SanDisk SDSDB-32-201-80 - Industrial Grade Flash Memory Card Product Manual page 66

Secure Digital (SD) Card Protocol Description
CRC16
When one DAT line is used (as in the MultiMediaCard), the CRC16 is used for payload protection in block transfer
mode. The CRC check sum is a 16-bit value and is computed as follows:
generator polynomial G(x) = x16 + x12 +x5 +1
M(x) = (first bit) * xn + (second bit)* xn-1 +...+ (last bit) * x0
CRC[15...0] = Remainder [(M(x) * x16 ) / G(x)]
The first bit is the first data bit of the corresponding block. The degree n of the polynomial denotes the number of
bits of the data block decreased by one. For example, n = 4,095 for a block length of 512 bytes. The generator
polynomial G(x) is a standard CCITT polynomial. The code has a minimal distance d=4 and is used for a payload
length of up to 2,048 bytes (n ≤ 16,383). The same CRC16 method is used in single DAT line mode and in wide bus
mode. In wide bus mode, the CRC16 is done on each line separately.
4-16
Figure 4-10. CRC7 Generator/Checker
Figure 4-11. CRC16 Generator/Checker
SanDisk Secure Digital (SD) Card Product Manual, Rev. 1.9 © 2003 SANDISK CORPORATION