原创地址:http://lidunyang2008.blog.163.com/blog/static/190676205201210551750805/
参考文档: http://blogimg.chinaunix.net/blog/upfile2/080702112233.pdf
NAND FLASH 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。
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(Parity Generation ( In case of 256 byte input )):
用数学表达式表示为:
P1=bit7(+)bit5(+)bit3(+)bit1(+)P1
P2=bit7(+)bit6(+)bit3(+)bit2(+)P2
P4=bit7(+)bit6(+)bit5(+)bit4(+)P4
这里(+)表示“位异或”操作
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。
- The result of XOR : all ECC code is `0`
* Correctable Error
- The result of XOR : total 11bits are `1`
- When main area has 1 bit error, each parity pair (ex. P8 & P8`) has 1 & 0 or 0 & 1
*ECC Error
- The result of XOR : only 1bit is `1`
- When ECC area has an error, call it ECC error
* Uncorrectable Error
- The result of XOR : random data
- When the flash memory has more than 2 bits error, data couldn’t be corrected
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;
}