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
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.
The original function, checkCRC, can also do "CRC-CCITT (XModem)".
Just set:
poly = 0x1021
reg = 0
Instead of
poly = 0x11021
reg = 0xFFFF
Here is a C version that you can translate to Python:
#define POLY 0x1021
/* CRC-16 XMODEM: polynomial 0x1021, init = 0, xorout = 0, no reflection */
unsigned crc16x(unsigned crc, unsigned char *buf, size_t len)
{
while (len--) {
crc ^= *buf++ << 8;
crc = crc & 0x8000 ? (crc << 1) ^ POLY : crc << 1;
crc = crc & 0x8000 ? (crc << 1) ^ POLY : crc << 1;
crc = crc & 0x8000 ? (crc << 1) ^ POLY : crc << 1;
crc = crc & 0x8000 ? (crc << 1) ^ POLY : crc << 1;
crc = crc & 0x8000 ? (crc << 1) ^ POLY : crc << 1;
crc = crc & 0x8000 ? (crc << 1) ^ POLY : crc << 1;
crc = crc & 0x8000 ? (crc << 1) ^ POLY : crc << 1;
crc = crc & 0x8000 ? (crc << 1) ^ POLY : crc << 1;
}
return crc & 0xffff;
}
crc is initialized to zero.
Here's a function that I use:
def crc16_ccitt(crc, data):
msb = crc >> 8
lsb = crc & 255
for c in data:
x = ord(c) ^ msb
x ^= (x >> 4)
msb = (lsb ^ (x >> 3) ^ (x << 4)) & 255
lsb = (x ^ (x << 5)) & 255
return (msb << 8) + lsb
I have developed a small python module to generate crc.
Give it a shot and check the source code it may help!
https://github.com/killercode/PythonCRC
For what you want you just need to use the following code
import crc
crccalc = crc.Crc()
crccalc.setCRCccitt() # Let's calculate the CRC CCITT of a value
crccalc.data = "My Data"
crccalc.compute()
print crccalc.result
Hope it helps :)
