# `Circuits.SPI.Backend`
[🔗](https://github.com/elixir-circuits/circuits_spi/blob/v2.2.0/lib/spi/backend.ex#L5)

Backends provide the connection to the real or virtual SPI controller

Select a backend with the `:circuits_spi, :default_backend` application
configuration. The value may be a backend module or a
`{backend_module, default_options}` tuple. See `Circuits.SPI` for details.

A backend's `c:open/2` callback returns a bus value that implements the
`Circuits.SPI.Bus` protocol. The protocol provides configuration, transfer,
read, write, close, and maximum transfer size operations for that value.

# `bus_names`

```elixir
@callback bus_names(options :: keyword()) :: [String.t()]
```

Return SPI bus names on this system

No options are supported.

# `info`

```elixir
@callback info() :: map()
```

Return information about this backend

# `open`

```elixir
@callback open(bus_name :: String.t(), [Circuits.SPI.spi_option()]) ::
  {:ok, Circuits.SPI.Bus.t()} | {:error, term()}
```

Open an SPI bus device

On success, `open/2` returns `{:ok, bus}`, where `bus` implements
`Circuits.SPI.Bus` and may be passed to `Circuits.SPI.transfer/2` and the
other bus operations.

Backends should release resources when the bus is garbage collected and
support explicit cleanup through `Circuits.SPI.Bus.close/1`.

SPI is not a standardized interface, so appropriate options will
differ from device to device. The defaults used here work on
many devices.

Parameters:
* `bus_name` is the name of the bus (e.g., "spidev0.0"). See `c:bus_names/1`
* The second argument is a keyword list to configure the bus

---

*Consult [api-reference.md](api-reference.md) for complete listing*