I am writing code for embedded devices. Many solutions for CRC-CCITT 16-bit calculations require libraries.
< p>Given that it is almost impossible to use the library and exhaust its resources, a function is needed.
Possible solutions
The following CRC calculation was found online. However, its implementation is not Correct.
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 +=' '
#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< br /> reg ^= poly
return reg
Existing online solutions
The following link calculates the 16-bit CRC correctly.
http:/ /www.lammertbies.nl/comm/info/crc-calculation.html#intr
The result under “CRC-CCITT(XModem)” is the correct CRC.
Specifications< /p>
I believe that the “CRC-CCITT(XModem)” in the existing online solution uses the polynomial 0x1021.
Question
If someone can write a new function or Provide directions to resolve the checkCRC function to the required specification. Please note that using a library or any import will not help.
this The library is very interesting for practical use cases because it pre-computes a crc table to improve speed.
Usage (with string or byte list):
crc('123456789')
crcb(0x31, 0x32, 0x33, 0x34, 0x35, 0x36, 0x37, 0x38, 0x39)
The test gives: ‘0x31c3’
< p>
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
< br /> tmp = (crc >> 8) ^ cc
crc = (crc << 8) ^ _tab[tmp & 0xff]
crc = crc & 0xffff
print (crc)< br />
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
If poly = 0x1021 replaces poly = 0x11021 at the beginning, the recommended checkCRC routine is the CRC-CCITT variable ‘1D0F’.
Question
I am writing code for embedded devices. Many solutions for CRC-CCITT 16-bit calculations require libraries.
Given It is almost impossible to use the library and exhaust its resources, a function is needed.
Possible solutions
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 +=' '
#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 solutions
The following link calculates the 16-bit CRC correctly.
http://www.lammertbies.nl/comm/info/crc-calculation.html#intr
“CRC-CCITT(XModem )” The result is the correct CRC.
Specifications
I believe the calculation of “CRC-CCITT(XModem)” in the existing online solutions uses the polynomial 0x1021.
Question
If someone can write a new function or provide directions to solve the checkCRC function to the required specification. Please note that using a library or any import will not help.
This is a python port of the C library for CRC-CCITT XMODEM starting from http://www.lammertbies.nl/comm/info/crc-calculation.html
< /p>
This library is very interesting for practical use cases, because it pre-calculates a crc table to improve speed.
Usage (with string or byte list):
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
If poly = 0x1021 replace poly = 0x11021 at the beginning , The recommended checkCRC routine is the CRC-CCITT variable ‘1D0F’.