crcmod

[gertjanvanhethof]

Does somebody know if there is an implementation of crcmod available for mircopython?
I need to calculate crc for 16bits values.

[Shaun]

@gertjanvanhethof
This is my CRC16 function based on a very common CRC16 method.
Perhaps it will help you.
You may need to change the seed value to 0x0000, depending on what CRC16 you are hoping to implement

def crc16Nib (data):
CRC16_TBL_SML = ([0x0000, 0xCC01, 0xD801, 0x1400, 0xF001, 0x3C00, 0x2800, 0xE401, 0xA001, 0x6C00, 0x7800, 0xB401, 0x5000, 0x9C01, 0x8801, 0x4400,])
crc = 0xFFFF # 0xFFFF seeded
for x in data:
crc = CRC16_TBL_SML[(x ^ crc) & 0x0F] ^ (crc >> 4) # because using the reflected algorithm, the low nibble is processed first
crc = CRC16_TBL_SML[((x >> 4) ^ crc) & 0x0F] ^ (crc >> 4)
return crc;

[gertjanvanhethof]

Thxs for you answer.

Do you also hava a CRC8 function which is able to do the following?

Name: CRC-8
Width: 8 bit
Polynomial 0x31 (x8 + x5 + x4 + 1)
Initialization 0xFF
Final XOR: 0x00
Examples: CRC (0xBEEF) = 0x92

[Shaun]

@gertjanvanhethof

I have a DALLAS 1Wire implementation.

I'm sure you can find look up values to change it to the CRC8 you require.

def crc8Nib (data):
#'''
CRC8_TABLE_SML1 = bytes([0x00, 0x5e, 0xbc, 0xe2, 0x61, 0x3f, 0xdd, 0x83, 0xc2, 0x9c, 0x7e, 0x20, 0xa3, 0xfd, 0x1f, 0x41])
CRC8_TABLE_SML2 = bytes([0x00, 0x9d, 0x23, 0xbe, 0x46, 0xdb, 0x65, 0xf8, 0x8c, 0x11, 0xaf, 0x32, 0xca, 0x57, 0xe9, 0x74])
#'''
try:
crc = 0xFF # 0xFF seeded
for x in data:
t = (x ^ crc) & 0xFF
crc = CRC8_TABLE_SML1[t & 0x0f] ^ CRC8_TABLE_SML2[t >> 4]
return crc;
except ValueError:
pass