RM0453 Rev 5 527/1450
RM0453 Cyclic redundancy check calculation unit (CRC)
531
The data size can be dynamically adjusted to minimize the number of write accesses for a
given number of bytes. For instance, a CRC for 5 bytes can be computed with a word write
followed by a byte write.
The input data can be reversed to manage the various endianness schemes. The reversing
operation can be performed on 8 bits, 16 bits and 32 bits depending on the REV_IN[1:0] bits
in the CRC_CR register.
For example, 0x1A2B3C4D input data are used for CRC calculation as:
• 0x58D43CB2 with bit-reversal done by byte
• 0xD458B23C with bit-reversal done by half-word
• 0xB23CD458 with bit-reversal done on the full word
The output data can also be reversed by setting the REV_OUT bit in the CRC_CR register.
The operation is done at bit level. For example, 0x11223344 output data are converted to
0x22CC4488.
The CRC calculator can be initialized to a programmable value using the RESET control bit
in the CRC_CR register (the default value is 0xFFFFFFFF).
The initial CRC value can be programmed with the CRC_INIT register. The CRC_DR
register is automatically initialized upon CRC_INIT register write access.
The CRC_IDR register can be used to hold a temporary value related to CRC calculation. It
is not affected by the RESET bit in the CRC_CR register.
Polynomial programmability
The polynomial coefficients are fully programmable through the CRC_POL register, and the
polynomial size can be configured to be 7, 8, 16 or 32 bits by programming the
POLYSIZE[1:0] bits in the CRC_CR register. Even polynomials are not supported.
Note: The type of an even polynomial is X+X
2
+..+X
n
, while the type of an odd polynomial is
1+X+X
2
+..+X
n
.
If the CRC data is less than 32-bit, its value can be read from the least significant bits of the
CRC_DR register.
To obtain a reliable CRC calculation, the change on-fly of the polynomial value or size can
not be performed during a CRC calculation. As a result, if a CRC calculation is ongoing, the
application must either reset it or perform a CRC_DR read before changing the polynomial.
The default polynomial value is the CRC-32 (Ethernet) polynomial: 0x4C11DB7.
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