NOR "bad blocks"

Hi all,

just a thought...
Same as NAND, NOR flashes mostly have an endurance of guaranted 100K program/erase cycles.
If a block reaches this (theoretical) value, it should be recognized as bad block if erase fails; e.g. not conaining only 0xFF after erase on NAND. (I don't know if the "100K" erase cycle boundary is mostly responsible for this) But obviously a block can wear out on NOR too after the max. number of erase cycles. (How) can this be recognized, handled?

Bernhard

Not easily. I haven't done the experiments myself, but wear-out effects were described to me. Basically, you can write to a block and read it back out in a tight loop and things will never fail. Not after 100k, not after 1M erases. But if you write to the block and wait long enough, bits will flip.

Bit-flipping will always happen from 0 to 1, similar to erases. Problem should be that the insulation around a flash cell has eroded and the charge will leak out over time - this cell is self-erasing now.

And if you want to check whether all this is actually true, you can try a testrun like this:

for (int i=1; ; i++) {
	for (int j=0; j<1000; j++)
		erase(block);
	write(block);
	sleep(10);
	read(block);
	if (read doesn't match write) {
		printf("block died at %d000 erases\n", i);
		break;
	}
}

Jörn

Geld macht nicht glücklich.
Glück macht nicht satt.

So this means that you have to hope that no block will wear out. And count on JFFS2's wear levelling.

What are the reasons that cause NAND blocks to die before they would "officially" wear out?
Dou you have a hint for good literature on this stuff (OK, let's better say NAND and NOR flash memory in general)?

Bernhard

all blocks will roughly wear out at the same time, which you can calculate beforehand. You need to know your average write rate and pick a device large enough to support that rate for your target lifetime.

JFFS2 also has checksums, so when the bitflipping happens, it will get noticed. But that doesn't mean that it is interpreted as bitflipping. Most checksum errors are caused by unfinished writes during power failure.

Dunno, not my area of expertise.

I'd doubt that there is even decent literature. Stuff is big-business, but not sexy for research papers. Try finding an unbiased explanation is such an environment. ;)

Jörn

Those who come seeking peace without a treaty are plotting.
-- Sun Tzu

Hehe, you may be right...
But - no doubt - there should be some decent _book_ about basic principles of flash memory. OK, let's serach...

Bernhard