[time-nuts] Re: PPS on Ubuntu server 20.04 LTS

[James Perkins]

x GPS shows that the GPS serial data is being received on /dev/ttyS0 with
reachability of 377 - so it's consistently coming in and being interpreted
correctly by something (presumably, gpsd service); however, the x means
that it is deemed a falseticker (inaccurate, compared to the chosen NTP
server, 192.168.0.20). This is probably because you need to adjust the
offset period to match the mean delay in serial data coming in from your
serial port to be in line with other NTP servers.

ppscheck is showing two events per second, the CTS event happening about
.9784 offset relative to the system clock, and the serial data about 100ms
later at .078. So, an offset of 0.100 would be a possible first try. The
fact ppscheck (part of the gpsd package's set of tools) sees this means
that gpsd was compiled with support for CTS edge detection. However, you're
not getting any pps input at chrony at all. What are your chrony.conf
refclock lines?

For example, on my server, I'm using an adafruit Ultimate GPS Hat on a
Raspberry Pi 4B. gpsd will recognize it as an NMEA0183 device. It provides
serial input to /dev/ttyS0 and PPS on gpio 4. In /boot/config.txt I added:

enable_uart=1
dtoverlay=pps-gpio,gpiopin=4

And rebooted. This will cause the kernel to activate the appropriate pins
for the GPS serial input/output, and run a software timestamp driver
(pps-gpio) which links the PPS gpio pin edge to an event on the /dev/pps0
device.

gpsd is run this way (setup in /etc/default/gpsd):

START_DAEMON="true"
USBAUTO="true"
DEVICES="/dev/ttyS0 /dev/pps0"
GPSD_OPTIONS="-n"

And start the service:

systemctl enable gpsd.service
systemctl restart gpsd.service

The 'systemctl status gpsd.service' command reveals it is running with
parameters: gpsd -n /dev/ttyS0 /dev/pps0 . This updates NTP shared memory 0
when the serial data becomes available (usually quite a while after PPS),
and NTP shared memory 2 when the PPS clock edge is found.

Then in /etc/chrony/chrony.conf, the big offset on the slow-to-arrive
imprecise serial data (GPS) and the accurate PPS:

refclock SHM 0 poll 3 refid GPS precision 1e-1 offset 0.5774 delay 0.2
refclock SHM 2 poll 3 refid PPS precision 1e-7

In the above, Shared memory 0 is the serial GPS source with low precision
and 0.5774 offset. Shared memory 2 is the PPS gpio source with high
precision (100ns precision was my ballpark guesstimate to use here, I need
to get a better oscilloscope for further testing).

Hope this example helps,
James Perkins