Re: [gpsd-dev] "Plain Jane" timing GPS is working!

[Gary E. Miller]

Yo Håkan!

On Tue, 3 Apr 2012 19:20:32 +0200
Håkan Johansson <address@hidden> wrote:

> Is it the pulse shape (i.e. length) or the pulse repetition that gpsd 
> rejects?  As I recall, a SiRf-III GPS delivers a PPS pulse that is
> only 1 us long, so does not record a full state change, but this is
> handled.

gpsd uses the leading edge of the shorter part of the pulse.  The
trailing edge is rejected as it is meaningless.  Unless the pulse is so
short that the kernel just logs the pulse.  The log messages call that
an 'invisible' pulse.  In that case we only have one event per second.

The case at hand is a 90/10 pulse, and the trailing edge is, of course,
rejected.  That is exactly the proper thing to do and it is working
as designed.

I should change the log message because people freak when they see all
the 'reject' messages.  Any suggestions?

> Timing over USB is distored by the inherent USB polling interval of 1
> ms, so could make the inter-pulse interval fail those tests.  (They
> actually allow +/-1 ms, so should make it?).  Logfile?

Could be, we'll cross that bridge when we get to it.
 
> Due to the USB effectively downsampling the measurement to the USB 
> 'polling clock', such a setup might be prone to seeing jumps.  I.e.
> if the USB polling frequency is close to a multiple of the PPS
> signal, then it for long times drifts slowly (which will not be
> detected), and then jumps one polling cycle.  (Possibly even usable.
> Unhandled it could make life miserable for chrony/ntp.)

Past experience shows this is not a problem.  If it crops up we'll
see how we can deal with it.

RGDS
GARY
---------------------------------------------------------------------------
Gary E. Miller Rellim 109 NW Wilmington Ave., Suite E, Bend, OR 97701
        address@hidden  Tel:+1(541)382-8588

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