The "Hello World" of the embedded world... blinking an on-board LED

• Boots to Elixir iex shell and blinks LEDs forever in background
• Uses Delux to control the LEDs
• Custom per-target configuration via config/#{target}.exs

Image credit

This example controls the built-in user-controllable LEDs on your target hardware so no additional hardware is needed.

If you've used an Arduino or Raspberry Pi Pico, you're probably expecting to see code that loops between turning an LED on, waiting, and then turning it off. This example doesn't do that. Instead, it tells the Delux library what it wants the LED to do, and Delux figures out what to tell the Linux kernel to do to make it happen. This results in an efficient way of controlling LEDs where Elixir code decides what should happen, but the actual timing happens at a low level.

This demo has two parts: board-specific LED configuration and initialization of Delux to blink at 2 Hz.

Configuration of each supported board is in config/<target>.exs. For example, the Raspberry Pi Zero's configuration looks like:

config :blinky,
  indicators: %{
    default: %{
      green: "ACT"
    }
  }

This provides configuration for the demo application, :blinky. The atom :indicators is used by Delux to group LEDs that are controlled together. The Normally this would be a red, green, and blue LED, but the Raspberry Pi just has a green LED. For convenience, we tell Delux that the :default indicator has a :green LED. It's known to Linux as "ACT". You can find the names by listing the /sys/class/leds directory.

The second part of the demo is to add Delux to this project's supervision tree. This happens in Blinky.Application and looks like:

    children = [
      {Delux, delux_options ++ [initial: Delux.Effects.blink(:on, 2)]}
    ]

    opts = [strategy: :one_for_one, name: Blinky.Supervisor]
    Supervisor.start_link(children, opts)

This code passes the options defined in the configuration and adds an initial program for the LEDs. In this case, it specifies that all LEDs on the default indicator should be blinked at 2 Hz.

The Delux.Effects module provides lots of helper functions for creating LED programs. You can explore these at the IEx prompt like this:

iex> Delux.render(Delux.Effects.blink(:green, 0.5))
:ok

1. Specify your target with the MIX_TARGET environment variable
2. Get dependencies with mix deps.get
3. Create firmware with mix firmware
4. Burn firmware to an SD card with mix firmware.burn
5. Insert the SD card into your target board and power it on
6. After about 10-30 seconds, the configured LED(s) should start blinking at 2 Hz a. Note: If the device normally blinks an LED to show a heartbeat or other status, you'll see that during the boot process until Delux takes over.

export MIX_TARGET=rpi0
mix deps.get
mix firmware
mix firmware.burn

• Official docs: https://hexdocs.pm/nerves/getting-started.html
• Official website: https://nerves-project.org/
• Discussion Slack elixir-lang #nerves (Invite)
• Source: https://github.com/nerves-project/nerves