I am trying to implement a crc CCITT16 algorithm. The c code is:[HTML]unsigned int crc_ccitt_update(unsigned int crc, unsigned char x){ unsigned int crc_new = (unsigned char)(crc >> 8)  (crc > 4; crc_new ^= crc_new
X

The generator ( as recommended by CCITT ) is X^16 + X^12 + X^5 + 1
so I'm not sure where the +1 is honoured in the rest of your code.
Try this and see if it works:
func crc_ccitt_update(var crc, var x)
var crc_new;
crc_new = byteSwap(crc);
crc_new ^= x;
crc_new ^= (crc_new & 0xFF) >> 4;
crc_new ^= crc_newRegards,
Dave

Hi Nerque and Dave,
some time ago I have implemented a file transfer function from and to the SD card. I was not sure how reliable this protocol would be, so I have decided to implement some sort of a CRC check. Although this may appear to be a simple operation, it turned out to be painfully slow for a reasonable large files (minutes for 100 kbyte file). I have tried to find an algorithm that would be as efficient as possible and I have found an excellent document by Terry Ritter. According to his directions, I have programmed two CRC functions, one is optimized for speed and the other for code size. The fast function is able to calculate the CRC for a single character in 30 us, which is in my opinion very fast and it takes only a few seconds to process a 100 kbyte file.
Please note that there are several versions of the 16bit CRC algorithm, which differ only in the CRC initial value.
I have appended a small test program with both CRC implementations. Note that the for loop is used only to enhance the timing accuracy for a single execution of a function.
#platform "uLCD32PT_GFX2"
var DataBlock[256];
func main()
var n,t,t0,CRC,pbDataBlock;
//INITIALIZE DATA BLOCK WITH SOME DATA
for(n:=0;n<256;n++)
DataBlock[n]:=n*777;
next
pbDataBlock:=str_Ptr(DataBlock);
//MEASURE LOOP OVERHEAD
t:=sys_T();
for(n:=0;n<100;n++)
next
t0:=sys_T()t;
//FAST CRC
t:=sys_T();
for(n:=0;n<100;n++) //for loop is used just for repeating the same operation 100 times, so more accurate timing can be obtained
CRC:=0xFFFF; //CRC initial value
CRC:=CRCFast(CRC,pbDataBlock,512);
next
t:=sys_T()t;
print("FAST CRC: ",[HEX4]CRC, " ",tt0,"\n");
//SHORT CRC
t:=sys_T();
for(n:=0;n<100;n++) //for loop is used just for repeating the same operation 100 times, so more accurate timing can be obtained
CRC:=0xFFFF; //CRC initial value
CRC:=CRCShort(CRC,pbDataBlock,512);
next
t:=sys_T()t;
print("SHORT CRC: ",[HEX4]CRC, " ",tt0,"\n");
repeat forever
endfunc
func CRCFast(var crc,var DataPtr,var DataLen)
//crc contains either the initial CRC value or the CRC result from previous data block
//DataPtr is a byte pointer to the data block, with the data to be checked. Data can be divided in several blocks, and processed one after the other
//DataLen is the length of the data block in bytes
//561 bytes, 30 us per character
#DATA
word CRC_Table 0x0000,0x1021,0x2042,0x3063,0x4084,0x50A5,0x60C6,0x70E7,0x8108,0x9129,0xA14A,0xB16B,0xC18C,0xD1AD,0x E1CE,0xF1EF,0x1231,0x0210,0x3273,0x2252,0x52B5,0x4294,0x72F7,0x62D6,0x9339,0x8318,0xB37B,0xA35A,0xD3 BD,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,0 xA77A,0x9719,0x8738,0xF7DF,0xE7FE,0xD79D,0xC7BC,0x48C4,0x58E5,0x6886,0x78A7,0x0840,0x1861,0x2802,0x3 823,0xC9CC,0xD9ED,0xE98E,0xF9AF,0x8948,0x9969,0xA90A,0xB92B,0x5AF5,0x4AD4,0x7AB7,0x6A96,0x1A71,0x0A5 0,0x3A33,0x2A12,0xDBFD,0xCBDC,0xFBBF,0xEB9E,0x9B79,0x8B58,0xBB3B,0xAB1A,0x6CA6,0x7C87,0x4CE4,0x5CC5, 0x2C22,0x3C03,0x0C60,0x1C41,0xEDAE,0xFD8F,0xCDEC,0xDDCD,0xAD2A,0xBD0B,0x8D68,0x9D49,0x7E97,0x6EB6,0x 5ED5,0x4EF4,0x3E13,0x2E32,0x1E51,0x0E70,0xFF9F,0xEFBE,0xDFDD,0xCFFC,0xBF1B,0xAF3A,0x9F59,0x8F78,0x91 88,0x81A9,0xB1CA,0xA1EB,0xD10C,0xC12D,0xF14E,0xE16F,0x1080,0x00A1,0x30C2,0x20E3,0x5004,0x4025,0x7046 ,0x6067,0x83B9,0x9398,0xA3FB,0xB3DA,0xC33D,0xD31C,0xE37F,0xF35E,0x02B1,0x1290,0x22F3,0x32D2,0x4235,0 x5214,0x6277,0x7256,0xB5EA,0xA5CB,0x95A8,0x8589,0xF56E,0xE54F,0xD52C,0xC50D,0x34E2,0x24C3,0x14A0,0x0 481,0x7466,0x6447,0x5424,0x4405,0xA7DB,0xB7FA,0x8799,0x97B8,0xE75F,0xF77E,0xC71D,0xD73C,0x26D3,0x36F 2,0x0691,0x16B0,0x6657,0x7676,0x4615,0x5634,0xD94C,0xC96D,0xF90E,0xE92F,0x99C8,0x89E9,0xB98A,0xA9AB, 0x5844,0x4865,0x7806,0x6827,0x18C0,0x08E1,0x3882,0x28A3,0xCB7D,0xDB5C,0xEB3F,0xFB1E,0x8BF9,0x9BD8,0x ABBB,0xBB9A,0x4A75,0x5A54,0x6A37,0x7A16,0x0AF1,0x1AD0,0x2AB3,0x3A92,0xFD2E,0xED0F,0xDD6C,0xCD4D,0xBD AA,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
#END
while(DataLen)
crc := CRC_Table[str_GetByte(DataPtr++) ^ (crc>>8)] ^ (crc<<8);
wend
return crc;
endfunc
func CRCShort(var crc,var DataPtr,var DataLen)
//crc contains either the initial CRC value or the CRC result from previous data block
//DataPtr is a byte pointer to the data block, with the data to be checked. Data can be divided in several blocks, and processed one after the other
//DataLen is the length of the data block in bytes
//99 bytes, 263 us per character
var n,m,character;
while(m<DataLen)
character:=str_GetByte(DataPtr+(m++)) << 7;
n:=8;
while(n)
character:=character << 1;
if((character ^ crc) <0)
crc := (crc << 1) ^ 0x1021;
else
crc := crc << 1;
endif
wend
wend
return crc;
endfunc
_________________________
Regards,
Valentin
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