reTerminal DM / Raspberry Pi CM4 (64-bit)

This is the base Nerves System configuration for the seeed studios reTerminal DM (CM4)

How to build

Prebuild firmware imags are available on github and will be used by nerves.

Webkitwpe is large and compilation is slow. In case it fails with a memory error just run make again.

reTermial DM

The DSI-Display might not start by itself. On some reTerminals it works out of the box, on others forcing modprobe to re-enable/restart enables the display. For best results do this directly after setting up udevd.

:os.cmd(~c"modprobe -r vc4")
:os.cmd(~c"modprobe vc4")

def setup_udev do
    :os.cmd(~c"udevd -d")
    :os.cmd(~c"udevadm trigger --type=subsystems --action=add")
    :os.cmd(~c"udevadm trigger --type=devices --action=add")
    :os.cmd(~c"udevadm settle --timeout=30")
    :os.cmd(~c"modprobe -r vc4")
    :os.cmd(~c"modprobe vc4")
end

Backlight

For now pin 13 (backlight) is pulled up (full brightness) via dtoverlay in config.txt. The brightness might be changed using hardware PWM if the overlay is disabled.

An example supervisor can be found in examples/kiosk.ex

Code for the light sensor and led can be found in examples/hardware.ex

Cog

cog needs a writeable directory. This can be configured – /data/nerves_weston or /run works fine

Firmware sizes

The artifact and firmwware are larger than a normal nerves image, starting at about 130MB. Because of this the fwup size for the A and B partion must be resized in fwup.conf

Using

The most common way of using this Nerves System is create a project with mix nerves.new and to export MIX_TARGET=rpi4. See the Getting started guide for more information.

If you need custom modifications to this system for your device, clone this repository and update as described in Making custom systems.

Supported WiFi devices

The base image includes drivers for the onboard Raspberry Pi 4 wifi module (brcmfmac driver).

Camera

This system supports the official Raspberry Pi camera modules via libcamera. The libcamera applications are included so it's possible to replicate many of the examples in the official Raspberry Pi Camera Documentation.

Here's an example commandline to run:

cmd("libcamera-jpeg -n -v -o /data/test.jpeg")

On success, you'll get an image in /data that you can copy off with sftp.

Since libcamera is being used instead of MMAL, the Elixir picam library won't work.

Audio

The Raspberry Pi has many options for audio output. This system supports the HDMI and stereo audio jack output. The Linux ALSA drivers are used for audio output.

The general Raspberry Pi audio documentation mostly applies to Nerves. For example, to force audio out the HDMI port, run:

cmd("amixer cset numid=3 2")

Change the last argument to amixer to 1 to output to the stereo output jack.

Provisioning devices

This system supports storing provisioning information in a small key-value store outside of any filesystem. Provisioning is an optional step and reasonable defaults are provided if this is missing.

Provisioning information can be queried using the Nerves.Runtime KV store's Nerves.Runtime.KV.get/1 function.

Keys used by this system are:

The normal procedure would be to set these keys once in manufacturing or before deployment and then leave them alone.

For example, to provision a serial number on a running device, run the following and reboot:

iex> cmd("fw_setenv nerves_serial_number 12345678")

This system supports setting the serial number offline. To do this, set the NERVES_SERIAL_NUMBER environment variable when burning the firmware. If you're programming MicroSD cards using fwup, the commandline is:

sudo NERVES_SERIAL_NUMBER=12345678 fwup path_to_firmware.fw

Serial numbers are stored on the MicroSD card so if the MicroSD card is replaced, the serial number will need to be reprogrammed. The numbers are stored in a U-boot environment block. This is a special region that is separate from the application partition so reformatting the application partition will not lose the serial number or any other data stored in this block.

Additional key value pairs can be provisioned by overriding the default provisioning.conf file location by setting the environment variable NERVES_PROVISIONING=/path/to/provisioning.conf. The default provisioning.conf will set the nerves_serial_number, if you override the location to this file, you will be responsible for setting this yourself.

Linux kernel and RPi firmware/userland

There's a subtle coupling between the nerves_system_br version and the Linux kernel version used here. nerves_system_br provides the versions of rpi-userland and rpi-firmware that get installed. I prefer to match them to the Linux kernel to avoid any issues. Unfortunately, none of these are tagged by the Raspberry Pi Foundation so I either attempt to match what's in Raspbian or take versions of the repositories that have similar commit times.

Linux kernel configuration

The Linux kernel compiled for Nerves is a stripped down version of the default Raspberry Pi Linux kernel. This is done to remove unnecessary features, select some Nerves-specific features like F2FS and SquashFS support, and to save space.