Wafer is an Elixir library to make writing drivers for i2c and SPI connected peripherals and interacting with GPIO pins easier.
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Build Status Hex.pm Hippocratic License HL3-FULL

Wafer is an OTP application that assists with writing drivers for peripherals using I2C, SPI and GPIO pins.

Wafer provides Elixir protocols for interacting with device registers and dealing with GPIO, so that you can use directly connected hardware GPIO pins or GPIO expanders such as the MCP23008 or the CD74HC595 SPI shift register.

Wafer implements the GPIO and Chip protocols for ElixirALE's GPIO and I2C drivers, Circuits.GPIO and Circuits.I2C. Implementing it for SPI should also be trivial, I just don't have any SPI devices to test with at the moment.

Documentation for the main branch can always be found here.

Some examples of how to use this project:

  • Augie, a hexapod robot.
  • PCA9641, an example of how easy it is to write a driver with Wafer.

Working with registers

Wafer provides the very helpful Registers macros which allow you to quickly and easily define your registers for your device:

Here's a very simple example:

defmodule HTS221.Registers do
  use Wafer.Registers

  defregister(:ctrl_reg1, 0x20, :rw, 1)
  defregister(:humidity_out_l, 0x28, :ro, 1)
  defregister(:humidity_out_h, 0x29, :ro, 1)

defmodule HTS221 do
  import HTS221.Registers
  use Bitwise

  def humidity(conn) do
    with {:ok, <<msb>>} <- read_humidity_out_h(conn),
         {:ok, <<lsb>} <- read_humidity_out_l(conn),
         do: {:ok, msb <<< 8 + lsb}

  def on?(conn) do
    case read_ctrl_reg1(conn) do
      {:ok, <<1::integer-size(1), _::bits>>} -> true
      _ -> false

  def turn_on(conn), do: write_ctrl_reg1(conn, <<1::integer-size(1), 0::integer-size(7)>>)
  def turn_off(conn), do: write_ctrl_reg1(conn, <<0>>)

Working with GPIO

Wafer provides a simple way to drive specific GPIO functionality per device.

Here's a super simple "blinky" example:

defmodule WaferBlinky do
  @derive [Wafer.GPIO]
  defstruct ~w[conn]a
  @behaviour Wafer.Conn
  alias Wafer.Conn
  alias Wafer.GPIO

  @type t :: %WaferBlinky{conn: Conn.t()}
  @type acquire_options :: [acquire_option]
  @type acquire_option :: {:conn, Conn.t()}

  @impl Wafer.Conn
  def acquire(options) do
    with {:ok, conn} <- Keyword.fetch(options, :conn) do
      {:ok, %WaferBlinky{conn: conn}}
      :error -> {:error, "`WaferBlinky.acquire/1` requires the `conn` option."}
      {:error, reason} -> {:error, reason}

  def turn_on(conn), do: GPIO.write(conn, 1)
  def turn_off(conn), do: GPIO.write(conn, 0)

And a simple mix task to drive it:

defmodule Mix.Tasks.Blink do
  use Mix.Task
  @shortdoc "GPIO LED Blink Example"

  alias Wafer.Driver.Circuits.GPIO

  def run(_args) do
    {:ok, led_pin_21} = GPIO.acquire(pin: 21, direction: :out)
    {:ok, conn} = WaferBlinky.acquire(conn: led_pin_21)

    Enum.each(1..10, fn _ ->

Running the tests

I've included stub implementations of the parts of ElixirALE and Circuits that are interacted with by this project, so the tests should run and pass on machines without physical hardware interfaces. If you have a Raspberry Pi with a Pi Sense Hat connected you can run the tests with the SENSE_HAT_PRESENT=true environment variable set and it will perform integration tests with two of the sensors on this device.


If available in Hex, the package can be installed by adding wafer to your list of dependencies in mix.exs:

def deps do
    {:wafer, "~> 1.0.3"}

Documentation can be generated with ExDoc and published on HexDocs. Once published, the docs can be found at https://hexdocs.pm/wafer.

Gitlab Mirror

This repository is mirrored on Gitlab from it's primary location on my Forgejo instance. Feel free to raise issues and open PRs on Gitlab.


This software is licensed under the terms of the HL3-FULL, see the LICENSE.md file included with this package for the terms.

This license actively proscribes this software being used by and for some industries, countries and activities. If your usage of this software doesn't comply with the terms of this license, then contact me with the details of your use-case to organise the purchase of a license - the cost of which may include a donation to a suitable charity or NGO.