After writing successfully a NAND block is this reliable ar reading as a NOR sector?

[Ricard Wanderlof]

On Mon, 11 Apr 2011, Ardelean, Andrei wrote:

> Hi,
>
> Am interested in using MT29F2G08AABWP NAND Flash memory for a new
> embedded design and I couldn't find a clear specification regarding how
> reliable is the NAND flash vs. NOR for reading operation.
>
> If I program successfully a NAND block, read back and verify
> successfully the information and never erase or program that particular
> block again, can I assume that block will remain a good block and the
> information written there is safe for READING, roughly as safe as a NOR
> sector?

In general, no. NAND flashes exhibit a phenomenon known as 'read disturb' 
which means that the data degrades every time it is written. It is also a 
process that happens over time even if no reads are performed.

It is a very slow process though, so for your particular application it 
may not be a problem.

Normally error correcting codes (ECC) are used when NAND flash is used as 
a medium to at least handle the odd bit flipping here and there, and to 
signal that it may be prudent to rewrite the data if the bits are 
decaying.

> Is it any reason to keep in NAND copies of Kernel, bootloader or RFS? Is
> it a standard practice in the industry to keep copies in NAND even
> though we do not erase/program those during the normal operation?

I don't know if there's a standard practice or even a recommended 
practice. One issue is that as chip geometries shrink with the 
introduction of new technologies, the bit error probabilities go up, so 
it really is a moving target, and it depends on your application too.

It is usually very difficult to get any form of reliability data from the 
manufacturers. Chips from different manufacturers can behave differently 
too even if their specs look identical.

The following document does a pretty good job of summing up NAND flash 
error modes:

http://download.micron.com/pdf/presentations/events/flash_mem_summit_jcooke_inconvenient_truths_nand.pdf

/Ricard
-- 
Ricard Wolf Wanderlöf                           ricardw(at)axis.com
Axis Communications AB, Lund, Sweden            www.axis.com
Phone +46 46 272 2016                           Fax +46 46 13 61 30