I am using the following function to generate a CRC sum and it doesn't appear to be returning the same checksum when compared to online CRC-CCITT calculators.
This function specifically uses the XMODEM CRC generation with a 0x8408 polynomial with an initial fcs of 0xFFFF.
uint16_t crc16(uint8_t byte, uint16_t fcs)
{
uint8_t bit;
for(bit=0; bit<8; bit++)
{
fcs ^= (byte & 0x01);
fcs = (fcs & 0x01) ? (fcs >> 1) ^ 0x8408 : (fcs >> 1);
byte = byte >> 1;
}
return fcs;
}
Am I doing something wrong? If I send 0xFF, or 0x00 I do not get the same checksum as I do on http://depa.usst.edu.cn/chenjq/www2/SDesign/JavaScript/CRCcalculation.htm
printf("%04X\n", crc16(0x31, 0xFFFF)); //returns 2F8D
CRC preset and residue. 5. CRC test vectors for CRC16 (CCITT)
CCITT - Consultative Committee for International Telegraphy and Telephony.
A cyclic redundancy check (CRC) is an error-detecting code commonly used in digital networks and storage devices to detect accidental changes to raw data. Blocks of data entering these systems get a short check value attached, based on the remainder of a polynomial division of their contents.
The 17 bit prime number in CRC Xmodem is 216 + 212 + 25 + 1 or 65536 + 4096 + 32 + 1 = 69665. So calculating the CRC is simple, just multiply the 128 byte data number by 65536, divide by 69665 and the low order 16 bits of the remainder are the CRC.
static const unsigned short CRC_CCITT_TABLE[256] =
{
0x0000, 0x1021, 0x2042, 0x3063, 0x4084, 0x50A5, 0x60C6, 0x70E7,
0x8108, 0x9129, 0xA14A, 0xB16B, 0xC18C, 0xD1AD, 0xE1CE, 0xF1EF,
0x1231, 0x0210, 0x3273, 0x2252, 0x52B5, 0x4294, 0x72F7, 0x62D6,
0x9339, 0x8318, 0xB37B, 0xA35A, 0xD3BD, 0xC39C, 0xF3FF, 0xE3DE,
0x2462, 0x3443, 0x0420, 0x1401, 0x64E6, 0x74C7, 0x44A4, 0x5485,
0xA56A, 0xB54B, 0x8528, 0x9509, 0xE5EE, 0xF5CF, 0xC5AC, 0xD58D,
0x3653, 0x2672, 0x1611, 0x0630, 0x76D7, 0x66F6, 0x5695, 0x46B4,
0xB75B, 0xA77A, 0x9719, 0x8738, 0xF7DF, 0xE7FE, 0xD79D, 0xC7BC,
0x48C4, 0x58E5, 0x6886, 0x78A7, 0x0840, 0x1861, 0x2802, 0x3823,
0xC9CC, 0xD9ED, 0xE98E, 0xF9AF, 0x8948, 0x9969, 0xA90A, 0xB92B,
0x5AF5, 0x4AD4, 0x7AB7, 0x6A96, 0x1A71, 0x0A50, 0x3A33, 0x2A12,
0xDBFD, 0xCBDC, 0xFBBF, 0xEB9E, 0x9B79, 0x8B58, 0xBB3B, 0xAB1A,
0x6CA6, 0x7C87, 0x4CE4, 0x5CC5, 0x2C22, 0x3C03, 0x0C60, 0x1C41,
0xEDAE, 0xFD8F, 0xCDEC, 0xDDCD, 0xAD2A, 0xBD0B, 0x8D68, 0x9D49,
0x7E97, 0x6EB6, 0x5ED5, 0x4EF4, 0x3E13, 0x2E32, 0x1E51, 0x0E70,
0xFF9F, 0xEFBE, 0xDFDD, 0xCFFC, 0xBF1B, 0xAF3A, 0x9F59, 0x8F78,
0x9188, 0x81A9, 0xB1CA, 0xA1EB, 0xD10C, 0xC12D, 0xF14E, 0xE16F,
0x1080, 0x00A1, 0x30C2, 0x20E3, 0x5004, 0x4025, 0x7046, 0x6067,
0x83B9, 0x9398, 0xA3FB, 0xB3DA, 0xC33D, 0xD31C, 0xE37F, 0xF35E,
0x02B1, 0x1290, 0x22F3, 0x32D2, 0x4235, 0x5214, 0x6277, 0x7256,
0xB5EA, 0xA5CB, 0x95A8, 0x8589, 0xF56E, 0xE54F, 0xD52C, 0xC50D,
0x34E2, 0x24C3, 0x14A0, 0x0481, 0x7466, 0x6447, 0x5424, 0x4405,
0xA7DB, 0xB7FA, 0x8799, 0x97B8, 0xE75F, 0xF77E, 0xC71D, 0xD73C,
0x26D3, 0x36F2, 0x0691, 0x16B0, 0x6657, 0x7676, 0x4615, 0x5634,
0xD94C, 0xC96D, 0xF90E, 0xE92F, 0x99C8, 0x89E9, 0xB98A, 0xA9AB,
0x5844, 0x4865, 0x7806, 0x6827, 0x18C0, 0x08E1, 0x3882, 0x28A3,
0xCB7D, 0xDB5C, 0xEB3F, 0xFB1E, 0x8BF9, 0x9BD8, 0xABBB, 0xBB9A,
0x4A75, 0x5A54, 0x6A37, 0x7A16, 0x0AF1, 0x1AD0, 0x2AB3, 0x3A92,
0xFD2E, 0xED0F, 0xDD6C, 0xCD4D, 0xBDAA, 0xAD8B, 0x9DE8, 0x8DC9,
0x7C26, 0x6C07, 0x5C64, 0x4C45, 0x3CA2, 0x2C83, 0x1CE0, 0x0CC1,
0xEF1F, 0xFF3E, 0xCF5D, 0xDF7C, 0xAF9B, 0xBFBA, 0x8FD9, 0x9FF8,
0x6E17, 0x7E36, 0x4E55, 0x5E74, 0x2E93, 0x3EB2, 0x0ED1, 0x1EF0
};
I use the following code to calculate a CRC-CCITT (0xFFFF):
unsigned short Calculate_CRC_CCITT(const unsigned char* buffer, int size)
{
unsigned short tmp;
unsigned short crc = 0xffff;
for (int i=0; i < size ; i++)
{
tmp = (crc >> 8) ^ buffer[i];
crc = (crc << 8) ^ CRC_CCITT_TABLE[tmp];
}
return crc;
}
I have read your questions and I also had the similar problem like you.
I have solved this issue for calculating CRC-CCITT in XMODEM. here I am attaching the sample program to calculate CRC-CCITT.
I have tried the data with the online converter and this program. Please use this if you wish.
unsigned short crc16(char *ptr, int count)
{
int crc;
char i;
crc = 0;
while (--count >= 0)
{
crc = crc ^ (int) *ptr++ << 8;
i = 8;
do
{
if (crc & 0x8000)
crc = crc << 1 ^ 0x1021;
else
crc = crc << 1;
} while(--i);
}
return (crc);
}
CRC should be defined as unsigned short since the crc16
function returns an unsigned short. CRC is defined as an int which on most systems is 4 bytes.
Take a look at Greg Cook's excellent catalog of CRCs. There is a variant often falsely identified as the CCITT CRC, which it isn't. That is what your code, with the 0xFFFF
initialization, appears to be computing, though reflected. The Kermit CRC is the actual CCITT CRC. To get the CCITT CRC, you should start with zero, not 0xFFFF
. The XMODEM CRC is different still, like the Kermit CRC, but unreflected (so bits go in the top, and you exclusive-or with 0x1021
).
KERMIT
width=16 poly=0x1021 init=0x0000 refin=true refout=true xorout=0x0000 check=0x2189 name="KERMIT"
XMODEM
width=16 poly=0x1021 init=0x0000 refin=false refout=false xorout=0x0000 check=0x31c3 name="XMODEM"
CRC-16/CCITT-FALSE
width=16 poly=0x1021 init=0xffff refin=false refout=false xorout=0x0000 check=0x29b1 name="CRC-16/CCITT-FALSE"
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