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Re: [gpsd-dev] PPS over USB

From: Gary E. Miller
Subject: Re: [gpsd-dev] PPS over USB
Date: Thu, 3 May 2012 13:42:58 -0700

Yo Hal!

I'm adding to the gpsd-dev list as this is of interest to many.

On Thu, 03 May 2012 00:27:28 -0700
Hal Murray <address@hidden> wrote:

> address@hidden said:
> > All packet flags are cleared at the start of each packet.  Then
> > PPSTIME_IS is set if the packet indicates a valid time.  Since it
> > is different in every driver you should check your fave driver if
> > it is not garmin or nmea. 
> Maybe it's a good thing I asked.  I couldn't find the code I was
> looking for.
> driver_nmea0183.c says:
>         if (session->fixcnt > 3)
>             retval |= PPSTIME_IS;
> That looks like what you described.
> I couldn't find the place that clears fixcnt when it gets a
> not-valid sentence.
> I did a grep fixcnt *.c
> Is there some other place I should have looked?  (I don't know my
> way around gpsd yet.)

Interesting, I could not find a clear fixcnt either.  So I added one
to git head.  I expect some corner case failures will appear as a result.

> There is another potential worm in this can.  GPZDA sentences don't
> have a not-valid field like most of the other sentences with time in
> them.  (I'm more interested in time.  I don't pay much attention to
> sentences that don't have time.)  So getting a GPZDA sentence that
> looks OK doesn't count as valid.

GPZDA does not set PPSTIME_IS, so is not part of the ntp calculations.

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

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