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Nand technology has some issues, which I find it a bit difficult to understand. It is said in nand devices that manufacture guaranty that a page will have no more than X (4/8/16,etc according to nand device) failed bit.

  1. Is it that the manufacture can really guarantee that there shall never be more than this amount of failed bits is a single page ?
  2. Is it that the failed ecc happens just randomly, after some time ?
  3. Is it that re-program the failed ecc page, shall recover its flip bits to the correct value ?

Thank you, Ran

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  1. No, it's all a game of probabilities. You just define a probability (e.g. \$1-10^{-14}\$) that all pages fulfill that criterion, the manufacturer models the stochastic process that leads to errors and makes observations of that process, and you so get to a probability that indeed your criterion is fulfilled.
  2. There's different kind of errors. But yes, spontaneous bit flips can happen for thermal, particle / ionizing radiation, or current leakage reasons. Other errors are not spontaneous, but always immediately manifest. Note that a flipped bit doesn't mean the ECC must fail: Error checks are only safe against certain patterns of bit errors, and if multiple bits within a block (or page, or whatever you call it) fail, the ECC might not "notice" it. It's, again, a stochastic thing.
  3. Only if
    1. some recovery mechanism recovered the original data correctly; again, patterns, probability, and
    2. the reason the page failed because a bit flipped randomly. Of course, if a bit cell is simply stuck to a value, then there's nothing rewriting could solve.
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  • \$\begingroup\$ I don't see the probability value in datasheet, so for example for a device such as MT29F4G08ABBDAH4, which is 4-biit ecc, the probability should be really low as you've described (e.g. 10^-14) ? \$\endgroup\$ – ransh Dec 21 '17 at 9:30
  • \$\begingroup\$ and another question, does oob (the additional info per page) is guaranteed to be correct ? Thanks! \$\endgroup\$ – ransh Dec 21 '17 at 10:24
  • \$\begingroup\$ you'll have to take a bitter pill: there's no certainty outside of math. Every device, every aspect of every device, just has an error probability. In good cases, this probability is close to 0. \$\endgroup\$ – Marcus Müller Dec 21 '17 at 12:08
  • \$\begingroup\$ That I understand, I just don't see why they don't write the probablity in datasheet. Myabe they mean that 5% there won't be error above 4-bit per page ? 5% is much more than 10^-14 \$\endgroup\$ – ransh Dec 21 '17 at 12:10
  • \$\begingroup\$ The y will say whatever they can, in the end, financially afford to disappoint a customer. When you buy memory modules for places where "nearly all work" isn't good enough, you call them, and get more expensive chips with a specific probability. \$\endgroup\$ – Marcus Müller Dec 21 '17 at 14:03

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