Syncing time on Upkie and your computer

[ubgk]

Time synchronisation

Some use cases may require (sub-)millisecond level of time synchronisation between your computer and Upkie.

For instance, you may be interested in collecting Motion Capture data or recording a video of the robot performing a certain task, and compare it against the robots proprioceptive logs. In such a scenario, you would typically need to align the timestamps on your robot logs, and the external ones, which is a tedious process.

Unless you are lucky enough to have a GPS clock in hand, it is most likely that your computer uses Network Time Protocol to update its time periodically, as its internal hardware clock (e.g., the RTC module on your motherboard) can gain or lose up to a few seconds a day.

Chrony

We can use the same protocol to synchronise the clock between Upkie and our computers. chrony is an implementation of NTP that can act both as a server and a client. It is available on most major package managers (e.g., apt, dnf or brew):

$ sudo apt install chrony

Setting up the server

You can set up your computer or the onboard Raspberry Pi to act as the NTP server, to do so, you need to edit your chrony configuration file. This will typically be in /etc/chrony.conf or /etc/chrony/chrony.conf

Add the following lines to your configuration:

# /etc/chrony/chrony.conf
local # allow chrony to act as a server
allow 192.168.0.0/16 # allow all incoming requests from 192.168.x.x

and restart the daemon:

 $ sudo systemctl restart chrony.service

Setting up the client

We can use chrony on the client to fetch the time from the server, although other NTP clients should work too.

You can see and edit the NTP servers (or server pools) in your config file:

# /etc/chrony/chrony.conf
server HOSTNAME_OR_IP

You can use chronyc to list all the sources and update your time:

$ sudo chronyc sources
MS Name/IP address         Stratum Poll Reach LastRx Last sample               
===============================================================================
^* 192.168.2.100                10   6   377    59    +49ms[ +168ms] +/- 6470us

and

$ sudo chronyc burst 1/2 192.168.2.100

The 1/2 argument will tell chronyc that we want at least 1 good measurement for a maximum of 2 attempts, and can be configured to reflect your needs. Another important point is, when chrony detects the system is out-of-sync with the server, it will adjust time gradually (rather than making a jump). You can add a directive in your config file to disable this:

# /etc/chrony/chrony.conf
makestep 0 -1 # Steps time if time difference is greater than 0 seconds

Nota Bene:

Every time you make changes to the config file, you will need to restart the service:

 $ sudo systemctl restart chrony.service