[Python] How to calculate CRC for ISO CAN-FD frames

I'm working on a study about CRC in ISO CAN-FD but I'm having issues with the calculation of the actual check sequence.

First of all, I have carefully read the specification ISO 11898-1:2015 that states how the calculation has to be carried out.

Then, I have defined a frame to try and calculate the CRC on:



Note that the red zeros are stuff-bits.

The same frame has been added to a CANalzyer configuration, and my CAPL script prints the FrameCRC = 0x6bd.

I have implemented the algorithm described in the ISO document using Numpy, but my result doesn't match what CANalyzer returns.

My code is the following (it probably isn't perfect numpy-ic coding, I'm new to this library):

def crc(data):
gen_poly = np.array([1, 0, 1, 1, 0, 1, 0, 0, 0, 0, 1, 0, 1, 1, 0, 1, 1])
crc_reg = np.array([1, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0])

for bit in data:
crc_next = abs(bit - crc_reg[0])
crc_reg = np.roll(crc_reg, -1) # Shift one position left (1)
crc_reg[-1] = 0 # Shift one position left (2)
if crc_next == 1:
reg_new = abs(crc_reg - gen_poly)
crc_reg = np.copy(reg_new)
return crc_reg.copy()


I also thought that maybe I misunderstood the document, and crc_reg and gen_poly have to be 18 bits long. So I modified the code this way:

• added 18 bits at zero at the end of the frame, instead of 17;
• gen_poly = np.array([1, 1, 0, 1, 1, 0, 1, 0, 0, 0, 0, 1, 0, 1, 1, 0, 1, 1]);
• crc_reg = np.array([1, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0]) (18 total bits);

but it didn't work as well. I really can't figure out where I'm making mistakes. I've reviewed everything many times.

Edit: as suggested by @Reinderien, I'm providing the input data and the expected output arrays:

input_data = np.array([
0, # SOF
1, 0, 1, 1, 0, 1, 0, 1, 0, 1, 0, # ID
0, # RTR
0, # IDE
1, # EDL
0, # RES
1, # BRS
0, # ESI
1, 0, 0, 0, # DLC
1, 1, 1, 1, 1, 0, 1, 1, 1, # Byte 1
1, 1, 0, 1, 1, 1, 1, 1, 0, 1, # Byte 2
1, 1, 1, 1, 0, 1, 1, 1, 1, # Byte 3
1, 0, 1, 1, 1, 1, 1, 0, 1, 1, # Byte 4
1, 1, 1, 0, 1, 1, 1, 1, 1, 0, # Byte 5
1, 1, 1, 1, 1, 0, 1, 1, 1, # Byte 6
1, 1, 0, 1, 1, 1, 1, 1, 0, 1, # Byte 7
1, 1, 1, 1, 0, 1, 1, 1, 1, # Byte 8
1, 1, 0, 0, # StuffCount + Parity
0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0 # 17 bits at zero
])

expected_output = np.array([0, 0, 0, 0, 0, 1, 0, 1, 0, 1, 0, 1, 1, 1, 1, 0, 1]) # 0x6bd

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