29 Jan
2009
29 Jan
'09
10:35 a.m.
On Jan 29, 2009, at 10:55 AM, Mr Ian Primus wrote:
I have to say I'm curious as to why you'd do this. Nicely-
packaged commercial EPROM erasers are available all over the place,
and they're cheap. I just bought a really, really nice one (I mean
REALLY nice) for ten bucks to replace my beat-up one, and then I sold
my old beat-up one for...ten bucks. I have two more in the closet
that I bought for similar amounts, at least ten years ago.
Keep in mind that all electronic devices are analog...and some of
them are specified in digital terms. EPROMs store a bit as a charge
on a tiny piece of conductive material floating in an insulator
("floating gate"), and when that charge is above a certain threshold,
it's a 0, and if it's below the threshold, it's a 1...so there's a
varying amount of charge on that floating gate.
Erasing an EPROM drains that charge away by turning that
surrounding insulator into a partial conductor. So, you see what
happens when you erase them to just below the bit transition threshold.
Three minites
seems awfully fast for complete erasure. For
example, an Intel 1982 databook
lists erase time for the 2764 as 15-20 mins at a distance
of 1 inch from the lamp (2537
Angstroms, 12000 uW/CM2).
Hmm. Well, either I got lucky with the previous EPROMs, or my lamp
is a little more powerful. I don't know the specs on it, I bought
it a couple years ago at a pet supply store. It's a 12" germicidal
UV tube, the thin kind, what's that, T6?. It's intended for use in
pond filters.
The EPROMs sit on a specially designed non-conductive UV absorbant
insulating surface (it's a piece of wood), right at about an inch
from the lamp. 3) Some EPROMs
are "less erased" than others.
Wow. Good to know. I had no idea. I always tried to give them the
least amount of UV that they needed to come up blank on my
programmer. It's a PC/clone based ISA programmer, so I'd assume
that the 5v it's giving the chip is just the 5v from the PC's
supply, so I would hope that it's close. But, yeah, I guess I'll try setting the timer for
longer now, to
ensure they're good and blank. I was always told that too much UV
would damage the chips - I've never left one in for more than say,
15 minutes before. Next time I need to erase some, I'll put 'em in
for longer than three or five minutes...
45 minutes sticks in my head as the rule of thumb from the
2708/2716 days, but we have no idea of how much UV light your lamp
emits.
-Dave
--
Dave McGuire
Port Charlotte, FL