# Elixir/ALE -- Elixir Actor Library for Embedded

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Elixir/ALE provides high level abstractions for interfacing to hardware
peripherals on embedded platforms. If this sounds similar to
[Erlang/ALE](, that's because it is. This
library is a Elixir-ized implementation of the library. It does differ from Erlang/ALE
in that the C port side has been simplified and all Raspberry PI-specific code
removed. It still runs on the Raspberry PI, but it interfaces to the hardware
only through Linux interfaces.

# Getting started

If you're natively compiling Elixir/ALE, everything should work like any other
Elixir library. Normally, you would include elixir_ale as a dependency in your
`mix.exs` like this:

    defp deps do
      [{:elixir_ale, "~> 0.4.1"}]

If you just want to try it out, you can do the following:

    git clone
    cd elixir_ale
    mix compile
    iex -S mix

If you're cross-compiling, you'll need to setup your environment so that the
right C compiler is called. See the `Makefile` for the variables that will need
to be overridden. At a minimum, you will need to set `CROSSCOMPILE`,

Elixir/ALE doesn't load device drivers, so you'll need to make sure that any
necessary ones for accessing I2C or SPI are loaded beforehand. On the Raspberry
Pi, the [Adafruit Raspberry Pi I2C
may be helpful.

# Examples

Elixir/ALE only supports simple uses of the GPIO, I2C, and SPI interfaces in
Linux, but you can still do quite a bit. The following examples were tested on a
Raspberry Pi that was connected to an [Erlang Embedded Demo
Board]( There's nothing special about
either the demo board or the Raspberry Pi, so these should work similarly on
other embedded Linux platforms.


A GPIO is just a wire that you can use as an input or an output. It can only be
one of two values, 0 or 1. A 1 corresponds to a logic high voltage like 3.3 V
and a 0 corresponds to 0 V. The actual voltage depends on the hardware.

Here's an example setup:

![GPIO schematic](doc/images/schematic-gpio.png)

To turn on the LED that's connected to the net labelled
`PI_GPIO18`, you can run the following:

    iex> {:ok, pid} = Gpio.start_link(18, :output)
    {:ok, #PID<0.96.0>}

    iex> Gpio.write(pid, 1)

Input works similarly:

    iex> {:ok, pid} = Gpio.start_link(17, :input)
    {:ok, #PID<0.97.0>}


    # Push the button down


If you'd like to get a message when the button is pressed or released, call the
`set_int` function. You can trigger on the `:rising` edge, `:falling` edge or

    iex> Gpio.set_int(pid, :both)

    iex> flush
    {:gpio_interrupt, 17, :rising}
    {:gpio_interrupt, 17, :falling}

## SPI

A SPI bus is a common multi-wire bus used to connect components on a circuit
board. A clock line drives the timing of sending bits between components. Bits
on the `MOSI` line go from the master (usually the processor running Linux) to
the slave, and bits on the `MISO` line go the other direction. Bits transfer
both directions simultanteously. However, much of the time, the protocol used
across the SPI bus has a request followed by a response and in these cases, bits
going the "wrong" direction are ignored.

The following shows an example ADC that reads from either a temperature sensor
on CH0 or a potentiometer on CH1.

![SPI schematic](doc/images/schematic-adc.png)

The protocol for talking to the ADC is described in the [MCP3203
Sending a 0x64 first reads the temperature and sending a 0x74 reads the

    # Make sure that you've enabled or loaded the SPI driver or this will
    # fail.
    iex> {:ok, pid} = Spi.start_link("spidev0.0")
    {:ok, #PID<0.124.0>}

    # Read the potentiometer

    # Use binary pattern matching to pull out the ADC counts (low 12 bits)
    iex> <<_::size(4), counts::size(12)>> = Spi.transfer(pid, <<0x74, 0x00>>)
    <<1, 197>>

    iex> counts

    # Convert counts to volts (1023 = 3.3 V)
    iex> volts = counts / 1023 * 3.3

## I2C

An I2C bus is similar to a SPI bus in function, but uses fewer wires. It
supports addressing hardware components and bidirectional use of the data line.

The following shows a bus IO expander connected via I2C to the processor.

![I2C schematic](doc/images/schematic-i2c.png)

The protocol for talking to the IO expander is described in the [MCP23008
Here's a simple example of using it.

    # On the Raspberry Pi, the IO expander is connected to I2C bus 1 (i2c-1).
    # Its 7-bit address is 0x20. (see datasheet)
    iex> {:ok, pid} = I2c.start_link("i2c-1", 0x20)
    {:ok, #PID<0.102.0>}

    # By default, all 8 GPIOs are set to inputs. Set the 4 high bits to outputs
    # so that we can toggle the LEDs. (Write 0x0f to register 0x00)
    iex> I2c.write(pid, <<0x00, 0x0f>>

    # Turn on the LED attached to bit 4 on the expander. (Write 0x10 to register
    # 0x09)
    iex> I2c.write(pid, <<0x09, 0x10>>)

    # Read all 11 of the expander's registers to see that the bit 0 switch is
    # the only one on and that the bit 4 LED is on.
    iex> I2c.write(pid, <<0>>)  # Set the next register to be read to 0

    iex>, 11)
    <<15, 0, 0, 0, 0, 0, 0, 0, 0, 17, 16>>

    # The operation of writing one or more bytes to select a register and
    # then reading is very common, so a shortcut is to just run the following:
    iex> I2c.write_read(pid, <<0>>, 11)
    <<15, 0, 0, 0, 0, 0, 0, 0, 0, 17, 16>>

    # The 17 in register 9 says that bits 0 and bit 4 are high
    # We could have just read register 9.

    iex> I2c.write_read(pid, <<9>>, 1)

# License

This library draws much of its design and code from the Erlang/ALE project which
is licensed under the Apache License, Version 2.0. As such, it is licensed