# `Circuits.SPI`
[🔗](https://github.com/elixir-circuits/circuits_spi/blob/v2.0.4/lib/spi.ex#L5)

This module enables Elixir programs to interact with hardware that's connected
via a SPI bus.

# `backend`

```elixir
@type backend() :: {module(), keyword()}
```

Backends specify an implementation of a Circuits.SPI.Backend behaviour

The second parameter of the Backend 2-tuple is a list of options. These are
passed to the behaviour function call implementations.

# `spi_option`

```elixir
@type spi_option() ::
  {:mode, 0..3}
  | {:bits_per_word, 8..16}
  | {:speed_hz, pos_integer()}
  | {:delay_us, non_neg_integer()}
  | {:lsb_first, boolean()}
```

SPI bus options

Options:

* `mode` - Set the clock polarity and phase to use:
  * Mode 0 (CPOL=0, CPHA=0) - Clock idle low/sample leading edge (default)
  * Mode 1 (CPOL=0, CPHA=1) - Clock idle low/sample trailing edge
  * Mode 2 (CPOL=1, CPHA=0) - Clock idle high/sample leading edge
  * Mode 3 (CPOL=1, CPHA=1) - Clock idle high/sample trailing edge
* `bits_per_word` - Set the bits per word on the bus. Defaults to 8 bit words.
* `speed_hz` - Set the bus speed. Supported speeds are device-specific. The
  default speed is 1 Mbps (1000000).
* `delay_us` - Set the delay between transactions (10)
* `lsb_first` - Set to `true` to send the least significant bit first rather
  than the most significant one. (false)
  The error message `unsupported mode bits 8` might be printed due to
  hardware that doesn't support the LSB-first mode, which can be ignored
  since Circuits.SPI handles it automatically.

# `spi_option_map`

```elixir
@type spi_option_map() :: %{
  mode: 0..3,
  bits_per_word: 8..16,
  speed_hz: pos_integer(),
  delay_us: non_neg_integer(),
  lsb_first: boolean(),
  sw_lsb_first: boolean()
}
```

SPI bus options as returned by `config/1`.

These mirror the options that can be passed to `open/2`. `:sw_lsb_first`
is set if `:lsb_first` is true, but Circuits.SPI is doing this in software.

# `bus_names`

```elixir
@spec bus_names() :: [binary()]
```

Return a list of available SPI bus names.  If nothing is returned,
it's possible that the kernel driver for that SPI bus is not enabled or the
kernel's device tree is not configured. On Raspbian, run `raspi-config` and
look in the advanced options.
```
iex> Circuits.SPI.bus_names
["spidev0.0", "spidev0.1"]
```

# `close`

```elixir
@spec close(Circuits.SPI.Bus.t()) :: :ok
```

Release any resources associated with the given file descriptor

# `config`

```elixir
@spec config(Circuits.SPI.Bus.t()) :: {:ok, spi_option_map()} | {:error, term()}
```

Return the configuration for this SPI bus

The configuration could be different that what was given to `open/2` if
the device had to change it for it to work.

# `info`

```elixir
@spec info(backend() | nil) :: map()
```

Return info about the low level SPI interface

This may be helpful when debugging SPI issues.

# `max_transfer_size`

```elixir
@spec max_transfer_size(Circuits.SPI.Bus.t() | nil) :: non_neg_integer()
```

Return the maximum transfer size in bytes

The number of bytes that can be sent and received at a time
may be capped by the low level SPI interface. For example,
the Linux `spidev` driver allocates its transfer buffer at
initialization based on the `bufsiz` parameter and rejects
requests that won't fit.

If you're sending large amounts of data over SPI, use this
function to determine how to split up large messages.

# `open`

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

Open a SPI bus device

On success, `open/2` returns a reference that may be passed to
with `transfer/2`. The device will be closed automatically when
the reference goes out of scope.

SPI is not a standardized interface so appropriate options will
different from device-to-device. The defaults use here work on
many devices.

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

# `transfer`

```elixir
@spec transfer(Circuits.SPI.Bus.t(), iodata()) :: {:ok, binary()} | {:error, term()}
```

Transfer data

Since each SPI transfer sends and receives simultaneously, the return value
will be a binary of the same length as `data`.

# `transfer!`

```elixir
@spec transfer!(Circuits.SPI.Bus.t(), iodata()) :: binary()
```

Transfer data and raise on error

---

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