[ntp:questions] fudge time1 for gps-18x-LVC?

David Lord snews at lordynet.org
Sun Feb 7 19:06:14 UTC 2010

Dave Hart wrote:
> On Feb 7, 12:24 UTC, David Lord wrote:
>> Downside of not having fudge is first few NMEA offsets are 350ms
>> until the NMEA driver sorts out its own handling of pps. As soon as
>> offset shifts to a few ms or less jitter then rockets to near 350ms
>> and works down from there fairly quickly.
>> It's probably worth trying with the fudge time1 in place as that
>> might just improve startup.
> I would suggest you upgrade to ntpd 4.2.6 or 4.2.7.  Based on your
> chronicles, I'm guessing you're using 4.2.5 or earlier, because you
> note that the NMEA driver is switching from serial end-of-line
> timestamps to PPS timestamps soon after starting, and because I'm
> assuming you don't have a fudge flag set for that driver to enable its
> PPSAPI use.  IIRC, that is the classic behavior of NMEA compiled on a
> system with PPSAPI support -- if PPSAPI works on the NMEA serial port,
> it's used unconditionally.  With 4.2.6, you have to specifically ask
> the NMEA driver to enable PPSAPI use with fudge 127.127.20.x flag1 1.
> Moreover, with older NMEA there is a single offset fudge available,
> used at least for serial end of line fudging but I think it was also
> used to offset the PPS timestamps.  With 4.2.6, fudge time1 affects
> only NMEA PPS timestamps, and fudge time2 affects only NMEA serial EOL
> timestamps.  That should enable you to cleanly fudge away the startup
> 350ms offset.
> If you do go to 4.2.6 or later, I suggest you do not attempt to use
> the same PPS signal via two different drivers at the same time, unless
> one is marked noselect.  That is, either use NMEA alone with fudge
> flag1 1 to enable PPSAPI, or use NMEA without fudge flag1 and marked
> prefer alongside PPS/ATOM (22).

I think you've explained/confirmed results I've had. I'm
using 4.2.4p6-o on the test box. I'm not likely to try install
of a more recent version unless it was going to solve problem
that I can't get parallel (or serial) pps to work  with just
ATOM driver flag3 0 | 1 (kernel discipline disable | enable)
and no other refclock, ie I'd like to just use another ntp
server as preferred with pps to condition the clock.



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