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Time Synchronization using PTP/IEEE1588

For a complicated robotic system, there may be multiple onboard computers and sensors working together to have the whole system functional. To ensure the data from different devices are synchronized in time, you may need to consider time synchronization among the devices. PTP is a synchronization protocol designed for this purpose and it allows sub-microsecond accuracy if properly configured. One example usage of PTP time synchronization is setting up an Ouster OS1-64 Lidar on a mobile robot for 3D mapping and navigation. To get the right timestamp for the pointcloud to be used with other parts of ROS stack, such as mapping and localization, a PTP grandmaster can be configured on the navigation computer to synchronize the time between Lidar and the computer.

How PTP works

Here are some good references:

You can either use a dedicated PTP grand master hardware or set up a Linux computer to act as the master. In this note, we mainly consider the latter case.

PTP Profiles

The following table is taken from [4] by peci1 published on ROS Discourse:

Profile BMCA Delay mech. Layer
Default Yes P2P/E2E L2/L3
gPTP (802.1AS) 17 Yes P2P L2
Automotive 10 No P2P L2
Autosar 9 No P2P L2
LXI 1 Yes P2P L3
IEC 62439-3 L2P2P Yes P2P L2
IEC 62439-3 L3E2E Yes E2E L3
Power Profile Yes P2P L2
GigE Vision 11 Yes P2P/E2E ?

Configure Linux PTP

Tutorials & manual

Install relevant packages

## install the packages
$ sudo apt install ethtool linuxptp chrony

The following list is taken from the Ouster documentation to give you a brief idea of the functions of the packages:

  • ethtool - A tool to query the hardware and driver capabilities of a given Ethernet interface.
  • linuxptp - Linux PTP package with the following components:
    • ptp4l daemon to manage hardware and participate as a PTP node
    • phc2sys to synchronize the Ethernet controller’s hardware clock to the Linux system clock or shared memory region
    • pmc to query the PTP nodes on the network.
  • chrony - A NTP and PTP time synchronization daemon. It can be configured to listen to both NTP time sources via the Internet and a PTP master clock such as one provided by a GPS with PTP support. This will validate the time configuration makes sense given multiple time sources.

Hardware and driver support

You can use the following command to check the hardware and driver support of the network interface:

$ ethtool -T eth0

Ideally the ethernet adapter and driver should support hardware timestamp. You may get something like this:

Time stamping parameters for eth0:
Capabilities:
    hardware-transmit     (SOF_TIMESTAMPING_TX_HARDWARE)
    software-transmit     (SOF_TIMESTAMPING_TX_SOFTWARE)
    hardware-receive      (SOF_TIMESTAMPING_RX_HARDWARE)
    software-receive      (SOF_TIMESTAMPING_RX_SOFTWARE)
    software-system-clock (SOF_TIMESTAMPING_SOFTWARE)
    hardware-raw-clock    (SOF_TIMESTAMPING_RAW_HARDWARE)
PTP Hardware Clock: 0
Hardware Transmit Timestamp Modes:
    off                   (HWTSTAMP_TX_OFF)
    on                    (HWTSTAMP_TX_ON)
    one-step-sync         (HWTSTAMP_TX_ONESTEP_SYNC)
Hardware Receive Filter Modes:
    none                  (HWTSTAMP_FILTER_NONE)
    ptpv1-l4-sync         (HWTSTAMP_FILTER_PTP_V1_L4_SYNC)
    ptpv1-l4-delay-req    (HWTSTAMP_FILTER_PTP_V1_L4_DELAY_REQ)
    ptpv2-l4-sync         (HWTSTAMP_FILTER_PTP_V2_L4_SYNC)
    ptpv2-l4-delay-req    (HWTSTAMP_FILTER_PTP_V2_L4_DELAY_REQ)
    ptpv2-l2-sync         (HWTSTAMP_FILTER_PTP_V2_L2_SYNC)
    ptpv2-l2-delay-req    (HWTSTAMP_FILTER_PTP_V2_L2_DELAY_REQ)
    ptpv2-event           (HWTSTAMP_FILTER_PTP_V2_EVENT)

Otherwise if only software timestamp is support, you may see

Time stamping parameters for eth0:
Capabilities:
    software-transmit
    software-receive
    software-system-clock
PTP Hardware Clock: none
Hardware Transmit Timestamp Modes: none
Hardware Receive Filter Modes: none

PTP configration summary

Set up ptp clock master or slave

  1. Update the configuration file /etc/linuxptp/ptp4l.conf

    If you're setting up a master clock, change the following lines:

    #clockClass     248
clockClass      128
# at very bottom of the file
boundary_clock_jbod 1
[eth0]

    If you're setting up a slave device, change the following lines: 

    #slaveOnly              0
slaveOnly               1
# at very bottom of the file
boundary_clock_jbod 1
[eth0]

  2. Create a systemd service for ptp4l: /etc/systemd/system/ptp4l.service

    [Unit]
Description=ptp4l service
Requires=network-online.target
After=network-online.target

[Service]
ExecStart=/usr/sbin/ptp4l -f /etc/linuxptp/ptp4l.conf
Restart=on-failure

[Install]
WantedBy=multi-user.target

  3. Start and enable the ptp4l service

    $ sudo systemctl daemon-reload
$ sudo systemctl start ptp4l.service
# if you get no errors starting the service
$ sudo systemctl enable ptp4l.service

Configure phc2sys to synchronize the system time to the PTP clock

  1. Create a systemd service for phc2sys: /etc/systemd/system/phc2sys.service

    [Unit]
Description=Synchronize system clock or PTP hardware clock (PHC)
Documentation=man:phc2sys
Requires=ptp4l.service
After=ptp4l.service

[Service]
ExecStart=/usr/sbin/phc2sys -w -s CLOCK_REALTIME -c eth0

[Install]
WantedBy=multi-user.target

  2. Start and enable the phc2sys service

    $ sudo systemctl daemon-reload
$ sudo systemctl start phc2sys.service
# if you get no errors starting the service
$ sudo systemctl enable phc2sys.service

Check clock synchronization

  • Check time synchronization between PHC and the Grandmaster clock

    Look at output of the ptp4l:

    ptp4l[5374018.735]: rms  787 max 1208 freq -38601 +/- 1071 delay  -14 +/-   0
ptp4l[5374019.735]: rms 1314 max 1380 freq -36204 +/- 346 delay   -14 +/-   0
ptp4l[5374020.735]: rms  836 max 1106 freq -35734 +/-  31 delay   -14 +/-   0
ptp4l[5374021.736]: rms  273 max  450 freq -35984 +/-  97 delay   -14 +/-   0
ptp4l[5374022.736]: rms   50 max   82 freq -36271 +/-  64 delay   -14 +/-   0
ptp4l[5374023.736]: rms   81 max   86 freq -36413 +/-  17 delay   -14 +/-   0

    If ptp4l consistently reports rms lower than 100 ns, the PHC is synchronized.

  • Check time synchronization between the PHC and the system clock

    Look at output of the phc2sys:

    phc2sys[5374168.545]: CLOCK_REALTIME phc offset   -372582 s0 freq    +246 delay   6649
phc2sys[5374169.545]: CLOCK_REALTIME phc offset   -372832 s1 freq      -4 delay   6673
phc2sys[5374170.547]: CLOCK_REALTIME phc offset        68 s2 freq     +64 delay   6640
phc2sys[5374171.547]: CLOCK_REALTIME phc offset       -20 s2 freq      -3 delay   6687
phc2sys[5374172.547]: CLOCK_REALTIME phc offset        47 s2 freq     +58 delay   6619
phc2sys[5374173.548]: CLOCK_REALTIME phc offset       -40 s2 freq     -15 delay   6680

    If phc2sys consistently reports offset lower than 100 ns, the System clock is synchronized.

More information about clock synchronization check can be found from [6].

Relevant Projects

Hardware Support

  • https://botblox.io/collections/frontpage

Reference