From: Ethan Dicks (ethan.dicks_at_gmail.com)
Date: 2007-04-04 20:46:43
On 4/4/07, Anders Carlsson <email@example.com> wrote: > Earlier today, I wrote: > > > If it helps, I might try to read them a few times more, perhaps > > at a lower speed and confirm that I really get consistent output. > > No matter speed or how many times I read the chip, the results are > exactly as I already posted, so no signs of reader inconsistency. Have you checked the Vcc at the pin on your target EPROMs? To test erasure and ghosts last year, I rigged up a double-stacked socket with a tunable voltage divider (pot) so I could turn down Vcc on a 27C256 and watch it behave. I found that using an ultraviolet inspection lamp (the only thing I had on hand - yes, I know I "need" a proper lamp, but I didn't have one and couldn't get one for several months), several hours under the lamp resulted in EPROMs that were blank at 5.00V, but had definite legible bits at 4.80V, and nearly the entire original contents still visible at 4.60V. At a certain point, the chip stopped working, but that was, IIRC, below 4.00V. What I'm wondering is that if you have weak cells _and_ your EPROM burner/reader isn't giving the full 5.00V, that you might be witnessing ageing effects from some of the weaker cells (especially given that you are holding a chip that was burned 24 years ago). The thought is that seeing one of the columns coming up wrong might be a case of the reverse of my experiment last year - in your case, not an intentional erasure being verified, but real data on the threshold of detectability. Another option *might* be to push Vcc above 5.00V (while still staying under the 10% tolerance threshhold) and see if the ROM image "makes more sense" with respect to bit 3. Read speed is probably not an issue here, so "slowing down" isn't likely to help. My initial impulse is to suggest that after 24 years, some of those insulating wells full of encapsulated electroncs are drying out. In a few years, there might be nothing legible remaining. -ethan Message was sent through the cbm-hackers mailing list
Archive generated by hypermail pre-2.1.8.