defmodule Circuits.I2C do
@moduledoc """
`Circuits.I2C` lets you communicate with hardware devices using the I2C
protocol.
"""
alias Circuits.I2C.Bus
# Public API
@typedoc """
I2C device address
This is a "7-bit" address for the device. Some devices specify an "8-bit"
address in their documentation. You can tell if you have an "8-bit" address
if it's greater than 127 (0x7f) or if the documentation talks about different
read and write addresses. If you have an 8-bit address, divide it by 2.
"""
@type address() :: 0..127
@typedoc """
Function to report back whether a device is present
See `discover/2` for how a custom function can improve device detection when
the type of device being looked for is known.
"""
@type present?() :: (Bus.t(), address() -> boolean())
@typedoc """
Backends specify an implementation of a Circuits.I2C.Backend behaviour
The second parameter of the Backend 2-tuple is a list of options. These are
passed to the behaviour function call implementations.
"""
@type backend() :: {module(), keyword()}
@type opt() :: {:retries, non_neg_integer()}
@typedoc """
Connection to a real or virtual I2C controller
"""
@type bus() :: Bus.t()
defmacrop unwrap_or_raise(call) do
quote do
case unquote(call) do
{:ok, value} -> value
{:error, reason} -> raise "I2C failure: " <> to_string(reason)
end
end
end
defmacrop unwrap_or_raise_ok(call) do
quote do
case unquote(call) do
:ok -> :ok
{:error, reason} -> raise "I2C failure: " <> to_string(reason)
end
end
end
@doc """
Open an I2C bus
I2C bus names depend on the platform. Names are of the form "i2c-n" where the
"n" is the bus number. The correct bus number can be found in the
documentation for the device or on a schematic. Another option is to call
`Circuits.I2C.bus_names/0` to list them for you.
The same I2C bus may be opened more than once. There is no need to share
it between modules.
On success, this returns a `Circuits.I2C.Bus.t()` struct for accessing the
I2C bus. Use this in subsequent calls to read and write I2C devices.
Options depend on the backend. The following are for the I2CDev (default)
backend:
* `:retries` - the default number of retries to automatically do on reads
and writes (defaults to no retries)
"""
@spec open(String.t(), keyword()) :: {:ok, Bus.t()} | {:error, term()}
def open(bus_name, options \\ []) when is_binary(bus_name) do
{module, default_options} = default_backend()
module.open(bus_name, Keyword.merge(default_options, options))
end
@doc """
Initiate a read transaction to the I2C device at the specified `address`
Options:
* `:retries` - number of retries before failing (defaults to no retries)
"""
@spec read(Bus.t(), address(), pos_integer(), [opt()]) :: {:ok, binary()} | {:error, term()}
def read(bus, address, bytes_to_read, opts \\ []) do
Bus.read(bus, address, bytes_to_read, opts)
end
@doc """
Initiate a read transaction and raise on error
"""
@spec read!(Bus.t(), address(), pos_integer(), [opt()]) :: binary()
def read!(bus, address, bytes_to_read, opts \\ []) do
unwrap_or_raise(read(bus, address, bytes_to_read, opts))
end
@doc """
Write `data` to the I2C device at `address`.
Options:
* `:retries` - number of retries before failing (defaults to no retries)
"""
@spec write(Bus.t(), address(), iodata(), [opt()]) :: :ok | {:error, term()}
def write(bus, address, data, opts \\ []) do
Bus.write(bus, address, data, opts)
end
@doc """
Write `data` to the I2C device at `address` and raise on error
Options:
* `:retries` - number of retries before failing (defaults to no retries)
"""
@spec write!(Bus.t(), address(), iodata(), [opt()]) :: :ok
def write!(bus, address, data, opts \\ []) do
unwrap_or_raise_ok(write(bus, address, data, opts))
end
@doc """
Write `data` to an I2C device and then immediately issue a read.
This function is useful for devices that want you to write the "register"
location to them first and then issue a read to get its contents. Many
devices operate this way and this function will issue the commands
back-to-back on the I2C bus. Some I2C devices actually require that the read
immediately follows the write. If the target supports this, the I2C
transaction will be issued that way. On the Raspberry Pi, this can be enabled
globally with `File.write!("/sys/module/i2c_bcm2708/parameters/combined", "1")`
Options:
* `:retries` - number of retries before failing (defaults to no retries)
"""
@spec write_read(Bus.t(), address(), iodata(), pos_integer(), [opt()]) ::
{:ok, binary()} | {:error, term()}
def write_read(bus, address, write_data, bytes_to_read, opts \\ []) do
Bus.write_read(bus, address, write_data, bytes_to_read, opts)
end
@doc """
Write `data` to an I2C device and then immediately issue a read. Raise on errors.
Options:
* `:retries` - number of retries before failing (defaults to no retries)
"""
@spec write_read!(Bus.t(), address(), iodata(), pos_integer(), [opt()]) :: binary()
def write_read!(bus, address, write_data, bytes_to_read, opts \\ []) do
unwrap_or_raise(write_read(bus, address, write_data, bytes_to_read, opts))
end
@doc """
close the I2C bus
"""
@spec close(Bus.t()) :: :ok
def close(bus) do
Bus.close(bus)
end
@doc """
Return a list of available I2C bus names. If nothing is returned, it's
possible that the kernel driver for that I2C bus is not enabled or the
kernel's device tree is not configured. On Raspbian, run `raspi-config` and
look in the advanced options.
```elixir
iex> Circuits.I2C.bus_names()
["i2c-1"]
```
"""
@spec bus_names() :: [String.t()]
def bus_names() do
{m, o} = default_backend()
m.bus_names(o)
end
@doc """
Scan the I2C bus for devices by performing a read at each device address and
returning a list of device addresses that respond.
```elixir
iex> Circuits.I2C.detect_devices("i2c-1")
[4]
```
The return value is a list of device addresses that were detected on the
specified I2C bus. If you get back `'Hh'` or other letters, then IEx
converted the list to an Erlang string. Run `i v()` to get information about
the return value and look at the raw string representation for addresses.
> #### Warning {: .warning}
> This is intended to be a debugging aid. Reading bytes from devices can
> advance internal state machines and might cause them to get out of sync
> with other code.
If you already have opened an I2C bus, then pass that to `detect_devices/1`
instead of the string name.
"""
@spec detect_devices(Bus.t() | binary()) :: [address()] | {:error, term()}
def detect_devices(bus) when is_struct(bus) do
Enum.filter(0x03..0x77, &device_present?(bus, &1))
end
def detect_devices(bus_name) when is_binary(bus_name) do
case open(bus_name) do
{:ok, bus} ->
devices = detect_devices(bus)
Bus.close(bus)
devices
error ->
error
end
end
@doc """
Convenience method to scan all I2C buses for devices
This is only intended to be called from the IEx prompt. Programs should
use `detect_devices/1`.
"""
@spec detect_devices() :: :"do not show this result in output"
def detect_devices() do
buses = bus_names()
total_devices = Enum.reduce(buses, 0, &detect_and_print/2)
IO.puts("#{total_devices} devices detected on #{length(buses)} I2C buses")
:"do not show this result in output"
end
@doc """
Scan all I2C buses for one or more devices
This function takes a list of possible addresses and an optional detection
function. It only scans addresses in the possible addresses list to avoid
disturbing unrelated I2C devices.
If a detection function is not passed in, a default one that performs a
simple read and checks whether it succeeds is used. If the desired device has
an ID register or other means of identification, the optional function should
try to query that. If passing a custom function, be sure to return `false`
rather than raise if there are errors.
A list of bus name and address tuples is returned. The list may be empty.
See also `discover_one/2`.
"""
@spec discover([address()], present?()) :: [{binary(), address()}]
def discover(possible_addresses, present? \\ &device_present?/2) do
Enum.flat_map(bus_names(), &discover(&1, possible_addresses, present?))
end
@spec discover(binary(), [address()], present?()) :: [{binary(), address()}]
defp discover(bus_name, possible_addresses, present?) when is_binary(bus_name) do
{module, options} = default_backend()
case module.open(bus_name, options) do
{:ok, bus} ->
result =
possible_addresses
|> Enum.filter(fn address -> present?.(bus, address) end)
|> Enum.map(&{bus_name, &1})
close(bus)
result
{:error, reason} ->
raise "I2C discovery error: Opening #{bus_name} failed with #{reason}"
end
end
@doc """
Scans all I2C buses for one specific device
This function and `discover_one!/2` are convenience functions for the use
case of helping a user find a specific device. They both call `discover/2` with
a list of possible I2C addresses and an optional function for checking whether
the device is present.
This function returns an `:ok` or `:error` tuple depending on whether one and
only one device was found. See `discover_one!/2` for the raising version.
"""
@spec discover_one([address()], present?()) ::
{:ok, {binary(), address()}} | {:error, :not_found | :multiple_possible_matches}
def discover_one(possible_addresses, present? \\ &device_present?/2) do
case discover(possible_addresses, present?) do
[actual_device] -> {:ok, actual_device}
[] -> {:error, :not_found}
_ -> {:error, :multiple_possible_matches}
end
end
@doc """
Same as `discover_one/2` but raises on error
"""
@spec discover_one!([address()], present?()) :: {binary(), address()}
def discover_one!(possible_addresses, present? \\ &device_present?/2) do
unwrap_or_raise(discover_one(possible_addresses, present?))
end
defp detect_and_print(bus_name, count) do
IO.puts("Devices on I2C bus \"#{bus_name}\":")
devices = detect_devices(bus_name)
Enum.each(devices, &IO.puts(" * #{&1} (0x#{Integer.to_string(&1, 16)})"))
IO.puts("")
count + length(devices)
end
@doc """
Return whether a device is present
This function performs a simplistic check for an I2C device on the specified
bus and address. It's not perfect, but works enough to be useful. Be warned
that it does perform an I2C read on the specified address and this may cause
some devices to actually do something.
"""
@spec device_present?(Bus.t(), address()) :: boolean()
def device_present?(bus, address) do
cond do
address in 0x30..0x37 -> probe_read(bus, address)
address in 0x50..0x5F -> probe_read(bus, address)
:supports_empty_write in Bus.flags(bus) -> probe_empty_write(bus, address)
true -> probe_read(bus, address)
end
end
defp probe_read(bus, address), do: match?({:ok, _}, read(bus, address, 1))
defp probe_empty_write(bus, address), do: :ok == write(bus, address, <<>>)
@doc """
Return info about the low level I2C interface
This may be helpful when debugging I2C issues.
"""
@spec info(backend() | nil) :: map()
def info(backend \\ nil)
def info(nil), do: info(default_backend())
def info({backend, _options}), do: backend.info()
defp default_backend() do
case Application.get_env(:circuits_i2c, :default_backend) do
nil -> {Circuits.I2C.NilBackend, []}
m when is_atom(m) -> {m, []}
{m, o} = value when is_atom(m) and is_list(o) -> value
end
end
end
