CRC-CCITT 16-bit Python Manual Calculation
Here is a python port of the C library from http://www.lammertbies.nl/comm/info/crc-calculation.html for CRC-CCITT XMODEM
This library is interesting for real use cases because it pre-computes a table of crc for enhanced speed.
Usage (with a string or a list of bytes) :
crc('123456789')
crcb(0x31, 0x32, 0x33, 0x34, 0x35, 0x36, 0x37, 0x38, 0x39)
The test gives : '0x31c3'
POLYNOMIAL = 0x1021
PRESET = 0
def _initial(c):
crc = 0
c = c << 8
for j in range(8):
if (crc ^ c) & 0x8000:
crc = (crc << 1) ^ POLYNOMIAL
else:
crc = crc << 1
c = c << 1
return crc
_tab = [ _initial(i) for i in range(256) ]
def _update_crc(crc, c):
cc = 0xff & c
tmp = (crc >> 8) ^ cc
crc = (crc << 8) ^ _tab[tmp & 0xff]
crc = crc & 0xffff
print (crc)
return crc
def crc(str):
crc = PRESET
for c in str:
crc = _update_crc(crc, ord(c))
return crc
def crcb(*i):
crc = PRESET
for c in i:
crc = _update_crc(crc, c)
return crc
Your proposed checkCRC
routine is CRC-CCITT variant '1D0F' if you replace poly = 0x11021
with poly = 0x1021
at the beginning.
The original function, checkCRC
, can also do "CRC-CCITT (XModem)".
Just set:
poly = 0x1021
reg = 0
Instead of
poly = 0x11021
reg = 0xFFFF