CRC 32 CALCULATION

[sallyakj]

Hi Alex ,
Can you please explain to me how you are calculating the crc for the mii packets ,
I saw the lfsr model and i don't quite understand it fully , in comparison to the crc32 Algorithms out there !
i need to implement a simple one that does the job and matches your result .
Thanks,
Sally

[alexforencich]

Basically it precomputes a matrix of which input bits need to be XORed to form each output bit, and then evaluates that with reduction-XOR operations. It's much faster in simulation than some of the alternatives. It's also parametrizable, unlike LFSR/CRC code generators, so you can use it in generate blocks and such.

[sallyakj]

I'm comparing your result to the calculators in the online calc and i'm not finding it similar to any of them , can you please tell me which algo is it , for example crc32 bzip2 , and the polynom is the msb one ? 4c11db7 taking the data from msb to lsb bit by bit ? data {mac_dst,mac_src,eth_type,payload} is this the right order??

Thanks ,
Sally

[alexforencich]

It implements the same CRC32 as specified in IEEE 802.3 that any Ethernet NIC needs to implement in order to be compliant with the spec. In Python, it's equivalent to the zlib crc32 of the complete frame, dest MAC, source MAC, etc. Note that the Ethernet header fields are packed in reverse byte order; you can use something like wireshark to check exactly how the data is packed into the frame. See https://github.com/alexforencich/cocotbext-eth/blob/master/cocotbext/eth/xgmii.py#L69 for how the CRC is computed in the testbenches. The implementation has been verified against commercial implementations from Xilinx, Intel, Mellanox, Arista, and others.