checksumClass.py

# Checksum Class
# Verifies Checksum (CRC) for reading Unique ID, temperature, and humidity.

crctab = [
          0, 94, 188, 226, 97, 63, 221, 131, 194, 156, 126, 32, 163, 253, 31, 65,
          157, 195, 33, 127, 252, 162, 64, 30, 95, 1, 227, 189, 62, 96, 130, 220,
          35, 125, 159, 193, 66, 28, 254, 160, 225, 191, 93, 3, 128, 222, 60, 98,
          190, 224, 2, 92, 223, 129, 99, 61, 124, 34, 192, 158, 29, 67, 161, 255,
          70, 24, 250, 164, 39, 121, 155, 197, 132, 218, 56, 102, 229, 187, 89, 7,
          219, 133, 103, 57, 186, 228, 6, 88, 25, 71, 165, 251, 120, 38, 196, 154,
          101, 59, 217, 135, 4, 90, 184, 230, 167, 249, 27, 69, 198, 152, 122, 36,
          248, 166, 68, 26, 153, 199, 37, 123, 58, 100, 134, 216, 91, 5, 231, 185,
          140, 210, 48, 110, 237, 179, 81, 15, 78, 16, 242, 172, 47, 113, 147, 205,
          17, 79, 173, 243, 112, 46, 204, 146, 211, 141, 111, 49, 178, 236, 14, 80,
          175, 241, 19, 77, 206, 144, 114, 44, 109, 51, 209, 143, 12, 82, 176, 238,
          50, 108, 142, 208, 83, 13, 239, 177, 240, 174, 76, 18, 145, 207, 45, 115,
          202, 148, 118, 40, 171, 245, 23, 73, 8, 86, 180, 234, 105, 55, 213, 139,
          87, 9, 235, 181, 54, 104, 138, 212, 149, 203, 41, 119, 244, 170, 72, 22,
          233, 183, 85, 11, 136, 214, 52, 106, 43, 117, 151, 201, 74, 20, 246, 168,
          116, 42, 200, 150, 21, 75, 169, 247, 182, 232, 10, 84, 215, 137, 107, 53]

def toIntList(message):
    intlist = []
    mlist = message.split()
    for i in xrange(len(mlist)):
        intlist.append(int(mlist[i]))
    return intlist

class Checksum:
    def __init__(self, message, istemp=0):
        self.message = message
        if istemp: # 1 = True for temp
            self.result = self.tempCRC(0)
        else: # 0 = False for uniqueID
            self.result = self.idCRC()

    # Check Sum function from Temp/Humi Documentation.
    def tempCRC(self, verbose):
        POLYNOMIAL = 0x131 # x^8 + x^5 + x^4 + 1 -> 9'b100110001 = 0x131
        crc = 0
        mList = toIntList(self.message)
        errorCode = mList[0]
        dataList = mList[1:-1]
        checksum = mList[-1]
        numBytes = len(dataList)
        if errorCode != 0:
            return 2 #'I2C_BUS_ERROR'
        # calculates 8-bit checksum with give polynomial
        for byteCtr in xrange(numBytes):
            crc ^= dataList[byteCtr]
            for bit in xrange(8,0,-1):
                if crc & 0x80: # True if crc >= 128, False if crc < 128
                    crc = (crc << 1) ^ POLYNOMIAL
                else: # crc < 128
                    crc = (crc << 1)
        if verbose:
            print 'CRC = ',crc
            print 'checksum = ',checksum
        if crc != checksum:
            return 1 # 'CHECKSUM_ERROR'
        return 0 # 'CHECKSUM_OK'

    # CRC (validate checksum)
    def idCRC(self):
        val = 0
        mlist = toIntList(self.message)
        error = mlist.pop(0)
        if int(error) != 0:
            print 'I2C_ERROR'
            return 2 # i2c bus error
        for x in mlist:
            val = crctab[val ^ x]
        if val != 0:
            print 'CHECKSUM_ERROR'
            return 1 # checksum error
        return 0 # checksum ok... crc = 0