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Configuring chrony

Chrony is an NTP implementation, but it has the ability to take advantage of hardware support designed for PTP in two ways:

  • it can read PPS input connected a pin of a PTP hardware clock (e.g. the SYNC_OUT pin on a CM4)
  • it can use hardware timestamping

This page is written assuming Fedora as the OS.

PPS input

Chrony can works very well using just the PPS signal from the GPS: the pulse says exactly when a second starts; chrony can figure out which second it is from network sources. Make sure you have chrony set up in way similar to the default getting time from the network, typically using pool directive.

To use the PPS as a source of time, add this line to /etc/chrony.conf:

refclock PHC /dev/ptp0:extpps:pin=0 poll 0 width 0.1 delay 6e-8 precision 2e-8 refid PPS

Here:

  • /dev/ptp0 is the device for the PHC (PTP hardware clock); see Verify PPS section for how to determine this
  • the extpps option enables a mode of operation in which chrony itself reads the timestamps of pulses on a pin on the PHC; without this option, chrony expects the PHC to have been synchronized to the correct time by another process and just reads the time from the PHC
  • pin=0 says to use pin 0; this is the physical pin that the PPS signal is connected to
  • poll 0 says to use samples immediately rather than store them (since the jitter should be very small)
  • width 0.1 is the pulse width in seconds; see Pulse Width section for how to determine this
  • delay 6e-8 specifies a delay of 60ns; this is suitable for a GPS with an accuracy of 30ns
  • precision 2e-8 specifies a precision of 20ns; this is the approximate expected stability of readings from the PHC
  • refid PPS names this refclock PPS

To use chrony as a server in your network, you will also need something like:

allow 192.168.1.0/24

On Fedora, you will also need to add firewall rules:

sudo firewall-cmd --add-service ntp
sudo firewall-cmd --add-service ntp --permanent

Then restart chrony:

sudo systemctl restart chronyd

Check that it started OK:

sudo systemctl status chronyd

Now run

chronyc sources

It should should a line starting with #* PPS. This means it has successfully synced to the PHC refclock.

Using serial connection from GPS

Connecting up the serial output from the GPS allows chrony to work even when there is no connection to any other NTP server. Unless you have this requirement, I do not recommend doing this: a serial connection to GPS output is generally less precise and more troublesome than using the internet with NTP.

Install gpsd:

sudo dnf install gpsd

Edit OPTIONS and USBAUTO lines in /etc/sysyconfig/gpsd:

OPTIONS="-n /dev/ttyUSB0"
USBAUTO=""

Now we are going to tweak the systemd unit file:

sudo systemctl edit gpsd.service

Then type

[Unit]
PartOf=chronyd.service

and write out the file and exit the editor. This installs an override in /etc/systemd/system/gpsd.service.d/override.conf, which ensures that when chronyd gets restarted, gpsd will also get restarted.

Now enable and start the service:

sudo systemctl enable --now gpsd.service

Now run

gpsmon

to check that gpsd is seeing the GPS. Use Ctrl-C to exit.

Now add this line to /etc/chrony.conf:

refclock SOCK /run/chrony.clk.ttyUSB0.sock offset 0 delay 0.1 poll 0 noselect refid UART

The meaning of this line is as follows:

  • the SOCK kind of refclock reads date-time samples from a socket
  • /run/chrony.clk.ttyUSB0.sock is the filename of the socket that chrony will create in order to receive information from gpsd; when gpsd starts reading from /dev/ttyX, it checks for the existence of the socket /run/chrony.clk.ttyX.sock; if this socket exists, then it will send the messages with the current date-time derived from messages received from the GPS over this socket
  • offset 0 means that chrony should assume that there are 0 seconds delay between the instant when the second started and the instant when it receives the date-time message for that second through the socket; this is certainly the wrong value; we will figure out the right value below
  • delay 0.1 means that the difference the minimum offset and maximum offset is 0.1s
  • poll 0 means samples are used immediately rather than being stored and filtered (we don't need to store since we have noselect)
  • noselect option means that this refclock is not going to be used as a time source on its own: it's too inaccurate for that. (Instead it's going to be used to supplement the pulse-per-second signal.)
  • refid UART named this refclock UART

Now

sudo systemctl restart chronyd

Let this run for a minute or so, and then do

chronyc sources

You should see lines similar to this:

#* PPS                           0   0   377     1     -6ns[  -19ns] +/-  867ns
#? UART                          0   3   377     9   +216ms[ +216ms] +/- 1434us

as well as multiple lines for NTP servers. The 216ms tells us the offset we need in the refclock SOCK line: 216ms is 0.216s, so we need 0.216 as the offset value.

Now we're ready to put the final tweaks to the two refclock lines. First in the reflock SOCK line, we edit the offset 0 to be e.g. offset 0.216. Then we can add a lock UART option to the refclock PHC line: this means that the pulse should be combined with the date-time from the UART refclock to provide a complete and accurate time sample.

The refclock lines now look like this:

refclock PHC /dev/ptp0:extpps:pin=0 poll 0 width 0.1 delay 6e-8 precision 2e-8 refid PPS lock UART
refclock SOCK /run/chrony.clk.ttyUSB0.sock delay 0.1 offset 0.216 poll 0 noselect refid UART

TODO: is it better to omit the lock UART from the PHC line and omit the noselect from the UART line?

Now we can restart chrony again:

sudo systemctl restart chronyd

The /run/chrony.clk.ttyX.sock feature was added in gpsd 3.25. Before that, it supported only a /run/chrony.ttyX.sock, which works only for PPS data. If you're using a version of gpsd that does not support /run/chrony.clk.ttyX.sock, then you should instead use the SHM 0 refclock.

refclock SHM 0 poll 0 delay 0.1 offset 0.216 noselect refid UART

Hardware timestamping

To make chrony use hardware timestamping, we just add the line

hwtimestamp enp1s0

But this won't work with clients whose NICs have the ability to timestamp only PTP packets. To support such clients, we also have to use NTP-over-PTP in order to use hardware timestamping. To enable this, we add the line

ptpport 319

We will also need to add firewall rules to allow inbound traffic on the PTP port:

sudo firewall-cmd --add-service ptp
sudo firewall-cmd --add-service ptp --permanent

On the client side, you will need something like this:

server 192.168.1.2 minpoll 0 maxpoll 0 xleave port 319
hwtimestamp enp1s0 rxfilter ptp
ptpport 319

You can also add extfield F323 to the server line to enable chrony's support for frequency transfer in NTP.

Chrony only supports NTP-over-PTP between server and client running the same versions of chrony.