Alex Stewart - 1 year ago 419
Python Question

# CRC-CCITT 16-bit Python Manual Calculation

Problem

I am writing code for an embedded device. A lot of solutions out there for CRC-CCITT 16-bit calculations require libraries.

Given that using libraries is almost impossible and a drain on its resources, a function is required.

Possible Solution

The following CRC calculation was found online. However, its implementation is incorrect.

http://bytes.com/topic/python/insights/887357-python-check-crc-frame-crc-16-ccitt

``````def checkCRC(message):
#CRC-16-CITT poly, the CRC sheme used by ymodem protocol
poly = 0x11021
#16bit operation register, initialized to zeros
reg = 0xFFFF
#pad the end of the message with the size of the poly
message += '\x00\x00'
#for each bit in the message
for byte in message:
#left shift by one
reg<<=1
#input the next bit from the message into the right hand side of the op reg
reg += 1
#if a one popped out the left of the reg, xor reg w/poly
if reg > 0xffff:
#eliminate any one that popped out the left
reg &= 0xffff
#xor with the poly, this is the remainder
reg ^= poly
return reg
``````

Existing Online Solution

The following link calculates a 16 bit CRC correctly.

http://www.lammertbies.nl/comm/info/crc-calculation.html#intr

The result under "CRC-CCITT (XModem)" is the correct CRC.

Specification

I believe the "CRC-CCITT (XModem)" calculation in the existing online solution uses a polynomial of
`0x1021`
.

Question

If someone could write a new function or provide direction to solve the
`checkCRC`
function to the required specification. Please note that the use of libraries or any
`import`
's would not help.

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.

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