crc hall_of_doom

[kjm1102]

This python function does CRCs

crc16 = crcmod.mkCrcFun(0x11021, rev=False, initCrc=0xffff, xorOut=0x0000)
def make_crc(bytestring):
    crc_tmp = '{:04x}'.format(crc16(bytestring))
    crc = crc_tmp[2]
    crc += crc_tmp[3]
    crc += crc_tmp[0]
    crc += crc_tmp[1]
    print('bytestring', bytestring, '  crc', crc)
    return crc

Sample output is

bytestring b'%\x124\x01\x02'   crc 36c6

I've been trying for a couple of days to come up with a micropython equivalent, my best effort so far

PRESET = 0xFFFF
POLYNOMIAL = 0x84088 # bit reverse of 0x11021
def  crc16(bytestring):
    crc = PRESET
    for c in bytestring:
        crc = crc ^ c
        for j in range(8):
            if crc & 0x01:
                crc = (crc >> 1) ^ POLYNOMIAL
            else:
                crc = crc >> 1
    print('bytestring', bytestring, 'crc', hex(crc)[2:]); return crc

is not even close

bytestring b'%\x124\x01\x02' crc 770f6

Can anyone suggest a upython fn that will give the 36c6 checksum?

[robert-hh]

CRC's are always a pain. There is pre- and postprocessing, different initial value settings, bit order, ....
But in the above example, you have to compare crc_tmp with your CRC result, because they to a strange postprocessing, reordering the nibbles.

[robert-hh]

About the Polynomail: The higgest order bit is skipped for the computation. So you have to take 0x1021 for the job, reversing it to 0x8408. And then you have to try 0 or 0xffff for the initial value. And check, whether the result is bit-reversed. Alternatively I had use the code snippet below for left.shift CRC:

def crc16l(str):
    crc = PRESET
    for c in str:
        crc = crc ^ (c << 8)
        for j in range(8):
            if crc & 0x8000:
                crc = (crc << 1) ^ POLYNOMIAL
            else:
                crc = crc << 1
        crc &= 0xffff

    return crc

[kjm1102]

Thnx for the input Rob. Stuff I google doesn't seem to verify when I try it out.
This function works

def crc16(data):
  table=[0x0000, 0x1021, 0x2042, 0x3063, 0x4084, 0x50A5, 0x60C6, 0x70E7, 0x8108, 0x9129, 0xA14A, 0xB16B, 0xC18C, 0xD1AD, 0xE1CE, 0xF1EF,
         0x1231, 0x0210, 0x3273, 0x2252, 0x52B5, 0x4294, 0x72F7, 0x62D6, 0x9339, 0x8318, 0xB37B, 0xA35A, 0xD3BD, 0xC39C, 0xF3FF, 0xE3DE,
         0x2462, 0x3443, 0x0420, 0x1401, 0x64E6, 0x74C7, 0x44A4, 0x5485, 0xA56A, 0xB54B, 0x8528, 0x9509, 0xE5EE, 0xF5CF, 0xC5AC, 0xD58D,
         0x3653, 0x2672, 0x1611, 0x0630, 0x76D7, 0x66F6, 0x5695, 0x46B4, 0xB75B, 0xA77A, 0x9719, 0x8738, 0xF7DF, 0xE7FE, 0xD79D, 0xC7BC,
         0x48C4, 0x58E5, 0x6886, 0x78A7, 0x0840, 0x1861, 0x2802, 0x3823, 0xC9CC, 0xD9ED, 0xE98E, 0xF9AF, 0x8948, 0x9969, 0xA90A, 0xB92B,
         0x5AF5, 0x4AD4, 0x7AB7, 0x6A96, 0x1A71, 0x0A50, 0x3A33, 0x2A12, 0xDBFD, 0xCBDC, 0xFBBF, 0xEB9E, 0x9B79, 0x8B58, 0xBB3B, 0xAB1A,
         0x6CA6, 0x7C87, 0x4CE4, 0x5CC5, 0x2C22, 0x3C03, 0x0C60, 0x1C41, 0xEDAE, 0xFD8F, 0xCDEC, 0xDDCD, 0xAD2A, 0xBD0B, 0x8D68, 0x9D49,
         0x7E97, 0x6EB6, 0x5ED5, 0x4EF4, 0x3E13, 0x2E32, 0x1E51, 0x0E70, 0xFF9F, 0xEFBE, 0xDFDD, 0xCFFC, 0xBF1B, 0xAF3A, 0x9F59, 0x8F78,
         0x9188, 0x81A9, 0xB1CA, 0xA1EB, 0xD10C, 0xC12D, 0xF14E, 0xE16F, 0x1080, 0x00A1, 0x30C2, 0x20E3, 0x5004, 0x4025, 0x7046, 0x6067,
         0x83B9, 0x9398, 0xA3FB, 0xB3DA, 0xC33D, 0xD31C, 0xE37F, 0xF35E, 0x02B1, 0x1290, 0x22F3, 0x32D2, 0x4235, 0x5214, 0x6277, 0x7256,
         0xB5EA, 0xA5CB, 0x95A8, 0x8589, 0xF56E, 0xE54F, 0xD52C, 0xC50D, 0x34E2, 0x24C3, 0x14A0, 0x0481, 0x7466, 0x6447, 0x5424, 0x4405,
         0xA7DB, 0xB7FA, 0x8799, 0x97B8, 0xE75F, 0xF77E, 0xC71D, 0xD73C, 0x26D3, 0x36F2, 0x0691, 0x16B0, 0x6657, 0x7676, 0x4615, 0x5634,
         0xD94C, 0xC96D, 0xF90E, 0xE92F, 0x99C8, 0x89E9, 0xB98A, 0xA9AB, 0x5844, 0x4865, 0x7806, 0x6827, 0x18C0, 0x08E1, 0x3882, 0x28A3,
         0xCB7D, 0xDB5C, 0xEB3F, 0xFB1E, 0x8BF9, 0x9BD8, 0xABBB, 0xBB9A, 0x4A75, 0x5A54, 0x6A37, 0x7A16, 0x0AF1, 0x1AD0, 0x2AB3, 0x3A92,
         0xFD2E, 0xED0F, 0xDD6C, 0xCD4D, 0xBDAA, 0xAD8B, 0x9DE8, 0x8DC9, 0x7C26, 0x6C07, 0x5C64, 0x4C45, 0x3CA2, 0x2C83, 0x1CE0, 0x0CC1,
         0xEF1F, 0xFF3E, 0xCF5D, 0xDF7C, 0xAF9B, 0xBFBA, 0x8FD9, 0x9FF8, 0x6E17, 0x7E36, 0x4E55, 0x5E74, 0x2E93, 0x3EB2, 0x0ED1, 0x1EF0]; crc = 0xFFFF
  for byte in data: crc=(crc<<8)^table[(crc>>8)^byte]; crc &= 0xFFFF                                                                                  # important, crc must stay 16bits all the way through
  crc_tmp = '{:04X}'.format(crc); crc=crc_tmp[2]; crc+=crc_tmp[3]; crc+=crc_tmp[0]; crc+=crc_tmp[1]
  print('crc', crc)

crc16(b'313233343536373839')

and returns a CRC of C1FC for the test string '123456789' (313233343536373839h)

Then, as luck would have it, I found the thread where this was being discussed. In that thread they claim that the above function is equivalent to http://www.sunshine2k.de/coding/javascript/crc/crc_js.html with predefined set to CRC16_AUG_CCITT.
While it's true they have the same lookup table, the javascript CRC is E5CC not C1FC.

I guess I should be happy I've something that works & move on but it bugs me that so little of this makes any sense to me.

[kjm1102]

OK a tiny bit of progress.

def crc_16(data):
  table=[0x0000, 0x1021, 0x2042, 0x3063, 0x4084, 0x50A5, 0x60C6, 0x70E7, 0x8108, 0x9129, 0xA14A, 0xB16B, 0xC18C, 0xD1AD, 0xE1CE, 0xF1EF,
         0x1231, 0x0210, 0x3273, 0x2252, 0x52B5, 0x4294, 0x72F7, 0x62D6, 0x9339, 0x8318, 0xB37B, 0xA35A, 0xD3BD, 0xC39C, 0xF3FF, 0xE3DE,
         0x2462, 0x3443, 0x0420, 0x1401, 0x64E6, 0x74C7, 0x44A4, 0x5485, 0xA56A, 0xB54B, 0x8528, 0x9509, 0xE5EE, 0xF5CF, 0xC5AC, 0xD58D,
         0x3653, 0x2672, 0x1611, 0x0630, 0x76D7, 0x66F6, 0x5695, 0x46B4, 0xB75B, 0xA77A, 0x9719, 0x8738, 0xF7DF, 0xE7FE, 0xD79D, 0xC7BC,
         0x48C4, 0x58E5, 0x6886, 0x78A7, 0x0840, 0x1861, 0x2802, 0x3823, 0xC9CC, 0xD9ED, 0xE98E, 0xF9AF, 0x8948, 0x9969, 0xA90A, 0xB92B,
         0x5AF5, 0x4AD4, 0x7AB7, 0x6A96, 0x1A71, 0x0A50, 0x3A33, 0x2A12, 0xDBFD, 0xCBDC, 0xFBBF, 0xEB9E, 0x9B79, 0x8B58, 0xBB3B, 0xAB1A,
         0x6CA6, 0x7C87, 0x4CE4, 0x5CC5, 0x2C22, 0x3C03, 0x0C60, 0x1C41, 0xEDAE, 0xFD8F, 0xCDEC, 0xDDCD, 0xAD2A, 0xBD0B, 0x8D68, 0x9D49,
         0x7E97, 0x6EB6, 0x5ED5, 0x4EF4, 0x3E13, 0x2E32, 0x1E51, 0x0E70, 0xFF9F, 0xEFBE, 0xDFDD, 0xCFFC, 0xBF1B, 0xAF3A, 0x9F59, 0x8F78,
         0x9188, 0x81A9, 0xB1CA, 0xA1EB, 0xD10C, 0xC12D, 0xF14E, 0xE16F, 0x1080, 0x00A1, 0x30C2, 0x20E3, 0x5004, 0x4025, 0x7046, 0x6067,
         0x83B9, 0x9398, 0xA3FB, 0xB3DA, 0xC33D, 0xD31C, 0xE37F, 0xF35E, 0x02B1, 0x1290, 0x22F3, 0x32D2, 0x4235, 0x5214, 0x6277, 0x7256,
         0xB5EA, 0xA5CB, 0x95A8, 0x8589, 0xF56E, 0xE54F, 0xD52C, 0xC50D, 0x34E2, 0x24C3, 0x14A0, 0x0481, 0x7466, 0x6447, 0x5424, 0x4405,
         0xA7DB, 0xB7FA, 0x8799, 0x97B8, 0xE75F, 0xF77E, 0xC71D, 0xD73C, 0x26D3, 0x36F2, 0x0691, 0x16B0, 0x6657, 0x7676, 0x4615, 0x5634,
         0xD94C, 0xC96D, 0xF90E, 0xE92F, 0x99C8, 0x89E9, 0xB98A, 0xA9AB, 0x5844, 0x4865, 0x7806, 0x6827, 0x18C0, 0x08E1, 0x3882, 0x28A3,
         0xCB7D, 0xDB5C, 0xEB3F, 0xFB1E, 0x8BF9, 0x9BD8, 0xABBB, 0xBB9A, 0x4A75, 0x5A54, 0x6A37, 0x7A16, 0x0AF1, 0x1AD0, 0x2AB3, 0x3A92,
         0xFD2E, 0xED0F, 0xDD6C, 0xCD4D, 0xBDAA, 0xAD8B, 0x9DE8, 0x8DC9, 0x7C26, 0x6C07, 0x5C64, 0x4C45, 0x3CA2, 0x2C83, 0x1CE0, 0x0CC1,
         0xEF1F, 0xFF3E, 0xCF5D, 0xDF7C, 0xAF9B, 0xBFBA, 0x8FD9, 0x9FF8, 0x6E17, 0x7E36, 0x4E55, 0x5E74, 0x2E93, 0x3EB2, 0x0ED1, 0x1EF0]; crc = 0xFFFF
  for byte in data: crc=(crc<<8)^table[(crc>>8)^byte]; crc &= 0xFFFF
  print('crc', '{:04X}'.format(crc))

crc_16(b'123456789')

gives a crc of 29B1. This is a match for http://www.sunshine2k.de/coding/javascript/crc/crc_js.html with predefined set to
CRC16_CCIT_FALSE (ie poly=0x1021, initial=0xFFFF). So I figure

PRESET = 0xFFFF; POLYNOMIAL = 0x8408 # bit reverse of 0x1021
def  crc16(bytestring):
    crc = PRESET
    for c in bytestring:
        crc = crc ^ c
        for j in range(8):
            if crc & 0x01: crc = (crc >> 1) ^ POLYNOMIAL
            else: crc = crc >> 1
    print('bytestring', bytestring, 'crc', hex(crc)[2:]); return crc

should give either 29B1 or B129 but it gives 6f91. Furthermore I figured the left shifter

PRESET = 0xffff; POLYNOMIAL = 0x1201
def crc16l(str):
    crc = PRESET
    for c in str:
        crc = crc ^ (c << 8)
        for j in range(8):
            if crc & 0x8000:
                crc = (crc << 1) ^ POLYNOMIAL
            else:
                crc = crc << 1
        crc &= 0xffff
    print (crc, '%.04X'%crc)

crc16l(b'123456789')

should at least agree with the 6f91 but it gives 5E1D so I'm still not understanding this. If anyone can see what's going on here I'd welcome some clues.