NandFlash ECC校验原理与实现

    大家应该都在用U盘，而U盘中的存储芯片就是NandFlash，你买的64G的U盘，实际并没有64G，其中一个原因就是存在坏块。

    因为工艺和其他方面的原因，不能保证NandFlash不存在坏块，因此就需要“挑选出坏块”。

本文就为大家讲述一下用于NandFlash的ECC校验原理与实现。

    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。

ECC原理

P4=D7(+)D6(+)D5(+)D4　　P4`=D3(+)D2(+)D1(+)D0P2=D7(+)D6(+)D3(+)D2　　P2`=D5(+)D4(+)D1(+)D0P1=D7(+)D5(+)D3(+)D1　　P1`=D6(+)D4(+)D2(+)D0

P8 = bit7(+)bit6(+)bit5(+)bit4(+)bit3(+)bit2(+)bit1(+)bit0(+)P8

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 paritystatic 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 blockvoid 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 blockint 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;}