[ntp:questions] NMEA driver using common PPSAPI code

Dave Hart hart at ntp.org
Wed Oct 6 23:49:34 UTC 2010

On Wed, Oct 6, 2010 at 09:42 AM, Venu Gopal <neo.venu at gmail.com> wrote:
> Hello Folks,
> I tried using time1 and time2 flags but things became worse :-(.
> Everything works fine with NMEA driver when using local PPSAPI.
> And I don't need use time1 or time2 flags (set to 0 by default).

I believe you are conflating two separate issues.  At the time the
NMEA driver had its own PPSAPI code, it used a single fudge for both
PPSAPI and serial end-of-line timestamp offsets.  Since then, the two
fudge factors were separated, to allow improved startup behavior by
bringing the serial timestamp closer to the PPS that is eventually

If you are switching back and forth between older and newer ntpd to
test using NMEA-local PPSAPI code, you are also switching between the
prior and current fudge behaviors.

The short-term goal you should have is to experiment with "fudge
127.127.20.x time2 y" varying y to attempt to reduce the size of the
step when switching from serial timestamps to PPS.  Do this without
hardpps, at least initially.  You should within a few tries be able to
get the difference between the two times in the tens of milliseconds
or better.

I'm pretty confident others are using the newer NMEA code, sine it has
been that way for well over a year.

Good luck,
Dave Hart

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