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CRC for 256 byte data

Can the following code be used for detecting an error in a data packet of 256 bytes?

/*
* Function: Do_CRC8
* * Description:
* Computes the CRC value given the byte and the old CRC value as a static value
*
* Return Value: CRC value
* Parameters: Input data byte to be CRC'ed
* Remarks:
* uses a table driven method to speed things up.
*
* */
unsigned char Do_CRC8(unsigned char xx)
{
// this procedure calculates
const unsigned char code Table[256]={
0x00,0x07,0x0E,0x09,0x1C,0x1B,0x12,0x15,0x38,0x3F,0x36,0x31,0x24,0x23,0x2A,0x2D,
0x70,0x77,0x7E,0x79,0x6C,0x6B,0x62,0x65,0x48,0x4F,0x46,0x41,0x54,0x53,0x5A,0x5D,
0xE0,0xE7,0xEE,0xE9,0xFC,0xFB,0xF2,0xF5,0xD8,0xDF,0xD6,0xD1,0xC4,0xC3,0xCA,0xCD,
0x90,0x97,0x9E,0x99,0x8C,0x8B,0x82,0x85,0xA8,0xAF,0xA6,0xA1,0xB4,0xB3,0xBA,0xBD,
0xC7,0xC0,0xC9,0xCE,0xDB,0xDC,0xD5,0xD2,0xFF,0xF8,0xF1,0xF6,0xE3,0xE4,0xED,0xEA,
0xB7,0xB0,0xB9,0xBE,0xAB,0xAC,0xA5,0xA2,0x8F,0x88,0x81,0x86,0x93,0x94,0x9D,0x9A,
0x27,0x20,0x29,0x2E,0x3B,0x3C,0x35,0x32,0x1F,0x18,0x11,0x16,0x03,0x04,0x0D,0x0A,
0x57,0x50,0x59,0x5E,0x4B,0x4C,0x45,0x42,0x6F,0x68,0x61,0x66,0x73,0x74,0x7D,0x7A,
0x89,0x8E,0x87,0x80,0x95,0x92,0x9B,0x9C,0xB1,0xB6,0xBF,0xB8,0xAD,0xAA,0xA3,0xA4,
0xF9,0xFE,0xF7,0xF0,0xE5,0xE2,0xEB,0xEC,0xC1,0xC6,0xCF,0xC8,0xDD,0xDA,0xD3,0xD4,
0x69,0x6E,0x67,0x60,0x75,0x72,0x7B,0x7C,0x51,0x56,0x5F,0x58,0x4D,0x4A,0x43,0x44,
0x19,0x1E,0x17,0x10,0x05,0x02,0x0B,0x0C,0x21,0x26,0x2F,0x28,0x3D,0x3A,0x33,0x34,
0x4E,0x49,0x40,0x47,0x52,0x55,0x5C,0x5B,0x76,0x71,0x78,0x7F,0x6A,0x6D,0x64,0x63,
0x3E,0x39,0x30,0x37,0x22,0x25,0x2C,0x2B,0x06,0x01,0x08,0x0F,0x1A,0x1D,0x14,0x13,
0xAE,0xA9,0xA0,0xA7,0xB2,0xB5,0xBC,0xBB,0x96,0x91,0x98,0x9F,0x8A,0x8D,0x84,0x83,
0xDE,0xD9,0xD0,0xD7,0xC2,0xC5,0xCC,0xCB,0xE6,0xE1,0xE8,0xEF,0xFA,0xFD,0xF4,0xF3

};

ucCRC=Table[ucCRC ^ xx];

return (ucCRC);
} 

Parents
  • "Can the following code be used for detecting an error in a data packet of 256 bytes?"

    Sure, but if you were listening at all to what I said here, http://www.keil.com/forum/docs/thread4290.asp , you will recall that:

    That is too much data to cover with only an 8-bit CRC. Generally speaking, an n-bit CRC's error detection properties degrade after 2**(n-1)-1 data bits. Thus, an 8-bit CRC should be avoided for data blocks over 15 bytes in length. For 256 byte data, you really should be using a 16-bit CRC.

    Is there any particular reason that you think that using the same 8-bit CRC and polynomial, but calculating it using a 256-byte feedback table instead of a 16-byte feedback table removes the > 2**(n-1)-1 data bit degradation?

Reply
  • "Can the following code be used for detecting an error in a data packet of 256 bytes?"

    Sure, but if you were listening at all to what I said here, http://www.keil.com/forum/docs/thread4290.asp , you will recall that:

    That is too much data to cover with only an 8-bit CRC. Generally speaking, an n-bit CRC's error detection properties degrade after 2**(n-1)-1 data bits. Thus, an 8-bit CRC should be avoided for data blocks over 15 bytes in length. For 256 byte data, you really should be using a 16-bit CRC.

    Is there any particular reason that you think that using the same 8-bit CRC and polynomial, but calculating it using a 256-byte feedback table instead of a 16-byte feedback table removes the > 2**(n-1)-1 data bit degradation?

Children