[ntp:questions] NTP, GPSD & PPS

Miroslav Lichvar mlichvar at redhat.com
Wed Dec 10 12:39:15 UTC 2014

On Wed, Dec 10, 2014 at 11:50:22AM +0000, David Taylor wrote:
> With -D 4 I get a list of devices ending with "PPS", but presumably that is
> not the same as "KPPS"?

In gpsd the PPS without K is the userspace timestamping. With kernel
timestamping the log looks like this:

gpsd:PROG: PPS edge: 1, cycle: 1000000 uSec, duration:  799998 uSec @ 1418214654.000007216
gpsd:INFO: PPS hooks called with accepted 1418214653.999999223 offset 0.000000777
gpsd:PROG: PPS edge accepted 1418214653.999999223 offset 0.000000777
gpsd:PROG: KPPS assert 1418214653.999999223, sequence: 73 - clear  1418214654.200000573, sequence: 73
gpsd:PROG: KPPS data: using clear
gpsd:PROG: KPPS cycle:  999999 uSec, duration:  200001 uSec @ 1418214654.200000573

> I did try an apt-get first to update gpsd but it
> seems I have the most recent available.  It seems I have 3.6.  Do I need a
> development version or...?

The kernel PPS support was added in 3.0 or so, but gpsd needs to be
compiled with the timepps.h header, similarly to ntpd for the ATOM and
other drivers. Also, some gpsd versions had bugs in the PPS/KPPS
support and I'm not sure if 3.6 was a good or bad. The latest version
- 3.11 is working well for me, 3.10 was not.

Miroslav Lichvar

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