GPSD PPS Jitter

[George Sexton]

I'm running into a problem using GPSD. What's happening is the PPS 
offset varies a great deal. Sometimes 7us, sometimes 250us. I got the 
values by using gpsmon, and reading the difference between the 
nano-seconds values on PPS outputs. I've also tried using Chrony 
configured with SHM refclock, and see the same kind of issue. Some 
consecutive values of the clock_nsec field from GPS mon are:

999 797 221
999 993 253
999 795 330
999 859 433
999 983 051
999 993 235
999 985 795

Configuration

GPS Unit: Garmin GPS-16X HVS
OS: OpenSUSE Leap 15.5 Kernel Version 5.14.21
GPSD Version: 3.25
Serial Port: MosChip Semicondiuctor Technology Ltd. PCIe 9912 Multi-I/O 
Controller 16550

The garmin unit is connected to a custom serial port interface board 
that's in turn connected to the 16550 serial port.

The Garmin/Serial port adapter were used very successfully with NTPD on 
an older computer and I would see variance of around 3-10us.

I tried adding nohz=off intel_idle.max_cstate=0 to my kernel boot 
command line, and it didn't seem to help.

I'm really not sure where to go next on this. I've searched the web and 
haven't run into anything that looks helpful.  Can anyone point me in 
the right direction?


--
George Sexton
(303) 438 9585 x102
MH Software, Inc.