A/D コンバータの MCP3002, MCP3208 を raspberry pi 上の Ruby から扱う ref: http://qiita.com/hotchpotch/items/5c47ae210a7a8e9fd6d1

file0.txt

require 'spi_mcp'

mcp = MCP3208.new
val = mcp.channel(0)
puts val # 0 ~ 4095

file1.txt

$ watch -n 1 sudo ruby spi_mcp.rb MCP3208

file2.rb

# spi_mcp.rb

require 'pi_piper'

class SpiMCP
  attr_reader :bit_resolution
  attr_reader :channels

  def initialize(options = {})
    @bit_resolution = options[:bit_resolution] or ArgumentError.new("require :bit_resolution")
    @channels = options[:channels] or ArgumentError.new("require :channels")
  end

  def channel(ch)
    raise ArgumentError.new("channel #{ch} is not support.") unless channels.include?(ch)
    result = 0
    PiPiper::Spi.begin do |spi|
      result = convert spi.write(*write_data(ch))
    end
    result
  end

  def convert(raw_data)
    ((raw_data[-2] << 8) + raw_data[-1]) & (2**bit_resolution - 1)
  end
end

class MCP3002 < SpiMCP
  def initialize
    super bit_resolution: 10, channels: 0..1
  end

  def write_data(ch)
    [
      0b01101000 | (ch<<4),
      0b00000000
    ]
  end
end

class MCP3208 < SpiMCP
  def initialize
    super bit_resolution: 12, channels: 0..7
  end

  def write_data(ch)
    control_bit = channel_control_bit(ch)
    [
      0b00000100 | (control_bit>>2), # 0b000001#{SINGLE/DIFF}#{D2}
      (control_bit & 0b11) << 6,     # 0b#{D1}#{D0}000000 (D1, D0)
      0b00000000
    ]
  end

  # return 4-bit (SINGLE/DIFF, D2, D1, D0)
  def channel_control_bit(ch)
    (0b1000 | ch) & 0b1111
  end
end

if __FILE__ == $0
  begin
    mcp_class = Object.const_get(ARGV[0])
  rescue NameError => e
    abort "usage: $ ruby #{__FILE__} MCP3208"
  end

  mcp = mcp_class.new
  mcp.channels.each do |ch|
    puts "ch #{ch}: #{mcp.channel(ch)}"
  end
end

file2.txt

git clone https://github.com/jwhitehorn/pi_piper.git
cd pi_piper
gem install bundler rake
bundle install
rake gem
sudo gem install pkg/pi_piper-2.0.beta.4.gem

file3.rb

# spi_mcp.rb

require 'pi_piper'

class SpiMCP
  attr_reader :bit_resolution
  attr_reader :channels

  def initialize(options = {})
    @bit_resolution = options[:bit_resolution] or ArgumentError.new("require :bit_resolution")
    @channels = options[:channels] or ArgumentError.new("require :channels")
  end

  def channel(ch)
    raise ArgumentError.new("channel #{ch} is not support.") unless channels.include?(ch)
    result = 0
    PiPiper::Spi.begin do |spi|
      result = convert spi.write(*write_data(ch))
    end
    result
  end

  def convert(raw_data)
    ((raw_data[-2] << 8) + raw_data[-1]) & (2**bit_resolution - 1)
  end
end

class MCP3002 < SpiMCP
  def initialize
    super bit_resolution: 10, channels: 0..1
  end

  def write_data(ch)
    [
      0b01101000 | (ch<<4),
      0b00000000
    ]
  end
end

class MCP3208 < SpiMCP
  def initialize
    super bit_resolution: 12, channels: 0..7
  end

  def write_data(ch)
    control_bit = channel_control_bit(ch)
    [
      0b00000100 | (control_bit>>2), # 0b000001#{SINGLE/DIFF}#{D2}
      (control_bit & 0b11) << 6,     # 0b#{D1}#{D0}000000 (D1, D0)
      0b00000000
    ]
  end

  # return 4-bit (SINGLE/DIFF, D2, D1, D0)
  def channel_control_bit(ch)
    (0b1000 | ch) & 0b1111
  end
end

if __FILE__ == $0
  begin
    mcp_class = Object.const_get(ARGV[0])
  rescue NameError => e
    abort "usage: $ ruby #{__FILE__} MCP3208"
  end

  mcp = mcp_class.new
  mcp.channels.each do |ch|
    puts "ch #{ch}: #{mcp.channel(ch)}"
  end
end