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Using an EPROM as a detector?
RuudL
#1 Posted : Wednesday, October 17, 2012 2:29:32 PM(UTC)
Rank: Newbie

Joined: 10/17/2012(UTC)
Posts: 3

I am experimenting with an EPROM as a detector.

I was planning to make a UV meter using an EPROM. The idea was that ultraviolet light erases an EPROM. When a fully programmed EPROM (all zoreo's) is exposed to uv light I should clearly see the number of erased bits (one's) increasing.
So I have built a small device that counts the number of erased bits in an EPROM.
I had guessed that the speed of increase of erased bits would certainly be a measure of the UV intensity.
But...
When exposing the EPROM to uv-light I expected a smooth and stable increase of bits, but that didn't happen. In stead, after some time of exposure to uv-light the bitcount begins to vary because of the bits (all Floating Gate devices) become unstable.

Knowing that the bits become unstable, I covered the EPROM window with aluminium and watched what would happen. As I expected the bitcount kept varying (I measure 64k bit 4 times per second). I have a built in threshold of 50, and everytime 2 measurements have a difference of 50 of more I show a flash on an LED.
The device is flasing very randomly between 20 to 60 flashes per minute. Now my question is: What is it that I measure? Clearly,some effects help to flip the very unstable bits, but I don't know what it is that makes them flip. It is more than just random flips, because I notice patterns over time. Can anyone give me a clue?
DugW
#2 Posted : Wednesday, December 05, 2012 9:57:03 AM(UTC)
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Joined: 12/5/2012(UTC)
Posts: 1

The EEPROM does have a finite life cycle, exposure is meant for intense but brief periods. I suspect that what you are observing is the breakdown of the junctions, providing you with interesting but useless data.

RuudL
#3 Posted : Saturday, December 15, 2012 6:19:19 AM(UTC)
Rank: Newbie

Joined: 10/17/2012(UTC)
Posts: 3

Hi DugW, Thanks for your reply, but I don't see your point. I understand that an EEPROM is not built for the purpose of my experiment. Also I don't believe in useless data because I do see patterns coming op in the data.
Apparently the breakdown of the memory cells is influenced by random and non random effects (I don't know: temperature, light, radiation, electrical fields?).
The patters must have a cause and should lead us to some conclusions once the influencing mechanism is known.
snail888
#4 Posted : Saturday, December 15, 2012 8:57:07 AM(UTC)
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Joined: 12/15/2012(UTC)
Posts: 2

When eeprom is exposured to uv light,it's erasing action may be always random,i think.
RuudL
#5 Posted : Saturday, December 15, 2012 9:24:22 AM(UTC)
Rank: Newbie

Joined: 10/17/2012(UTC)
Posts: 3

Hi snail888. Yes I guess so. But in my case I stop the erasure process once I detect instable memory cells. I remove the uv-source and I even cover the window with aluminium sheet, and still changes in the number of erased cells occur, meaning that some cells flip on and off. The flippingcounts show patterns when measuring for some time. I still have to do some more experiments. it is quite fun!
snail888
#6 Posted : Friday, December 21, 2012 4:00:22 AM(UTC)
Rank: Newbie

Joined: 12/15/2012(UTC)
Posts: 2

A better consideration is to query the eeprom manufacture.Without deep erasing mechanism explanation,it's too hard to understand and consider.
jgruszynski2
#7 Posted : Thursday, March 21, 2013 3:40:39 PM(UTC)
Rank: Newbie

Joined: 3/21/2013(UTC)
Posts: 3

I worked at Intel in their EPROM group as a product test engineer years ago. I've been involved in floating gate memory technologies for many years since.

You could in theory use a cell erasure to indicate UV exposure but it would be a VERY noisy measurement. The problem is that as you erase, the threshold shift causes the sense amplifiers to pass through the metastable logic threshold and that's going to give you instability in the measured value for a pretty wide range of UV exposures (exposure time or net energy).

When we erased EPROMs the expected exposure to attain "all 1's" (which is actually what the erase state usually is), you could have nearly an order of magnitude in variance. So we simply pounded them towards the long side of erasure. As you push further into the EPROM's endurance, the amount of time/exposure required usually changes also.

Honestly I'd recommend using a UV sensor designed specifically measure UV light instead if you are trying to accomplish something practical. Just to learn and experiment - that's different.

BTW if you have an Intel 27128B EPROM, it was tested with code (8085A assembly to be specific) that I wrote in 1982.
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