[Nand Flash]软件ECC

/*
---------------------------------------------------------------
作    者:温子祺
联系方式:wenziqi@hotmail.com

说     明:软件ECC

文件来源:Microsoft
---------------------------------------------------------------
*/

文件路径 C:\WINCE600\PLATFORM\Mini2440\src\common\nandflash\fmd\ECC.c

源码：

#include <windows.h>

UCHAR CountNumberOfOnes(UCHAR num)
{
    UCHAR count = 0;

    while(num)
    {
        num=num&(num-1);
        count++;
    }

    return count;
}


#define DATA_BUFF_LEN            512
#define ECC_BUFF_LEN            3                    // # of bytes in ECC


#define    NO_DATA_ERROR            0
#define ECC_ERROR                1
#define CORRECTABLE_ERROR        12                    // Half of the ECC bits are 1

/*++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
Function:        ECC_CorrectData()

Description:    Corrects any errors (if possible) in the specified data.

Notes:            This implemention uses 3 bytes of ECC info for every 512 bytes
                of data.  Furthermore, a only single-bit error can be corrected
                for every 512 bytes of data.

                This code is based on the ECC algorithm publicly available on
                Samsung's FLASH media website.

Returns:        Boolean indicating if the data was corrected.
-------------------------------------------------------------------------------*/
BOOL ECC_CorrectData(LPBYTE pData, LPBYTE pExistingECC, LPBYTE pNewECC)
{
    DWORD i, numOnes, byteLocation, bitLocation;
    BYTE xorECC[ECC_BUFF_LEN];

    //----- 1. Check the parameters -----
    if((pData == NULL) || (pExistingECC == NULL) || (pNewECC == NULL))
    {
        return FALSE;
    }

    //----- 2. First, determine if this is a single-bit, correctable, error -----
    //           NOTE: To answer this question, the two ECC values are XOR'd 
    //                 together and the total # of 1 bits is counted, which 
    //                 then tell us if we can correct the erroneous single-bit 
    //                 transition in the data.
    for(i=0; i<ECC_BUFF_LEN; i++)
    {
        xorECC[i] = pExistingECC[i] ^ pNewECC[i];
    }

    numOnes = 0;
    for(i=0; i<ECC_BUFF_LEN; i++)        
    {
        numOnes += CountNumberOfOnes(xorECC[i]);
    }

    switch(numOnes)
    {
        case NO_DATA_ERROR:                    // Data doesn't contain an error
        return TRUE;

        case ECC_ERROR:                        // Existing ECC value has gone bad!
        return FALSE;

        case CORRECTABLE_ERROR:                // Single-bit error
        break;

        default:                            // More than a single-bit error
        return FALSE;
    }
        
    //----- 3. Compute the location of the single-bit error -----
    byteLocation = ( ((xorECC[2]&0x02)<<7) |
                     ((xorECC[1]&0x80))    | ((xorECC[1]&0x20)<<1) | 
                     ((xorECC[1]&0x08)<<2) | ((xorECC[1]&0x02)<<3) |
                     ((xorECC[0]&0x80)>>4) | ((xorECC[0]&0x20)>>3) | 
                     ((xorECC[0]&0x08)>>2) | ((xorECC[0]&0x02)>>1) );

    bitLocation  = (((xorECC[2]&0x80)>>5) | ((xorECC[2]&0x20)>>4) | ((xorECC[2]&0x08)>>3) );

    //----- 4. Correct the single-bit error (set the bit to its complementary value) -----
    if(pData[byteLocation] & (0x01 << bitLocation))
    {
        pData[byteLocation] &= ~(0x01 << bitLocation);        // 0->1 error, set bit to 0
    }else
    {
        pData[byteLocation] |= (0x01 <<  bitLocation);        // 1->0 error, set bit to 1
    }

    return TRUE;
}

代码分析：

1）

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