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- Subject: [chrony-users] Synchronizing clock with GPS with PPS
- From: Ryan Govostes <rgovostes@xxxxxxxx>
- Date: Tue, 15 Sep 2020 14:30:51 +0000
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- Thread-topic: Synchronizing clock with GPS with PPS
Hi all,
I am setting up chronyd on an embedded Linux device to synchronize the system clock using a GPS module. The GPS device sends NMEA strings over the character device /dev/ttyAMA1 and I have also configured /dev/pps0, both of which appear to be working OK.
The system is running Ubuntu 18.04 and the latest package versions are chronyd 3.2 and gpsd 3.17.
I configured gpsd to listen to the serial device and then added these lines to my chrony.conf:
refclock SHM 0 refid GPS precision 1e-1 offset 0.9999 delay 0.2
refclock PPS /dev/pps0 refid PPS
When I run `chronyc sources` they both seem to be kind of working:
210 Number of sources = 2
MS Name/IP address Stratum Poll Reach LastRx Last sample
===============================================================================
#- GPS 0 4 377 12 +128ms[ +128ms] +/- 200ms
#* PPS 0 4 377 12 +6ns[ +119ns] +/- 203ns
However it looks like the GPS source is “not combined”. Is this a degraded state, e.g., it is using one of these two sources?
Also, I am not sure why the LastRx from the PPS (or frankly either) ticks upwards so long—shouldn’t it constantly be receiving updates?
I am just using the precision / offset / delay figures that several examples use. Is there documentation on calibrating these values?
Finally, I read that using Unix sockets rather that shared memory is preferable, but my chronyd does not seem to create those sockets.
Thanks,
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