Erasing UV EPROM under power - Page 1
Forum section: electronics / projects

author: Artlav
author_rank: Frequent Contributor
timestamp: March 09, 2014, 08:38:14 pm
content: One day i was curious on how the content of an UV-eraseable EPROM chip degrade when it's being erased.Surprisingly, i could not find any description of that.So, i put a picture into a chip from a TV test signal generator (M27C512), powered it up, and put an UV erasing lamp over it, expecting pixels to go pop.... pop... pop pop popopopopop.Turns out it erases differently than i expected.At first for a minute or so nothing happens.Then the pixels on one side of the screen start to flicker.The flicker form vertical lines.Lines become whiter and whiter.Then the second side start to flicker.Here is the video of the interesting part (better to watch in HD):What exactly is going on there?As far as i understand it, there is some charge level that is defined as 1, and below that - 0, but the programmed bit level is much higher than that threshold.As the UV light shines, it erodes the charge, but there is a long way from fully programmed to the threshold, so a minute pass before anything start to happen.If you turn off the UV mid-erase, the flickering remains - as if the bits are stuck in a half-charged state.I later heard that this sort of intermittent state was widely used as copy protection.It also suggests that the chip left in the sun or staying for decades in the dark would develop intermittents, probably in a form of weird bugs in the code, instead of just failing.I guess the half of the screen at a time is due to the chip being made out of two slightly different banks?But then why some lines persist for much longer than the rest?

author: uwezi
author_rank: Unknown
timestamp: March 09, 2014, 09:44:44 pm
content: Scotty - beam me upNice experiment and thank you for sharing!The information in the individual EPROM cells is stored as charge on a tiny insulated, floating gate. This charge shifts the threshold voltage of the field-effect transistor, which is then either turned on by the read-impulse (programmed bit, or 0) or not (unprogrammed/erased bit, or 1).There are differences in the exact amount of charge needed as well as in the exact charge transferred during writing. Most modern programming schemes would use an individual programming on a per-byte basis: program with a tiny amount of charge (i.e. a short pulse time) until you get a stable read-out, then repeat for a certain multiple of the time needed so far.Internally there are certainly different resistive losses depending on the exact location of the memory cells relative to the read-out amplifiers and the supply of the programming voltage, which can lead to individual differences in the amount of charge.Also there is no guarantee that neighboring pixels in your image are actually stored in neighboring memory cells on the chip.If you have the possibility: try to shade some part of the chip during UV exposure, i.e. a diagonal edge over the window, and see how this affects the erasing.

author: krivx
author_rank: Frequent Contributor
timestamp: March 09, 2014, 09:50:55 pm
content: Quote from: Artlav on March 09, 2014, 08:38:14 pmIt's possible that some bits require more energy to erase than others due to variations in the fab process, but I think it's much more likely that the eraser is the casue for this. UV erasers aren't precision instruments - they just flood the chip with UV for a time that's "long enough", which is determined by checking if the chip is blank afterwards. I would guess that certain parts of the die receive a higher UV intensity than others, there is no reason to try and design for even distribution of light across the die so it probably varies quite a bit.

author: calexanian
author_rank: Super Contributor
timestamp: March 09, 2014, 10:26:24 pm
content: I was selling at the de anza swap meet yesterday and came across a EPROM eraser. I felt old. Then I saw a bunch of data books being given away and remember us making state of the art designs out o those books. I felt older. Then I saw some TRS 80's, apples, comodores, etc, and old 12" disk drives and such and was wondering when am I going to end up on one of those piles of discount scrap! Bought a bottle of beard and moustache dye on my way home and used it this morning!Come on time, take a load off and slow down every now and then!!! I am not ready for my quickly greying beard! I still don't have all the right tounge angles yet!

author: krivx
author_rank: Frequent Contributor
timestamp: March 09, 2014, 11:14:28 pm
content: Some of us are greying before we can even grow a decent beard. Maybe it's inevitable.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: March 09, 2014, 11:15:04 pm
content: Very cool!

author: Artlav
author_rank: Frequent Contributor
timestamp: March 10, 2014, 10:30:39 am
content: Quote from: uwezi on March 09, 2014, 09:44:44 pmThe die is tiny, and the lamp is large - there is no way to shade it partially.Quote from: krivx on March 09, 2014, 09:50:55 pmVariations explain the bits disappearing randomly, but aren't quite enough to explain the line grouping.Quote from: uwezi on March 09, 2014, 09:44:44 pmThe difference between the theory and practice is small in theory, but large in practice.So, it's always fun to actually take a look at what really happens.Quote from: calexanian on March 09, 2014, 10:26:24 pmYou're only as old as you feel.I had a co-worker who looks and dresses like he's 40, but with gray hair. He moves easily, thinks quickly, and wins every time in in-house arm wrestling competitions.And you can never tell he is actually over 80 years old until he starts telling stories about how he was fixing vacuum tube computers in early 50s.

author: Rerouter
author_rank: Super Contributor
timestamp: March 10, 2014, 10:40:18 am
content: Something interesting from before my timethanks for sharing,I was listening to radiohead - no surprises, and started watching this just as he says silence, it matched it well

author: tom66
author_rank: Super Contributor
timestamp: March 10, 2014, 10:42:44 am
content: Could the lines be something like address lines on another layer - thus slightly shading part of the memory? It can still be erased with enough UV, but requires more energy as the address line shades part or absorbs some energy.If it is 8-bit addressed, they look about right to be column select lines.

author: cosmos
author_rank: Regular Contributor
timestamp: March 10, 2014, 11:27:33 am
content: The lines could perhaps be differences in the switching point of sense amplifiers.They could register a written 0 longer than the other amplifiers around it even though the remaining charge in the gates may still be the same.

author: Kjelt
author_rank: Super Contributor
timestamp: March 10, 2014, 11:38:46 am
content: Thanks for sharing, very nice demo.

author: amyk
author_rank: Super Contributor
timestamp: March 10, 2014, 12:01:06 pm
content: When the excess electrons on the floating gate are liberated by UV they have to go somewhere, right? That's what I think the vertical lines are coming from - having left their original bit cells, they've acquired enough energy to enter another one, with the voltage being applied to the chip helping them go that way. If you look carefully, you can see that the bits on the lines in the top right getprogrammed- they turn black, then only later do they turn white. The bits on the line closest to an originally dark portion of the image also get programmed like this. It's like the bits are moving toward these lines and then exiting them from the top and bottom... the lines could represent the bits next to the wordlines on the chip.As for the flickering, that's easy to explain: a half-programmed cell is just at the threshold where any noise will cause the sense amplifier to flip one way or the other.

author: Artlav
author_rank: Frequent Contributor
timestamp: March 10, 2014, 08:02:46 pm
content: Quote from: amyk on March 10, 2014, 12:01:06 pmInteresting, i haven't noticed that before.So, the charge is actually leaking all over the place before dissipating.

author: nctnico
author_rank: Super Contributor
timestamp: March 11, 2014, 09:09:30 am
content: Very cool experiment!