Character device GPIO library for Elixir.

Since Linux 4.8 the sysfs interface is deprecated in preference of the character device GPIO API.

The advantages of the character device API over the sysfs interface are:

• The allocation of the GPIO is tied to the process that is using it, which prevents many process from trying to control a GPIO at once.
• Since the GPIO is tied to the process, if the process ends for any reason the GPIO will be cleaned up automatically.
• It is possible to read or write many GPIOs at once.
• It is possible to configure the state of the GPIO (open-source, open-drain, etc).
• The polling process to catch event is reliable.

The way to drive a GPIO line (a pin) is by requesting a line handle from a GPIO chip. A system may have many GPIO chips and these can be named or they default to using the name gpiochipN where N is the chip number starting a 0.

For example the main GPIO chip on the Raspberry PI systems is gpiochip0 located at "/dev/gpiochip0".

def deps do
  [{:circuits_cdev, "~> 0.1.0"}]
end

First request a line handle from the GPIO chip:

{:ok, line_handle} = Circuits.Cdev.request_line("gpiochip0", 17, :output)

After getting a line handle can now set the value of the line:

Circuits.Cdev.read_value(line_handle)
0

To set the value of the line:

Circuits.Cdev.set_value(line_handle, 1)
:ok

First request a line handle from the GPIO chip:

{:ok, line_handle} = Circuits.Cdev.request_line("gpiochip0", [17, 27 20], :output)

After getting the line handle you can set the values of the lines. Notice that you to provide all the values in the same order as you requested the lines:

Circuits.Cdev.set_values(line_handle, [0, 1, 1])

When reading the values of a line handle that controls more than one line you will receive a list of values in the order of that you requested the lines:

Circuits.Cdev.read_values(line_handle)
{:ok, [0, 1, 1]}

You can listen for events on an GPIO line by calling the listen_event/2 function:

Circuits.Cdev.listen_event("gpiochip0", 27)
:ok

When an event is received from the line it will be in the form of:

{:circuits_cdev, pin_number, timestamp, new_value}

The timestamp is the unix time in nanoseconds. In order to convert it into a DateTime you will need to pass the :nanoseconds as the unit to DateTime.from_unix/3 function:

DateTime.from_unix(timestamp, :nanoseconds)
{:ok, ~U[1990-07-24 07:30:03.123456Z]}

Also when calculating the time delta between two events keep in mind the difference will be nanoseconds.