EPROM erase times and lifespan

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). This is pretty consistant with what I've observed - I normally bake em for about 15 mins. I've seen some take as long as 30 mins (usually older "fixed program time" devices). Keep in mind that although the output is digital, the actually process of an EPROM is analog in nature - the output is decided based on the amount of charge on the gate. This brings a few important points to understand: 1) Some EPROMS are "more programmed" than others. Early devices with a fixed proramming time per cell normally are saturated, and well programmed. Later "intelligent" algorithms would watch the device, note how much charge was needed to make a 1->0 transition occur in the cell, add a margin and "your done". Much faster - but depending on the equipment, sometimes "less programmed". 2) The comparitor is voltage dependant on most devices. An EPROM bit near the 0/1 threshold may change state with a difference in power supply voltage - Intel spec'd a supply of 6V when using their "intelligent algorithm" because that would raise the comparitor voltage and cause it to require more charge to get a zero bit. This insured that you were programmed "well past" the normal boundary at 5V. You can sometimes read an EPROM that is suffering from "bit rot" by powering it at a lower voltage. A side effect of this is that: 3) Some EPROMs are "less erased" than others. An "erased" EPROM read at 5.2v might not be erased when read at 4.8v ... so just because it all reads FF's doesn't mean it's really erased. The fact that you see some "mostly erased" suggests you are a little close to the boundary. Just like programming, you need to add a margin to insure adaquate erasure. My guess is that "lightly" programmed devices are showing just under the 1/0 threshold after 3 mins, but probably aren't fully erased. You might also want to try and find out the specs for your lamp and see if it's close to a normal eraser, and adjust your distance/time accordingly. Dave -- dave09 (at) Dave Dunfield dunfield (dot) Firmware development services & tools: www.dunfield.com com Collector of vintage computing equipment: http://www.classiccmp.org/dunfield/index.html