
That New Memory Smell: Tech Can Tell If Your Flash Is New or Recycled - jonbaer
https://spectrum.ieee.org/tech-talk/semiconductors/memory/that-new-memory-smell-tech-can-tell-you-if-your-flash-is-new-or-recycled
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larelli
From the headline I assumed that technicians were literally able to smell
whether a chip is new, possibly by it releasing gases shortly after
production. It reminded me of the distinct smell of new hard drives when you
open the factory sealed pouch.

I was slightly disappointed when I found that the article describes software
based testing instead.

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ChuckMcM
I can confirm from experience that if you apply too much voltage it takes on
that “never going to work again” smell :-)

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nfriedly
"All diodes can be light-emitting - once"

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femto
Funnily enough, all diodes actually are light emitting, and light receiving,
to some extent. It's why diodes are generally in opaque encapsulation. Whether
a PN junction is an LED, PV cell or rectifier/switch is a matter of what the
design optimises for. Try putting a voltmeter across a glass encased signal
diode and shining a strong light on it to see it act as a PV cell. One
wouldn't normally use a diode as a PV cell, but if the desired power was very
small, a glass encased diode might be used as a cheap PV cell.

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userbinator
_These effects show up as a slowdown in the memory’s erase time._

This effect, along with a _decrease_ in write time, was noticed a long time
ago:

[https://www.usenix.org/event/fast10/tech/full_papers/boboila...](https://www.usenix.org/event/fast10/tech/full_papers/boboila.pdf)

One of the little-known fascinating facts about standard NAND flash is that it
contains its own tiny microcontroller and internal oscillator, usually a
custom 4-bit architecture, to manage the command interface and do the
program/erase algorithms. If you're interested in this stuff, the book _Inside
NAND Flash Memories_ is highly recommended, and may be the only publicly
accessible documentation of the instruction set of one such microcontroller
along with some interesting snippets of the actual source code of the NAND
flash firmware.

~~~
janekm
That book sounds really interesting. Shame about the crazy price (but I
suppose they only expected a handful of readers...).

Curiously, if you connect a microSD card to an Arduino, there's more compute
power in the card than the Arduino (typically an ARM micro controller or
something of similar complexity to handle all the protocols and wear leveling
that microSD cards support).

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hugey010
Seems really useful, but if my knowledge of flash memory is accurate, this
testing process could highly degrade the memory you run it on. Due to quantum
tunneling, erasures are very hard on flash memory, often the limiting
endurance factor. That is one method mentioned for testing the chip in the
article. Writing all 1's then erasing to all 0's multiple times to see what
percentage are correct. On an already dated device this could be catastrophic!

~~~
kevin_thibedeau
A few dozen erases for testing are inconsequential. They are meant to be used.

Old chips are rated for 10k erases. Newer is 100k+. Only the most ancient
parts have lower wear limits and nobody designs them into new products. The
10k parts are all going EOL now.

~~~
ac29
> Old chips are rated for 10k erases. Newer is 100k+.

Isn't this backwards? I was under the impression that as NAND moved to store
more bits per cell (SLC -> MLC -> TLC), endurance went down, not up.

~~~
userbinator
Yes, endurance and retention decreases _exponentially_ with increasing bits
per cell and is correlated with the capacity process size too --- higher
density NAND made on a smaller process obviously has less area to store a
charge in the first place, and the smaller gate oxide means less endurance. In
the "good old days" SLC was typically 100K cycles and MLC 10K, now even good
SLC is around 10K with MLC at 5K, and the newest TLC is ~1K or less.
Manufacturers are, not surprisingly, increasingly secretive about the
decreasing reliability of their products. It's only with complex error
correction that such chips can even be used at all.

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moopling
This is almost a shame. If the memory is well below it's expected life then it
seems like a good thing to reduce electronic waste by recycling. It seems like
detection would only discourage this.

~~~
pilsetnieks
On the other hand, this enables one to reliably grade the recycled chips. For
example, you could use grade A for low cost servers, grade B for consumer
computers, grade C for phones and tablets, etc. Before this, you'd have no
indication of how much a chip has been used and maybe would be more reluctant
to reuse it.

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pkaye
This was already known in the flash industry for a long time. They
characterize the NAND to a great degree and know how the behavior changes as
the flash cells wear down.

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kazinator
> _Older flash, even if it’s only gone through 1 percent of its lifetime, will
> reliably have more of these flipped bits than new flash will, his team
> discovered._

Creatively ironic use of "reliably" duly noted. :)

