CRC-CCITT Implementation

Question

I use the following function to generate the CRC sum, and it does not seem to return the same checksum compared to the CRC-CCITT online calculators.

This function specifically uses XCODEM CRC generation with the polynomial 0x8408 with initial fcs 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 will not receive the same checksum as on http://depa.usst.edu.cn/chenjq/www2/SDesign/JavaScript/CRCcalculation.htm

 printf("%04X\n", crc16(0x31, 0xFFFF)); //returns 2F8D 

Answer 1

Check out Greg Cook's excellent CRC directory . There is an option that is often falsely identified as CRIT CCITT, which is not. This is what your code, with initialization of 0xFFFF , seems to be computed, although it is reflected. CRC Kermit is the actual CRCT CCITT. To get CRC CCITT, you must start from scratch, not 0xFFFF . XCODEM CRC is still different, like Kermit CRC, but not reflected (so the bit goes up and you're exclusive - or from 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" 

Answer 2

 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 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; } 

Answer 3

I read your questions and I also had a similar problem like you.

I solved this problem to calculate CRC-CCITT in XMODEM. here I am attaching an example program for calculating CRC-CCITT.

I tried the data with an online converter and this program. Please use this if you want.

 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 int, which on most systems is 4 bytes.