[LinuxPPS] LinuxPPS 5.3.2 strange behaviour

[Cirilo Bernardo]

On Tue, Sep 30, 2008 at 8:00 PM, George Spelvin <linux at horizon.com> wrote:
> I've been trying to get linuxpps 5.3.2 working.
>
[snip]
> - Stopping ntpd causes assert to keep incrementing, but clear rarely if
>  ever increments.  (Note that it's a 10 us pulse, and this would be expected
>  from 10 us interrupt latency missing the "clear" edge.)
>  This produces output like:
>        assert: 1222762974.001136739#219
>        clear:  1222762963.001112475#112
> - Restarting ntpd stops them again.
[snip]

10us is a very short pulse; the second edge may occur even before the
first edge was processed, so the interrupt is missed.  You need
circuitry to extend the length of the pulse but without introducing
too much variance in the propagation of the edge used for timing; a
typical GPS would have the pulse extended to about 4ms.
Alternatively, if it is the 'assert' edge which you need for timing,
just ignore the 'clear' edge altogether; the ntpd config file can be
used to select 'assert' or 'clear' events.

- Cirilo