[Yaffs] bit error rates

[Richard A. Smith]

Charles Manning wrote:

 > The best guide I have read is the Toshiba NAND flash applications design
 > guide, available at various locations including
 > http://www.edn.com/contents/images/ToshibaNANDFlash1.pdf

Thank you! that was _exactly_ what I needed.  It answered all my 
questions and even some I had not though of yet.

> I don't believe that there is any "read disturb". Once written, AFAIK
> only other writes are likely to mess things up.

Nope.  See page 22 of the doc you pointed me to.

Read Disturb — In this failure mode, a read operation can disturb the 
memory contents causing a “1” to change to
a “0.” The bit error occurs on another page in the block, not the page 
being read.

Its non-permanent though an erase will fix it and _really_ unlikely.

The ROM section of the document discussed that in their testing it was 
3ppm over 10 years.  So 3 blocks out of every million blocks will have a 
1 bit error in 10 years.

As you said the program-disturb is more common.  Although still pretty 
rare. 1E-10 or 1 bit per 10 billion

> YAFFS2 does no rewrites (ie only one write per page and no deletion
> markers.

Is YAFFS2 ready for production?  I've been looking through the code and 
I see a lot of FIXMEs and TODOs.

The no deletion markers is a bit confusing.  I've not yet groked how 
YAFFS2 does this.  Care to enlighten me?

> NAND flash seems to be getting more reliable all the time. I did some
> accelerated lifetime testing where I wrote and verified over 100Gbytes
> of data without a single bit being damaged.

Good news.  I'll bet the 1e-10 error rate is at the max rated operating 
temp of the part.  So in the normal temp range the error rate is 
probably far lower.

> YAFFS direct is vanilla C and should compile fine for just about
> anything.

Excellent.  Has anyone actually done it though?
Hopefully, with no weird niosII compiler bugs or linker problems.

> need about 1k of RAM per 1MB of NAND. Plus expenses :-).
> Hope that helps
> 

Yes.  Very helpful.  Thanks again.

-- 
Richard A. Smith
Bitworks, Inc