1. First steps with an LED strip

Mix.install([
  {:circuits_spi, "~> 2.0"}
])

Search and open the SPI bus

First of all we try to figure out what kind of SPI buses exist on the device

_ = Circuits.SPI.bus_names()

As a next step we open the first SPI bus and close it again. Let's make sure this works without any error

{:ok, ref} =
  Circuits.SPI.open("spidev0.0",
    mode: 0,
    bits_per_word: 8,
    speed_hz: 1_000_000,
    delay_us: 10,
    lsb_first: false
  )

:ok = Circuits.SPI.close(ref)

Define a data structure

Mext we define a data structure as required by the LED strip. The documentation states that we simply have to send the bytes for the different LEDs in a row. If it's too short only a couple of LEDs will be addressed and the rest will not get any info. If the data structure is longer all LEDs will receive their update and the rest of the data will be ignored.

To start with we define 3 LEDs with the color red

range = Enum.to_list(1..3)

data =
  Enum.reduce(range, <<>>, fn _index, leds ->
    leds <> <<0xFF, 0x00, 0x00>>
  end)

Send the data to the LED strip

For this we need to combine the above steps, i.e.

1. We open the SPI bus
2. We define a datastructure with only zeros (to delete the strip, make it completely black)
3. We send the binary to the SPI bus
4. We define our data structure with some colors (see above)
5. We send the binary to the SPI bus
6. We close the SPI bus

Let's see what the result will be

# 1
{:ok, ref} =
  Circuits.SPI.open("spidev0.0",
    mode: 0,
    bits_per_word: 8,
    speed_hz: 1_000_000,
    delay_us: 10,
    lsb_first: false
  )

# 2 (we got a 3m strup with 96 LEDs, 3x 32 LEDs)
data_black =
  Enum.reduce(Enum.to_list(1..96), <<>>, fn lindex, leds ->
    leds <> <<0, 0, 0>>
  end)

# 3
{:ok, _} = Circuits.SPI.transfer(ref, data_black)

# 4
data_3red =
  Enum.reduce(Enum.to_list(1..3), <<>>, fn _index, leds ->
    leds <> <<0xFF, 0x00, 0x00>>
  end)

# 5
{:ok, _} = Circuits.SPI.transfer(ref, data_3red)

# 6
:ok = Circuits.SPI.close(ref)