🛠 Horodatage industriel : PTP IEEE 1588 sur le PHY Ethernet du CM4

Objectives

A unique feature in the Pi range: the CM4 embeds a Gigabit Ethernet PHY compatible with IEEE 1588 (PTP) on the module — designed for industrial real-time synchronization (flagship cards use discrete PHYs without this claim). This tutorial sets up a two-node PTP domain with linuxptp: one grandmaster, one slave, and measures the residual offset in microseconds.

Hardware

• 2 × CM4 (with Ethernet, so not a CM4S) on an IO Board, running 64-bit Raspberry Pi OS
• 1 straight-through Ethernet cable between the two RJ45 ports (direct topology: a non-transparent switch significantly degrades precision)
• Separate control network (Wi-Fi or second port) for SSH configuration
• sudo apt install -y linuxptp on both nodes

Complete code

# --- Nœud A : grandmaster ---
sudo tee /etc/linuxptp/master.conf >/dev/null <<'EOF'
[global]
slaveOnly 0
networkTransportAddress eth0:500
summaryRatio -1
syncInterval 0
EOF
sudo ptp4l -f /etc/linuxptp/master.conf -m &

# --- Nœud B : esclave ---
sudo tee /etc/linuxptp/slave.conf >/dev/null <<'EOF'
[global]
slaveOnly 1
networkTransportAddress eth0:500
EOF
sudo ptp4l -f /etc/linuxptp/slave.conf -m &
# Ajuste CLOCK_REALTIME depuis la hardware clock de eth0
sudo phc2sys -s eth0 -c CLOCK_REALTIME -O 0 -m &

# --- Sur l'esclave : interroger l'état de synchronisation ---
sudo pmc -u -b eth0 'GET TIME_STATUS_NP' | grep -E 'master_offset|gmIdentity'

Expected verification

In the output of ptp4l -m on the slave side, the master offset lines converge to values in the order of microseconds in a direct topology; pmc 'GET TIME_STATUS_NP' displays a stable master_offset and the grandmaster identity; phc2sys no longer reports jumps. The two date commands at both ends agree to within a millisecond.

Common errors

• Going through a home switch: without transparent PTP support, traversal asymmetry causes the offset to plateau at hundreds of microseconds or more.
• Trying to run PTP over Wi-Fi: the wireless medium (CYW43455) does not offer timestamp stability; only the wired PHY is covered by 1588 compatibility.
• Forgetting phc2sys: PTP synchronizes the interface's hardware clock, not CLOCK_REALTIME; without this daemon, date does not change.
• Conflict with systemd-timesyncd/NTP: disable sudo timedatectl set-ntp false on the slave, otherwise NTP constantly "corrects" the PTP offset.