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CM4/CM5 OS installation and configuration

These instructions are for the Raspberry Pi OS Lite. Raspberry Pi OS used to be called Raspbian.

As of the time of writing (early 2025), the current version of Raspberry Pi OS is based on Debian Bookworm. The legacy version of Raspberry Pi OS is based on Debian Bullseye. The CM5 requires Bookworm. We are using Raspberry Pi OS, since it is optimized for the Raspberry Pi hardware. We are using the Lite version, since we do not need or want a desktop environment for this application. We are also using the 64-bit version, since this takes best advantage of the CM4 hardware (particularly with 8Gb RAM).

OS installation

Install Raspberry Pi OS Lite 64-bit.

If your CM4/CM5 has eMMC, follow these instructions. When using the Raspberry Pi Imager, select Raspberry Pi OS (other) and then Raspberry Pi OS Lite (64-bit).

TODO: installation without eMMC

TODO: What is the minimum amount of RAM for 64-bit to be a better choice than 32-bit? Not sure if 1Gb RAM would work better with a 32-bit OS.

OS configuration

If you want to do this using SSH from your main machine, then

  • run raspi-config to enable SSH (under Interfacing)
  • find the current IP address using ifconfig

Update packages

sudo apt update
sudo apt upgrade

and reboot.

On the CM5, it is necessary to have at least kernel version 6.12 for PTP hardware timestamping to work. You can check your kernel version with uname -r. If that says you are running something earlier than 6.12 (e.g. 6.6.x), then you will need to update to a more recent kernel. At the time of writing (January 2025), this can be done using the command sudo rpi-update next. See this forum thread. This is not necessary nor recommended for the CM4.

Run raspi-config:

  • enable serial port (under Interface/Serial Port); answer
    • No to login shell accessible over serial
    • Yes to enable serial port hardware

For the CM4, but not the CM5, add the following at the end of /boot/firmware/config.txt (on Bullseye, it's /boot/config.txt). This should not

# realtime clock
dtoverlay=i2c-rtc,pcf85063a,i2c_csi_dsi
# fan
dtoverlay=i2c-fan,emc2301,i2c_csi_dsi
# Make /dev/ttyAMA0 be connected to GPIO header pins 8 and 10
# This always disables Bluetooth
dtoverlay=disable-bt

Disable the system service that initialises the modem:

sudo systemctl disable hciuart

Set the timezone:

sudo dpkg-reconfigure tzdata

Use raspi-config to set

  • wifi country (under System Options > Wireless LAN)
  • hostname (under System Options)

Reboot.

Static IP address

Although it's not essential, you probably want a static IP address.

Network Manager

Raspberry Pi OS Bookworm by default uses Network Manager to manage network connections.

You can see the current connections using

nmcli c show

Assuming the connection on the interface is named Wired connection 1, you can change it to a static IP using a command like:

nmcli c mod "Wired connection 1" ipv4.method manual ipv4.addresses 192.168.0.5/24 ipv4.gateway 192.168.0.1 \
   ipv4.dns 8.8.8.8 ipv4.dns-search lan

You can activate this by doing:

nmcli d reapply eth0

dhcpcd

Raspberry Pi OS Bullseye by default uses dhcpcd to manage the network. Edit the section of /etc/dhcpcd.conf starting with Example static IP configuration.

Verify OS setup

Check that your kernel includes the necessary have ethernet PTP hardware support

ethtool -T eth0

You should see:

Time stamping parameters for eth0:
Capabilities:
        hardware-transmit
        hardware-receive
        hardware-raw-clock
PTP Hardware Clock: 0
Hardware Transmit Timestamp Modes:
        off
        on
        onestep-sync
        onestep-p2p
Hardware Receive Filter Modes:
        none
        ptpv2-event

Note that PTP Hardware Clock: 0 means that this interface uses /dev/ptp0 as its hardware clock.

Check the RTC

sudo hwclock --show

Check that the current date is correct:

date

Check support for the fan controller. Do

ls /sys/class/thermal

You should see:

cooling_device0  thermal_zone0

Verify GPS connection

You should verify that the GPS is properly connected. There are two connections

  • the serial connection
  • the PPS connection

Serial connection

Assuming you have specified dtoverlay=disable-bt above and you have connected the GPS RX (white) and TX (green) pins to pins 8 and 10 respectively on the J8 HAT connector, then the serial device will be /dev/ttyAMA0.

Do:

(stty 9600 -echo -icrnl; cat) </dev/ttyAMA0

Here 9600 is the speed. The most common default speed is 9600, but some receivers default to 38400 or 115200.

You should see lines starting with $. In particular look for a line starting with $GPRMC or $GNRMC. The number following that should be the current UTC time; for example, 025713.00 means 02:57:13.00 UTC. After another 8 commas, there will be a field that should have the current UTC date; for example, 140923 means 14th Septemember 2023.

PPS connection

To verify that the PPS connection is working, first configure the SYNC_OUT for input:

echo 1 0 | sudo tee /sys/class/ptp/ptp0/pins/SYNC_OUT

Replace the 0 in ptp0 with whatever ethtool said was the number. SYNC_OUT here is the name of the pin to which the PPS is connected. In the echo 1 0, 1 means to use the pin for input and 0 means the pin should use input channel 0.

Now do:

echo 0 1 | sudo tee /sys/class/ptp/ptp0/extts_enable

This means to enable timestamping of pulses on channel 0. In the echo 0 1, 0 means channel 0 and 1 means to enable timestamping.

Now see if we're getting timestamps:

sudo cat /sys/class/ptp/ptp0/fifo

The cat command should output a line, which represents a timestamp of an input pulse and consists of 3 numbers: channel number, which is zero in this case, seconds count, nanoseconds count. Repeating the last command will give lines for successive input timestamps.

If cat outputs nothing, then it's not working.