>-----Original Message-----
>From: piclist-bounces@mit.edu [mailto:piclist-bounces@mit.edu] 
>Sent: 07 April 2005 03:15
>To: microcontroller discussion list - Public.
>Subject: [PIC] Hamming ECC again... In C, this time
>
>
>Just wrote this for a friend who needed to do simple error- 
>correction.  I've previously posted (4,7) Hamming encode/decode 
>routines in assembly language, but he needed it in C, so...
>
>It's two small tables and two lines of code.  Maybe someone else will 
>find it useful.
>
>    const uint8 encode[16] = {0x00, 0x31, 0x52, 0x63,
>                              0x64, 0x55, 0x36, 0x07,
>                              0x78, 0x49, 0x2A, 0x1B,
>                              0x1C, 0x2D, 0x4E, 0x7F}
>
>    const uint8 fixbits[8] = {0x00, 0x90, 0xA0, 0x81,
>                              0xC0, 0x82, 0x84, 0x88}
>
>    uint8 transmit(uint8 x)
>    {
>        return encode[x];
>    }
>
>    uint8 receive(uint8 x)
>    {
>        return x ^ fixbits[(encode[x & 0x0F] ^ x) >> 4];
>    }
>
>The transmit() function takes as input a 4-bit number (0x00-0x0F) and 
>returns a 7-bit codeword (original input unchanged in bits 0-3, three 
>check bits in bits 4-6, and bit 7 clear).  
>
>The receive() function takes as input a 7-bit codeword (0x00-0x7F) 
>with up to two errors.  If the codeword contains no errors, the 
>function returns the codeword unchanged (original data in bits 0-3, 
>correct check bits in bits 4-6, bit 7 clear).  If the codeword 
>contains one error, the function returns a corrected codeword 
>(original data in bits 0-3, correct check bits in bits 4-6, but bit 7 
>set to indicate that an error was detected).  If the codeword 
>contains two errors, the function returns garbage in bits 0-6, but 
>with bit 7 set to indicate that an error was detected.       
>
>-Andy

That's been added to my usefull code snippets collection, thanks for
posting!  Maybe a stupid question, but if bit 7 is set after calling
receive(), how would you tell if the data had been corrected or was
garbage?

Regards

Mike

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