botbook_mcp3002.py

# botbook_mcp3002.py - read analog values from mcp3002
# (c) BotBook.com - Karvinen, Karvinen, Valtokari

# Installing spidev: 
#  sudo apt-get update
#  sudo apt-get -y install git python-dev
#  git clone https://github.com/doceme/py-spidev.git
#  cd py-spidev/
#  sudo python setup.py install

import spidev   # installation help in botbook_mcp3002.py comments
import time

def readAnalog(device = 0,channel = 0):
    assert device in (1, 0)
    assert channel in (1, 0)
    #open spi
    spi = spidev.SpiDev()
    spi.open(0, device)
    """
    Protocol start bit (S), sql/diff (D), odd/sign (C), MSBF (M)
    Use leading zero for more stable clock cycle
    0000 000S DCM0 0000 0000 0000
    Sending 3 8bit packages so xpi.xfer2 will return the same amount.
    start bit = 1
    sql/diff = 1 SINGLE ENDED MODE  (2 channel mode) 
    odd/sign = channel 0/1
    MSBF = 0
    """
    command = [1, (2 + channel) << 6, 0]
    #2 + channel shifted 6 to left
    #10 or 11 << 6 = 1000 0000 or 1100 0000
    reply = spi.xfer2(command)
    """
    Parse right bits from 24 bit package (3*8bit)
    We need only data from last 2 bytes.
    And there we can discard last two bits to get 10 bit value 
    as MCP3002 resolution is 10bits
    Discard reply[0] byte and start from reply[1] where our data starts
    """ 
    value = reply[1] & 31   
    #31 = 0001 1111 with & operation makes sure that we have all data from XXXX DDDD and nothing more. 0001 is for signed in next operation.
    value = value << 6  #Move to left to make room for next piece of data.
    #000D DDDD << 6 = 0DDD DD00 0000
    #Now we get the last of data from reply[2]
    value = value + (reply[2] >> 2)
    #Here we discard last to bits
    #DDDD DDXXX >> 2 = 00DD DDDD
    #0DDD DD00 0000 + 00DD DDDD = 0DDD DDDD DDDD
    spi.close()
    return value


def main():
    #read channel 0 on device 0
    value = readAnalog(0, 0)
    print(value)
    time.sleep(10)

if __name__ == "__main__":
    main()