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作者：龙林 EMAIL：dragon_hn@sohu.com WEB：
ECC简介
??由于NAND Flash的工艺不能保证NAND的Memory Array在其生命周期中保持性能的可靠，因此，在NAND的生产中及使用过程中会产生坏块。为了检测数据的可靠性，在应用NAND Flash的系统中一般都会采用一定的坏区管理策略，而管理坏区的前提是能比较可靠的进行坏区检测。
??如果操作时序和电路稳定性不存在问题的话，NAND Flash出错的时候一般不会造成整个Block或是Page不能读取或是全部出错，而是整个Page（例如512Bytes）中只有一个或几个bit出错。
??对数据的校验常用的有奇偶校验、CRC校验等，而在NAND Flash处理中，一般使用一种比较专用的校验——ECC。ECC能纠正单比特错误和检测双比特错误，而且计算速度很快，但对1比特以上的错误无法纠正，对2比特以上的错误不保证能检测。
ECC原理
??ECC一般每256字节原始数据生成3字节ECC校验数据，这三字节共24比特分成两部分：6比特的列校验和16比特的行校验，多余的两个比特置1，如下图所示：
??
??ECC的列校验和生成规则如下图所示：
??用数学表达式表示为：
????P4=D7(+)D6(+)D5(+)D4??P4`=D3(+)D2(+)D1(+)D0
????P2=D7(+)D6(+)D3(+)D2??P2`=D5(+)D4(+)D1(+)D0
????P1=D7(+)D5(+)D3(+)D1??P1`=D6(+)D4(+)D2(+)D0
??这里(+)表示“位异或”操作
??
??ECC的行校验和生成规则如下图所示：
??用数学表达式表示为：
????P8 = bit7(+)bit6(+)bit5(+)bit4(+)bit3(+)bit2(+)bit1(+)bit0(+)P8
????……………………………………………………………………………………
??这里(+)同样表示“位异或”操作
?
??当往NAND Flash的page中写入数据的时候，每256字节我们生成一个ECC校验和，称之为原ECC校验和，保存到PAGE的OOB（out-of-band）数据区中。
??当从NAND Flash中读取数据的时候，每256字节我们生成一个ECC校验和，称之为新ECC校验和。
??校验的时候，根据上述ECC生成原理不难推断：将从OOB区中读出的原ECC校验和新ECC校验和按位异或，若结果为0，则表示不存在错（或是出现了ECC无法检测的错误）；若3个字节异或结果中存在11个比特位为1，表示存在一个比特错误，且可纠正；若3个字节异或结果中只存在1个比特位为1，表示OOB区出错；其他情况均表示出现了无法纠正的错误。
ECC算法的实现
??static const u_char nand_ecc_precalc_table[] =
??{
????0x00, 0x55, 0x56, 0x03, 0x59, 0x0c, 0x0f, 0x5a, 0x5a, 0x0f, 0x0c, 0x59, 0x03, 0x56, 0x55, 0x00,
????0x65, 0x30, 0x33, 0x66, 0x3c, 0x69, 0x6a, 0x3f, 0x3f, 0x6a, 0x69, 0x3c, 0x66, 0x33, 0x30, 0x65,
????0x66, 0x33, 0x30, 0x65, 0x3f, 0x6a, 0x69, 0x3c, 0x3c, 0x69, 0x6a, 0x3f, 0x65, 0x30, 0x33, 0x66,
????0x03, 0x56, 0x55, 0x00, 0x5a, 0x0f, 0x0c, 0x59, 0x59, 0x0c, 0x0f, 0x5a, 0x00, 0x55, 0x56, 0x03,
????0x69, 0x3c, 0x3f, 0x6a, 0x30, 0x65, 0x66, 0x33, 0x33, 0x66, 0x65, 0x30, 0x6a, 0x3f, 0x3c, 0x69,
????0x0c, 0x59, 0x5a, 0x0f, 0x55, 0x00, 0x03, 0x56, 0x56, 0x03, 0x00, 0x55, 0x0f, 0x5a, 0x59, 0x0c,
????0x0f, 0x5a, 0x59, 0x0c, 0x56, 0x03, 0x00, 0x55, 0x55, 0x00, 0x03, 0x56, 0x0c, 0x59, 0x5a, 0x0f,
????0x6a, 0x3f, 0x3c, 0x69, 0x33, 0x66, 0x65, 0x30, 0x30, 0x65, 0x66, 0x33, 0x69, 0x3c, 0x3f, 0x6a,
????0x6a, 0x3f, 0x3c, 0x69, 0x33, 0x66, 0x65, 0x30, 0x30, 0x65, 0x66, 0x33, 0x69, 0x3c, 0x3f, 0x6a,
????0x0f, 0x5a, 0x59, 0x0c, 0x56, 0x03, 0x00, 0x55, 0x55, 0x00, 0x03, 0x56, 0x0c, 0x59, 0x5a, 0x0f,
????0x0c, 0x59, 0x5a, 0x0f, 0x55, 0x00, 0x03, 0x56, 0x56, 0x03, 0x00, 0x55, 0x0f, 0x5a, 0x59, 0x0c,
????0x69, 0x3c, 0x3f, 0x6a, 0x30, 0x65, 0x66, 0x33, 0x33, 0x66, 0x65, 0x30, 0x6a, 0x3f, 0x3c, 0x69,
????0x03, 0x56, 0x55, 0x00, 0x5a, 0x0f, 0x0c, 0x59, 0x59, 0x0c, 0x0f, 0x5a, 0x00, 0x55, 0x56, 0x03,
????0x66, 0x33, 0x30, 0x65, 0x3f, 0x6a, 0x69, 0x3c, 0x3c, 0x69, 0x6a, 0x3f, 0x65, 0x30, 0x33, 0x66,
????0x65, 0x30, 0x33, 0x66, 0x3c, 0x69, 0x6a, 0x3f, 0x3f, 0x6a, 0x69, 0x3c, 0x66, 0x33, 0x30, 0x65,
????0x00, 0x55, 0x56, 0x03, 0x59, 0x0c, 0x0f, 0x5a, 0x5a, 0x0f, 0x0c, 0x59, 0x03, 0x56, 0x55, 0x00
??};
??//?Creates non-inverted ECC code from line parity
??static void nand_trans_result(u_char reg2, u_char reg3,u_char *ecc_code)
??{
????u_char a, b, i, tmp1, tmp2;
????/* Initialize variables */
????a = b = 0x80;
????tmp1 = tmp2 = 0;
????/* Calculate first ECC byte */
????for (i = 0; i < 4; i++)
????{
??????if (reg3 & a)??? /* LP15,13,11,9 --> ecc_code[0] */
????????tmp1 |= b;
??????b >>= 1;
??????if (reg2 & a)??? /* LP14,12,10,8 --> ecc_code[0] */
????????tmp1 |= b;
??????b >>= 1;
??????a >>= 1;
????}
????/* Calculate second ECC byte */
????b = 0x80;
????for (i = 0; i < 4; i++)
????{
??????if (reg3 & a)??? /* LP7,5,3,1 --> ecc_code[1] */
????????tmp2 |= b;
??????b >>= 1;
??????if (reg2 & a)??? /* LP6,4,2,0 --> ecc_code[1] */
????????tmp2 |= b;
??????b >>= 1;
??????a >>= 1;
????}
????/* Store two of the ECC bytes */
????ecc_code[0] = tmp1;
????ecc_code[1] = tmp2;
??}
??//?Calculate 3 byte ECC code for 256 byte block
??void nand_calculate_ecc (const u_char *dat, u_char *ecc_code)
??{
????u_char idx, reg1, reg2, reg3;
????int j;
????/* Initialize variables */
????reg1 = reg2 = reg3 = 0;
????ecc_code[0] = ecc_code[1] = ecc_code[2] = 0;
????/* Build up column parity */
????for(j = 0; j < 256; j++)
????{
??????/* Get CP0 - CP5 from table */
??????idx = nand_ecc_precalc_table[dat[j]];
??????reg1 ^= (idx & 0x3f);
??????/* All bit XOR = 1 ? */
??????if (idx & 0x40) {
????????reg3 ^= (u_char) j;
????????reg2 ^= ~((u_char) j);
??????}
????}
????/* Create non-inverted ECC code from line parity */
????nand_trans_result(reg2, reg3, ecc_code);
????/* Calculate final ECC code */
????ecc_code[0] = ~ecc_code[0];
????ecc_code[1] = ~ecc_code[1];
????ecc_code[2] = ((~reg1) << 2) | 0x03;
??}
??//?Detect and correct a 1 bit error for 256 byte block
??int nand_correct_data (u_char *dat, u_char *read_ecc, u_char *calc_ecc)
??{
????u_char a, b, c, d1, d2, d3, add, bit, i;
????/* Do error detection */
????d1 = calc_ecc[0] ^ read_ecc[0];
????d2 = calc_ecc[1] ^ read_ecc[1];
????d3 = calc_ecc[2] ^ read_ecc[2];
????if ((d1 | d2 | d3) == 0)
????{
??????/* No errors */
??????return 0;
????}
????else
????{
??????a = (d1 ^ (d1 >> 1)) & 0x55;
??????b = (d2 ^ (d2 >> 1)) & 0x55;
??????c = (d3 ^ (d3 >> 1)) & 0x54;
??????/* Found and will correct single bit error in the data */
??????if ((a == 0x55) && (b == 0x55) && (c == 0x54))
??????{
????????c = 0x80;
????????add = 0;
????????a = 0x80;
????????for (i=0; i<4; i++)
????????{
??????????if (d1 & c)
????????????add |= a;
??????????c >>= 2;
??????????a >>= 1;
????????}
????????c = 0x80;
????????for (i=0; i<4; i++)
????????{
??????????if (d2 & c)
????????????add |= a;
??????????c >>= 2;
??????????a >>= 1;
????????}
????????bit = 0;
????????b = 0x04;
????????c = 0x80;
????????for (i=0; i<3; i++)
????????{
??????????if (d3 & c)
????????????bit |= b;
??????????c >>= 2;
??????????b >>= 1;
????????}
????????b = 0x01;
????????a = dat[add];
????????a ^= (b << bit);
????????dat[add] = a;
????????return 1;
??????}
??????else
??????{
????????i = 0;
????????while (d1)
????????{
??????????if (d1 & 0x01)
????????????++i;
??????????d1 >>= 1;
????????}
????????while (d2)
????????{
??????????if (d2 & 0x01)
????????????++i;
??????????d2 >>= 1;
????????}
????????while (d3)
????????{
??????????if (d3 & 0x01)
????????????++i;
??????????d3 >>= 1;
????????}
????????if (i == 1)
????????{
??????????/* ECC Code Error Correction */
??????????read_ecc[0] = calc_ecc[0];
??????????read_ecc[1] = calc_ecc[1];
??????????read_ecc[2] = calc_ecc[2];
??????????return 2;
????????}
????????else
????????{
??????????/* Uncorrectable Error */
??????????return -1;
????????}
??????}
????}
????/* Should never happen */
????return -1;
??}
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