Function crc32_brown

Function Documentation

uint32_t crc32_brown(const uint8_t *buf, uint32_t crc, size_t size)

Compute a 32-bit CRC using the work of Gary S. Brown.

COPYRIGHT (C) 1986 Gary S. Brown. You may use this program, or code or tables extracted from it, as desired without restriction.

First, the polynomial itself and its table of feedback terms. The polynomial is X^32+X^26+X^23+X^22+X^16+X^12+X^11+X^10+X^8+X^7+X^5+X^4+X^2+X^1+X^0

Note that we take it “backwards” and put the highest-order term in the lowest-order bit. The X^32 term is “implied”; the LSB is the X^31 term, etc. The X^0 term (usually shown as “+1”) results in the MSB being 1

Note that the usual hardware shift register implementation, which is what we’re using (we’re merely optimizing it by doing eight-bit chunks at a time) shifts bits into the lowest-order term. In our implementation, that means shifting towards the right. Why do we do it this way? Because the calculated CRC must be transmitted in order from highest-order term to lowest-order term. UARTs transmit characters in order from LSB to MSB. By storing the CRC this way we hand it to the UART in the order low-byte to high-byte; the UART sends each low-bit to hight-bit; and the result is transmission bit by bit from highest- to lowest-order term without requiring any bit shuffling on our part. Reception works similarly

The feedback terms table consists of 256, 32-bit entries. Notes 

The table can be generated at runtime if desired; code to do so
is shown later.  It might not be obvious, but the feedback
terms simply represent the results of eight shift/xor opera
tions for all combinations of data and CRC register values

The values must be right-shifted by eight bits by the "updcrc
logic; the shift must be size_t (bring in zeroes).  On some
hardware you could probably optimize the shift in assembler by
using byte-swap instructions
polynomial $edb88320

Parameters:

  • buf – Buffer to compute over

  • size – # bytes in buffer

  • crc – Starting value for crc

Returns:

The CRC