[ntp:questions] Re: NEMA driver on Fedora Core 3 or 4

Hal Murray hmurray at suespammers.org
Sat Nov 19 09:36:50 UTC 2005

My query from a month ago...

>I've got a GPSClock 200.  The serial port is off by 500 or 600 ms.
>That's running FC 4.  I've got a similar setup at work running FC 3.
>It has the same quirk.

I think I have figured out what's going on...

The NMEA driver has this code in the nmea_receive routine:

         * If the PPSAPI is working, rather use its timestamps.
         * assume that the PPS occurs on the second so blow any msec
        if (nmea_pps(up, &rd_tmp) == 1) {
                pp->lastrec = up->tstamp = rd_tmp;
                pp->nsec = 0;
#endif /* HAVE_PPSAPI */

The new FC 3 and 4 boxes that I'm having troubles with don't
have the PPS kernel mods so that code gets run on the good/normal
case but not on my new kernels that are causing troubles.

Is that a bug or feature?  Should other refclocks do the same thing?

Is smashing the nsec the right thing?  Assuming the second is right,
won't that make it look like all the NMEA samples arrive at exactly
the right time?  How can you correct the clock if the samples all
appear to arrive at the right time?  (I know, use the PPS/ATOM driver,
but it needs to know that the seconds are close enough.)

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