http://www.atmel.com/dyn/resources/prod_documents/doc1143.pdf
Several standards are used today for CRC detection. The characteristics of the divisor
vary from 8 to 32 bits, and the ability to detect errors varies with the width of the divisor
used. Some commonly used CRC divisors are:
CRC-16 = 1 1000 0000 0000 0101= 8005(hex)
CRC-CCITT = 1 0001 0000 0010 0001= 1021 (hex)
CRC-32 = 1 0000 0100 1100 0001 0001 1101 1011 0111 = 04C11DB7 (hex)
Observe that in 16 bits divisors, the actual numbers of bits are 17, and in a 32 bits divi-
sor the number of bits are 33. The MSB is always 1.
