Alex Stewart - 6 months ago 68

Python Question

**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:

mask = 0x80

while(mask > 0):

#left shift by one

reg<<=1

#input the next bit from the message into the right hand side of the op reg

if ord(byte) & mask:

reg += 1

mask>>=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

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.

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

`0x1021`

If someone could write a new function or provide direction to solve the

`checkCRC`

`import`

Answer

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.