NAND ECC Pitfall

If you are using the NAND ECC engine with BCH4 or BCH8, there is a common pitfall you want to avoid:

In case of an EMPTY PAGE (Content==FF, ECC==FF), there is NO VALID ECC in this page.

So the most simplyfied code to check the ECC in your page_read() function is:

if (ECC == FFFFFF....FF)

   ecc_valid = 1;

else if (check(ECC) == VALID)

  ecc_valid = 1;

else if (correct(ECC) == OK)

  ecc_valid = 1;

else

  error():

This check for ECC == FF has a serious drawback: what if there is a single bit error in the ECC area? Remember: we have a NAND flash! Single bit errors are common! This single bit error will give you an uncorrectable ECC failure. Not good in production devices...

So a better check is:

if (ECC == FFFFFF....FF)

   ecc_valid = 1;

else if (check(ECC) == VALID)

  ecc_valid = 1;

else if (number_of_0_bits_in_ecc() < acceptable_wrong_ecc_bits)

  ecc_valid = 1;

else .....

I choose acceptable_wrong_ecc_bits == 4 for BCH8.

This will give your code a certain degree of immunity against single bit errors.

regards

Wolfgang