 58534A. Without any other
> way
> >> of
> >> testing, I would probably trust the output of the timing receiver more
> >> than the SkyNav module. Anyway the SkyNav board is an inexpensive unit
> and
> >> I wouldn't mind setting an offset in ntpd.
> >>
> >> I don't have a scope yet, and a low jitter PPS GPIO is the closest
> thing I
> >> have
> >> to a TIC.
> >>
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> >
> >
> >
> > --
> >
> > Chris Albertson
> > Redondo Beach, California
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