# Circuits - GPIO
[](https://hex.pm/packages/circuits_gpio)
[](https://hexdocs.pm/circuits_gpio/Circuits.GPIO.html)
[](https://circleci.com/gh/elixir-circuits/circuits_gpio)
[](https://api.reuse.software/info/github.com/elixir-circuits/circuits_gpio)
`Circuits.GPIO` lets you use GPIOs in Elixir.
*This is the v2.0 development branch. If you're still using v1, please see the [maint-v1.x branch](https://github.com/elixir-circuits/circuits_gpio/tree/maint-v1.x).*
`Circuits.GPIO` v2.0 is an almost backwards compatible update to `Circuits.GPIO`
v1.x. Here's what's new:
* Linux or Nerves are no longer required. In fact, the NIF supporting them won't
be compiled if you don't want it.
* GPIOs can be enumerated to see what's available (See `Circuits.GPIO.enumerate/0`)
* Linux and Nerves now use the Linux GPIO cdev subsystem rather than sysfs
* GPIO pull mode setting for all platforms that support it rather than only Raspberry Pi
* Develop using simulated GPIOs to work with LEDs and buttons with
[CircuitsSim](https://github.com/elixir-circuits/circuits_sim)
If you've used `Circuits.GPIO` v1.x, nearly all of your code will be the
same.`Circuits.GPIO` offers a substantial improvement by more descriptive GPIO
specs for identifying GPIOs. You can still refer to GPIOs by number. However,
you can also refer to GPIOs by labels and by which GPIO controller handles them.
The new `enumerate/0` can help with this.
Please review the [porting guide](PORTING.md) when upgrading from v1.x.
## Getting started on Nerves and Linux
If you're natively compiling `circuits_gpio` using Nerves or using a Linux-based
SBC like a Raspberry Pi, everything should work like any other Elixir library.
Normally, you would include `circuits_gpio` as a dependency in your `mix.exs`
like this:
```elixir
def deps do
[{:circuits_gpio, "~> 2.0"}]
end
```
One common error on RaspberryPi OS is that the Erlang headers are missing
(`erl_nif.h`), you may need to install erlang with `apt-get install erlang-dev`
or build Erlang from source per instructions [here](http://elinux.org/Erlang).
## Examples
While `Circuits.GPIO` can support non-Nerves and non-Linux systems, the examples
below were made using Nerves. Operation on other devices mostly differs on how
to refer to GPIOs.
The following examples were tested on a Raspberry Pi that was connected to an
[Erlang Embedded Demo Board](http://solderpad.com/omerk/erlhwdemo/). There's
nothing special about either the demo board or the Raspberry Pi.
## GPIO
A [General Purpose
Input/Output](https://en.wikipedia.org/wiki/General-purpose_input/output) (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 of turning an LED on or off:
To turn on the LED that's connected to the net (or wire) labeled `GPIO18`, you
need to open it first. The first parameter to `Circuits.GPIO.open/2` is called a
GPIO spec and identifies the GPIO. The Raspberry Pis are nice and provide string
names for GPIOs. Other boards are not as nice so you always have to check. The
string name for this GPIO is `"GPIO18"` (use `"PIN12"` on a Raspberry Pi 5).
```elixir
iex> {:ok, gpio} = Circuits.GPIO.open("GPIO12", :output)
{:ok, %Circuits.GPIO.CDev{...}}
iex> Circuits.GPIO.write(gpio, 1)
:ok
iex> Circuits.GPIO.close(gpio)
:ok
```
The call to `Circuits.GPIO.close/1` is not necessary, since the garbage
collector will free up unreferenced GPIOs. It's a good practice, though,
since backends can enforce exclusivity and prevent future opens from
working until the GC occurs.
Input works similarly. Here's an example of a button with a pull down resistor
connected.
If you're not familiar with pull up or pull down resistors, they're resistors
whose purpose is to drive a wire high or low when the button isn't pressed. In
this case, it drives the wire low. Many processors have ways of configuring
internal resistors to accomplish the same effect without needing to add an
external resistor. If you're using a Raspberry Pi, you can use [the built-in
pull-up/pull-down resistors](#internal-pull-uppull-down).
The code looks like this in `Circuits.GPIO`:
```elixir
iex> {:ok, gpio} = Circuits.GPIO.open("GPIO17", :input)
{:ok, %Circuits.GPIO.CDev{...}}
iex> Circuits.GPIO.read(gpio)
0
# Push the button down
iex> Circuits.GPIO.read(gpio)
1
```
If you'd like to get a message when the button is pressed or released, call the
`set_interrupts` function. You can trigger on the `:rising` edge, `:falling`
edge or `:both`.
```elixir
iex> Circuits.GPIO.set_interrupts(gpio, :both)
:ok
iex> flush
{:circuits_gpio, "GPIO17", 1233456, 1}
{:circuits_gpio, "GPIO17", 1234567, 0}
:ok
```
Note that after calling `set_interrupts`, the calling process will receive an
initial message with the state of the pin. This prevents the race condition
between getting the initial state of the pin and turning on interrupts. Without
it, you could get the state of the pin, it could change states, and then you
could start waiting on it for interrupts. If that happened, you would be out of
sync.
### Internal pull-up/pull-down
To connect or disconnect an internal [pull-up or pull-down resistor](https://github.com/raspberrypilearning/physical-computing-guide/blob/master/pull_up_down.md) to a GPIO
pin, call the `set_pull_mode` function.
```elixir
iex> Circuits.GPIO.set_pull_mode(gpio, pull_mode)
:ok
```
Valid `pull_mode` values are `:none` `:pullup`, or `:pulldown`
Note that `set_pull_mode` is platform dependent, and currently only works for
Raspberry Pi hardware. Calls to `set_pull_mode` on other platforms will have no
effect. The internal pull-up resistor value is between 50K and 65K, and the
pull-down is between 50K and 60K. It is not possible to read back the current
Pull-up/down settings, and GPIO pull-up pull-down resistor connections are
maintained, even when the CPU is powered down.
## GPIO Specs
`Circuits.GPIO` v2.0 supports a new form of specifying how to open a GPIO called
a `t:gpio_spec/0`. These specs are very flexible and allow for GPIOs to be
opened by number, a string label, or a tuple that includes both the GPIO
controller hardware name and a line offset.
The contents of a `gpio_spec` depend on the backend. When running on Nerves or
a Linux machine, `Circuits.GPIO` uses the Linux gpio-cdev backend. This backend
prefers the use of GPIO controller/line offset tuples and labels. For backwards
compatibility, it somewhat supports use of the *older* pin numbering scheme.
The GPIO controller part of the tuple is usually some variation on `"gpiochip0"`
that depends on what controllers are available under `/dev`. The line offset is
a the line offset of the GPIO on that controller.
```elixir
iex> {:ok, gpio} = Circuits.GPIO.open({"gpiochip0", 1}, :input)
{:ok, %Circuits.GPIO.CDev{...}}
```
When the Linux device tree is configured with GPIO labels, you can use those instead:
```elixir
iex> {:ok, gpio} = Circuits.GPIO.open("special-name-for-pin-1")
{:ok, %Circuits.GPIO.CDev{...}}
```
If you're deploying to multiple types of devices and you can set labels in the
device tree, labels make it really easy for code using `Circuits.GPIO` to just
use the right GPIO.
Labels are not guaranteed to be unique, so if your device defines one twice,
`Circuits.GPIO` will use the first GPIO it finds that has the specified label.
See [Enumeration](#enumeration) for listing out all available `gpio_specs` for
your device.
## Enumeration
`Circuits.GPIO` v2.0 supports a new function, `enumerate/0`, which lists every
known GPIO pin.
For Nerves and Linux users, the `gpio-cdev` subsystem maintains the official
list. See the [Official DeviceTree documentation for
GPIOs](https://elixir.bootlin.com/linux/v6.6.6/source/Documentation/devicetree/bindings/gpio/gpio.txt)
for more information on how to configure the fields of this struct for your own
system.
Here's an example:
```elixir
iex> Circuits.GPIO.enumerate()
[
%{
location: {"gpiochip0", 0},
label: "ID_SDA",
controller: "pinctrl-bcm2835"
},
%{
location: {"gpiochip0", 1},
label: "ID_SCL",
controller: "pinctrl-bcm2835"
},
%{
location: {"gpiochip0", 2},
label: "SDA1",
controller: "pinctrl-bcm2835"
},
...
]
```
The `:location` can always be passed as the first parameter to
`Circuits.GPIO.open/3`. You may find the `:label` field more descriptive to use,
though.
## Convenience functions
Having to `open` *then* `read` and `write` can be a cumbersome for one-off GPIO
access in code and when working at the IEx prompt. Circuits v2.0 has a pair of
new functions to help:
```elixir
iex> alias Circuits.GPIO
iex> GPIO.write_one("special-name-for-pin-1", 1)
:ok
iex> GPIO.read_one("special-name-for-pin-2")
1
```
These functions get passed a `t:gpio_spec/0` just like `open/3` and internally
open the GPIO and read or write it. Importantly, they `close` the GPIO when done
to avoid reserving the GPIO any longer than necessary.
Please note that this is not a performant way of reading or writing the same
GPIO more than once. Opening a GPIO takes much longer than reading or writing an
already opened one, so if these are used in tight loops, the open overhead will
dominate (>99% of the time taken in a trivial benchmark.)
## Testing
`Circuits.GPIO` supports a "stub" hardware abstraction layer on platforms
without GPIO support and when `MIX_ENV=test`. The stub allows for some limited
unit testing without real hardware.
To use it, first check that you're using the "stub" HAL:
```elixir
iex> Circuits.GPIO.backend_info()
%{name: :stub, pins_open: 0}
```
The stub HAL has 64 GPIOs. Each pair of GPIOs is connected. For example, GPIO 0
is connected to GPIO 1. If you open GPIO 0 as an output and GPIO 1 as an input,
you can write to GPIO 0 and see the result on GPIO 1. Here's an example:
```elixir
iex> {:ok, gpio0} = Circuits.GPIO.open({"gpiochip0", 0}, :output)
{:ok, %Circuits.GPIO.CDev{...}}
iex> {:ok, gpio1} = Circuits.GPIO.open({"gpiochip0", 1}, :input)
{:ok, %Circuits.GPIO.CDev{...}}
iex> Circuits.GPIO.read(gpio1)
0
iex> Circuits.GPIO.write(gpio0, 1)
:ok
iex> Circuits.GPIO.read(gpio1)
1
```
The stub HAL is fairly limited, but it does support interrupts.
If `Circuits.GPIO` is used as a dependency the stub may not be present. To
manually enable it, set `CIRCUITS_MIX_ENV` to `test` and rebuild
`circuits_gpio`.
## FAQ
### Where can I get help?
Most issues people have are on how to communicate with hardware for the first
time. Since `Circuits.GPIO` is a thin wrapper on the Linux CDev GPIO interface,
you may find help by searching for similar issues when using Python or C.
For help specifically with `Circuits.GPIO`, you may also find help on the nerves
channel on the [elixir-lang Slack](https://elixir-lang.slack.com/). Many
[Nerves](http://nerves-project.org) users also use `Circuits.GPIO`.
### I tried turning on and off a GPIO as fast as I could. Why was it slow?
Please don't do that - there are so many better ways of accomplishing whatever
you're trying to do:
1. If you're trying to drive a servo or dim an LED, look into PWM. Many
platforms have PWM hardware and you won't tax your CPU at all. If your
platform is missing a PWM, several chips are available that take I2C commands
to drive a PWM output.
2. If you need to implement a wire level protocol to talk to a device, look for
a Linux kernel driver. It may just be a matter of loading the right kernel
module.
3. If you want a blinking LED to indicate status, `gpio` really should
be fast enough to do that, but check out Linux's LED class interface. Linux
can flash LEDs, trigger off events and more. See [nerves_leds](https://github.com/nerves-project/nerves_leds).
If you're still intent on optimizing GPIO access, you may be interested in
[gpio_twiddler](https://github.com/fhunleth/gpio_twiddler). Note that the
twiddler doc is dated. Circuits.GPIO v2 should be faster than v1.
### Can I develop code that uses GPIO on my laptop?
The intended way to support this is to have a custom `Circuits.GPIO.Backend`
that runs on your laptop. The
[CircuitsSim](https://github.com/elixir-circuits/circuits_sim) is an example of
a project that provides simulated LEDs and buttons.
## License
All original source code in this project is licensed under Apache-2.0.
Additionally, this project follows the [REUSE recommendations](https://reuse.software)
and labels so that licensing and copyright are clear at the file level.
Exceptions to Apache-2.0 licensing are:
* Configuration and data files are licensed under CC0-1.0
* Documentation files are CC-BY-4.0
* Erlang Embedded board images are Solderpad Hardware License v0.51.
