Can you overwrite an OTP ROM?

[mikeselectricstuff]

This question reaches back 35-40 years, which is almost the stone age in integrated circuits.

40 years ago the only viable choice for high density memory was a mask-programmed ROM.  A memory chip was fabricated up to the final metal layer, which contained the links to encode the program.  A customer would provide the programming, a final mask was made, and the chips would be finished and packaged.

The was way less involved than you might imagine.  The difficult and risky steps are creating the semiconductors with deposition and diffusion.  These steps can take weeks.  Specifically each diffusion step can take up to a day of high temperature baking, and an entire batch can be ruined by an impurity.  In contrast a metal deposition layer can be done in a few minutes.  It's just exposing the photoresist (under a minute), cleaning, flashing the metal layer (five minutes to pull a vacuum, a few seconds to flash) and cleaning again. If you screw up, you can etch off the metal and try again.

And the mask wasn't expensive.  The feature sizes were relatively huge, and masks could be made in-house.  Sometimes it was as simple as rubylith film dots laid out by hand.  A pair of clerical-level staff would call and lay out the rubylith dots in an hour or so.

The other approach was fused PROMs.  They had thin metal strips at each matrix point that could be vaporized.  The downside was that the chip had to be physically larger, and the voltage and impulse energy had to be carefully managed to vaporize the fuse completely without damaging the chip structure.  PROMs were usually limited to low capacity chips that held configuration information rather than program memory.

Once EPROMs arrived, the game changed completely.  They die was more expensive than mask ROM, but masks were getting expensive.  Decreasing feature size and increasing memory size meant that making masks was outsourced to specialty companies.  That increased cost and turn-around time.  Pretty quickly OTP EPROMs in cheap plastic packages dominated, with mask-programmed ROMs only used for large-volume parts (primarily game cartridges).

On a personal note, I built my first EPROM programmer in 1980 or 1981 using three "precision discharged" 9V batteries for the 24V power supply needed for programming (25-26V volts supplied).  I was extremely careful because I didn't want to waste the then-expensive parts.  Luckily for my student budget it worked the first time.

One oddity at the height of the EPROM era was that because of the huge production volumes of windowed EPROMs, OTPs were, at least for the casual low-quantity purchaser, not significantly cheaper. Maybe this was because it was only high volume users buying OTPs.
I very, very rarely see OTPs in stuff I do teardowns of, though they tend to be higher-end bits of gear. Of course back in those days anything with an EPROM of any sort was pretty high-end as all consumer stuff was mask, and industrial stuff was expensive enough not to care about savings from OTP. .

[ali6x944]

theoretically, yes.

most PROMs use the same die as UV erasable PROMs, but packaged in a plastic DIP without a window. I know Atmel did this for decades until they obsoleted their ceramic DIP with a window.

This question reaches back 35-40 years, which is almost the stone age in integrated circuits.

40 years ago the only viable choice for high density memory was a mask-programmed ROM.  A memory chip was fabricated up to the final metal layer, which contained the links to encode the program.  A customer would provide the programming, a final mask was made, and the chips would be finished and packaged.

The was way less involved than you might imagine.  The difficult and risky steps are creating the semiconductors with deposition and diffusion.  These steps can take weeks.  Specifically each diffusion step can take up to a day of high temperature baking, and an entire batch can be ruined by an impurity.  In contrast a metal deposition layer can be done in a few minutes.  It's just exposing the photoresist (under a minute), cleaning, flashing the metal layer (five minutes to pull a vacuum, a few seconds to flash) and cleaning again. If you screw up, you can etch off the metal and try again.

And the mask wasn't expensive.  The feature sizes were relatively huge, and masks could be made in-house.  Sometimes it was as simple as rubylith film dots laid out by hand.  A pair of clerical-level staff would call and lay out the rubylith dots in an hour or so.

The other approach was fused PROMs.  They had thin metal strips at each matrix point that could be vaporized.  The downside was that the chip had to be physically larger, and the voltage and impulse energy had to be carefully managed to vaporize the fuse completely without damaging the chip structure.  PROMs were usually limited to low capacity chips that held configuration information rather than program memory.

Once EPROMs arrived, the game changed completely.  They die was more expensive than mask ROM, but masks were getting expensive.  Decreasing feature size and increasing memory size meant that making masks was outsourced to specialty companies.  That increased cost and turn-around time.  Pretty quickly OTP EPROMs in cheap plastic packages dominated, with mask-programmed ROMs only used for large-volume parts (primarily game cartridges).

On a personal note, I built my first EPROM programmer in 1980 or 1981 using three "precision discharged" 9V batteries for the 24V power supply needed for programming (25-26V volts supplied).  I was extremely careful because I didn't want to waste the then-expensive parts.  Luckily for my student budget it worked the first time.

One oddity at the height of the EPROM era was that because of the huge production volumes of windowed EPROMs, OTPs were, at least for the casual low-quantity purchaser, not significantly cheaper. Maybe this was because it was only high volume users buying OTPs.
I very, very rarely see OTPs in stuff I do teardowns of, though they tend to be higher-end bits of gear. Of course back in those days anything with an EPROM of any sort was pretty high-end as all consumer stuff was mask, and industrial stuff was expensive enough not to care about savings from OTP. .

Wow this is rather interesting I didn't know that PROMs had fuses in them...
The even more interesting part is why the programming voltage dropped from 25-26v in the older chips to around 12 in the newer chips? And how do the newer chips "EPROMs" store memory?

[SeanB]

Drop in programming voltage was process driven, they changed to a smaller process, which meant a thinner oxide could store the charge, and needed a lower voltage to tunnel the electrons across the oxide layer. Older ones are however slightly more reliable, simply because the bucket of charge is so many more electrons in size, and thus leaks away slower. However in most situations this only will show up after 100 years or so, as most will keep data for at least 25 years in practise, provided you use a proper opaque label over the window, not a plain paper one but one with an integrated aluminium foil inner layer to block light.

Later on EEProm was developed to a more reliable method, and this, along with some of the later Eprom variants, have a built in voltage generation source, used to generate both the internal reference voltages ( the chip internally is not digital, it is analogue, with a single bit AD converter for EPROM and now with MLC flash a few bits more) and the higher voltages required to erase ( for EEPROM) the required cells/blocks before programming, and to program the cell when they are blank.

in all cases EProm has a built in voltage converter to generate substrate bias, except for the very early types, which needed a -21V substrate bias voltage, and were very unhappy if it was not present, but did not blow up till it was at around +2v, when the chip would crowbar quite nicely. Was a very big advertising headline with the 2716, advertised with bold "Single 5V supply voltage only needed" as a selling point.

The good Eprom programmers verify the devices after writing, using a first pass at 5V00, a second pass at 6V00 and a third pass at 4V50, to test that the programming will be valid data over the full voltage range the device will experience in use. Military ones repeated the test 2 times again, at -55c and at +125C, to ensure they would work under all circumstances.

[Howardlong]

The first microprocessor system I built in the mid 70s used a PROM, ISTR 128 or 256 x 8 bits. They were about 50 quid** a pop. You didn't want to get your boot monitor wrong, it was an expensive business. But equally there wasn't any means to test it beforehand, or fix it if you did get it wrong, other than spend another 50 quid. PCs didn't exist to run simulators, and there were no in circuit emulators. In those days, making a ROM emulator was an expensive proposition, even if you had something to program it with.

The remnants of that machine are below, hopefully my metalwork skills have improved since I was a 12yo.



** about £280 now.

[timb]

But what it can't be...
The laser is of different wavelength to the erase uv wavelength required?

Yes, many years ago, I tried different UV lights in an effort to erase an eprom, including sunlight.  Only the official eprom eraser fluorescent tube would actually erase the eproms.

It needs a very particular wavelength - from memory around 250nm. The only thing that will do it is a germicidal lamp - I think that's what UV erasers generally used. Bright sunlight might do it after a very long time but I think a lot of that wavelength is filtrered by the Ozone layer

Awhile back I needed to program a new UVEPROM for some test equipment I was restoring. It just so happened I had an old PC motherboard on hand with a windowed EEPROM (for the BIOS) that would work. Unfortunately I didn't have an EPROM eraser handy...

What I *did* have, however, was a humidifier that contained a UV bulb in the bottom, designed to kill germs in the water between the tank and filter. So, I simply attached the EPROM to the lamp housing with a piece of Kapton tape, turned it on for 10 minutes and voila, I had an erased EPROM!

[mikeselectricstuff]

in all cases EProm has a built in voltage converter to generate substrate bias, except for the very early types, which needed a -21V substrate bias voltage, and were very unhappy if it was not present, but did not blow up till it was at around +2v, when the chip would crowbar quite nicely. Was a very big advertising headline with the 2716, advertised with bold "Single 5V supply voltage only needed" as a selling point.

.. and not just for writing, they needed wacky voltages for reading as well.

Speaking of process changes, this is a nice illustration of how the same part number changed over about 10 years

[SeanB]

in all cases EProm has a built in voltage converter to generate substrate bias, except for the very early types, which needed a -21V substrate bias voltage, and were very unhappy if it was not present, but did not blow up till it was at around +2v, when the chip would crowbar quite nicely. Was a very big advertising headline with the 2716, advertised with bold "Single 5V supply voltage only needed" as a selling point.

.. and not just for writing, they needed wacky voltages for reading as well.

Speaking of process changes, this is a nice illustration of how the same part number changed over about 10 years

Quite a lot of die shrink over the 11 years difference, but I bet ST still has a few thousand of those old large dies for the military customers, as they are stuck on that process, and a smaller die means a recertification of the process, as it likely has a slightly different program voltage, write method and more importantly a different readout of chip ID during programming. They might have them already diced, tested and in CERDIP as well, as they probably stopped ordering that large window long time ago, quartz glass is a lot more expensive than ceramic, but storage blank ( or with a factory test pattern written in for long term degradation test as a free check, paid for by the customer of course) in tubes and in hermetic package probably is not a lot more store space than the dies in a case along with the blank ceramic cases.

Bet the customers still have a few dozen old 396 machines running MSDOS5.0 with a programmer plugged into the ISA bus as well for them, though they probably have nice new 19in LCD displays with a CGA/HGC/MDA/EGA to VGA converter in the cable.

[ali6x944]

But what it can't be...
The laser is of different wavelength to the erase uv wavelength required?

Yes, many years ago, I tried different UV lights in an effort to erase an eprom, including sunlight.  Only the official eprom eraser fluorescent tube would actually erase the eproms.

It needs a very particular wavelength - from memory around 250nm. The only thing that will do it is a germicidal lamp - I think that's what UV erasers generally used. Bright sunlight might do it after a very long time but I think a lot of that wavelength is filtrered by the Ozone layer

Awhile back I needed to program a new UVEPROM for some test equipment I was restoring. It just so happened I had an old PC motherboard on hand with a windowed EEPROM (for the BIOS) that would work. Unfortunately I didn't have an EPROM eraser handy...

What I *did* have, however, was a humidifier that contained a UV bulb in the bottom, designed to kill germs in the water between the tank and filter. So, I simply attached the EPROM to the lamp housing with a piece of Kapton tape, turned it on for 10 minutes and voila, I had an erased EPROM!

Amazing, I'm searching about a uv lap to do the same but I saw this video, in the video is using what looks like a discharge tube or a Xeon flash of some sort to erase the chip...
See video:
https://youtu.be/_sSuzDg4ntA

[james_s]

The germicidal lamps used in EPROM erasers are internally the same as any ordinary fluorescent tube except the glass is made of quartz which transmits the short wave UV from the mercury vapor discharge that regular glass is quite opaque to. The windows in UV erasable EPROMs are also made of quartz for this reason.

[bitseeker]

What I *did* have, however, was a humidifier that contained a UV bulb in the bottom, designed to kill germs in the water between the tank and filter. So, I simply attached the EPROM to the lamp housing with a piece of Kapton tape, turned it on for 10 minutes and voila, I had an erased EPROM!

What an uncanny place to find the solution. 

So, what's the preferred solution for restoring old equipment that used EPROMs that have since failed? Vintage gear archaeology, EEPROM, Flash? I recall seeing some OTP ROMs at Mouser or somewhere.

[mikeselectricstuff]

Amazing, I'm searching about a uv lap to do the same but I saw this video, in the video is using what looks like a discharge tube or a Xeon flash of some sort to erase the chip...
See video:
https://youtu.be/_sSuzDg4ntA

Many years ago one of the UK programmer manufacturers company sold an eraser that worked like this - it was a larger handheld unit than in the video and worked pretty well. I still have one somewhere.
Found this pic of one that has been badged by someone else :

I think it was basically a standard flash tube but with a quartz envelope - may have had a special gas mix. Or it could be they found that normal flash tubes would work as-is.
Something I recall was they were rather vague about tube life, but said they would supply replacements at reasonable cost if it died.

[timb]

What I *did* have, however, was a humidifier that contained a UV bulb in the bottom, designed to kill germs in the water between the tank and filter. So, I simply attached the EPROM to the lamp housing with a piece of Kapton tape, turned it on for 10 minutes and voila, I had an erased EPROM!

What an uncanny place to find the solution. 

So, what's the preferred solution for restoring old equipment that used EPROMs that have since failed? Vintage gear archaeology, EEPROM, Flash? I recall seeing some OTP ROMs at Mouser or somewhere.

You can replace some UVEPROMs with EEPROMs (may require you to make an adapter board if the pinout is different or if you want to use a SMD part to replace a DIP one).

Generally though, the easiest method is to simply buy New Old Stock UVEPROMs off eBay. I bought an entire tube of brand new 256kbit ones awhile back for $25 (even though I only needed two; it was cheaper that way).

I also picked up a cheap Chinese UV eraser for $10 from Amazon. The quality isn't very good, but it works for the amount of chips I erase. (I can't use my humidifier to erase them in the winter, since that's when it's actually full of water!)

[bitseeker]

Thanks, Tim. I have some old gear in the queue and I suspect one of them might be having problems reading its EPROM. I'll check out eBay if it turns out to need replacement.

[timb]

Thanks, Tim. I have some old gear in the queue and I suspect one of them might be having problems reading its EPROM. I'll check out eBay if it turns out to need replacement.

Feel free to PM me first, as I've actually got several tubes full of NOS EPROMs; I'd be more than happy to send you as many you need for a much better price than eBay. I can even pre-erase or program them if you need. (All this assumes I have the size you need!)

[bitseeker]

Feel free to PM me first, as I've actually got several tubes full of NOS EPROMs; I'd be more than happy to send you as many you need for a much better price than eBay. I can even pre-erase or program them if you need. (All this assumes I have the size you need!)

Will do. Thanks, Tim!

[ali6x944]

Thanks, Tim. I have some old gear in the queue and I suspect one of them might be having problems reading its EPROM. I'll check out eBay if it turns out to need replacement.

Feel free to PM me first, as I've actually got several tubes full of NOS EPROMs; I'd be more than happy to send you as many you need for a much better price than eBay. I can even pre-erase or program them if you need. (All this assumes I have the size you need!)

Thanks alot man, this is really kind of u

[ali6x944]

Amazing, I'm searching about a uv lap to do the same but I saw this video, in the video is using what looks like a discharge tube or a Xeon flash of some sort to erase the chip...
See video:
https://youtu.be/_sSuzDg4ntA

Many years ago one of the UK programmer manufacturers company sold an eraser that worked like this - it was a larger handheld unit than in the video and worked pretty well. I still have one somewhere.
Found this pic of one that has been badged by someone else :

I think it was basically a standard flash tube but with a quartz envelope - may have had a special gas mix. Or it could be they found that normal flash tubes would work as-is.
Something I recall was they were rather vague about tube life, but said they would supply replacements at reasonable cost if it died.

Where the tubes like this:
https://plus.google.com/115108013327841958420/posts/LdtYh4pXTpU
I noticed when I had the  light from my phone a silvery looking thing inside the tube I suspect it to be just reflections or what I hope it is mercury vapor...
I know it is unlikely but just hoping...

[mikeselectricstuff]

It looks like a standard mid-sized xenon tube, don't recall if it was linear or U shape. 

[ali6x944]

I think it is possible after all to erase it using Xeon flash:
http://www.protonbasic.co.uk/archive/index.php/t-48209.html

[helius]

Be careful bodging together UV erasers. The UV-C wavelengths they use are germicidal, and lethal to human cells as well (particularly the delicate ones in your cornea). They are powerful inducers of skin cancers.
Properly designed erasers (like these) have interlocks, so the bulb is only energized if the light-tight EPROM drawer is closed.

[SeanB]

The flash erasers just used a regular linear or U shape flash tube, standard part as they are quartz glass to withstand the high pressure and thermal shock of regular flash use. Then they used a standard 120uF photoflash capacitor as power bank, with a modified charge circuit to get high recharge speeds, so you could do 1 pulse per second to erase. Typically 4 flashes and the PROM was like it left the factory, all FF.

You could use just a regular flash unit, just take the front soda glass or plastic window out, as it is a UV filter for the tube. Then place on window of the EPROM, trigger 5 times and it is done.

[BrianHG]

For strobe eraser, use an old camera with xenon flash, working from 2xAA cells.  Though they are a flat tube, you have the driver circuit, it comes packages in a reflector, just remove the front plastic lens and place over the eprom & all of the strobe will be focused right into the window thanks to the reflector housing.

If you want shorter bursts, just shrink the 330uf power cap.  It will also charge faster this way.

[bitseeker]

That's pretty cool. I had never tried a flash before. Beats waiting 15 min.

[Gyro]

Just a quick warning that most camera flashes use the reflector as the external trigger electrode. Without the insulation of the plastic lens you might give the EPROM (or yourself) a few kV spike. Just be careful of the spacing.

[BrianHG]

Just a quick warning that most camera flashes use the reflector as the external trigger electrode. Without the insulation of the plastic lens you might give the EPROM (or yourself) a few kV spike. Just be careful of the spacing.

Unfortunately the plastic lens may block a lot of the UV range needed for the erasing.