At 00:00 26/06/97 -0400, Terry Ewing wrote: >This is not only off topic, but probably a question WAY below the level of > expertise of the majority of the group. I haven't found a difinitive answer > anywhere, so I'm turning here. No need to apologise. Even if the question is basic, it's surprising how simple questions can turn up very interesting answers - although I doubt my answer qualifies! >My question is about EPROMS. In a project I'm making I need to interface a set > of digital lines to a 14 segment display. The only problem is this... The > lines are not your normal hex. They have a funky logic. I could use a bunch > of gates and make the output I want, or I could use a PGA. Neither of which > are very good solutions for the small size of this application. > >I was wondering if I could use an EPROM in this way. Put the input into the > address lines and feed the 14 segment display off the output lines. Would this > be possible? I know EPROMS are slower than other ICs, but am I going to be > able to use this to multiplex displays? It's very unlikely that programmable logic other than EPROM can give you a "neater" design, unless there's some sequencing involved. For example, I've seen fast bipolar ROMs programmed with a look-up table used as fast 4x4 multiply chips. >Also, can I drive the display directly from the outputs of the eprom, or do I > need to use transistors? You should check out low-current LEDs and LED displays. You don't state exactly what display you intend using, although it sounds like a starburst alpha-numeric - or is it a dual-digit 7 seg? I've been using low-current LEDs for years. They are a bit more expensive, but running at 1mA rather than 10 makes life much easier. Unfortunately, many display types and configurations aren't available in low-current - for instance, I can't find a source of the 16 pin 4-bar types in green low current. Another alternative is to use the multi-character dot-matrix type of display. Some of these have character RAM included for a user-defined display set. You'd need something like a PIC to initialise that RAM on power-up, and to decode/encode your parallel data lines into display address/data/control signals. Again, I'm assuming you are displaying special characters, if not, things are easier and cheaper. As for speed, what's the problem? The slowest EPROM ever made can decode your signals at display rates thousands of times faster than a human can hope to read them! If you go for an EPROM, you'll need a programmer and an eraser. If you are using PICs you probably have an eraser, but unless your PIC progger is a "Universal" type, it probably won't program EPROMs. A benefit of having a proper EPROM programmer is that it will allow you to edit the EPROM contents. That may be laborious, but it saves having to write a little program to generate the data file. On the subject of LED displays, I often use the multi-LED HEX displays (like 7 segment but with the corners separately controlled so that B and 8 are distinct). The TIL911 has been around for more than a decade, and is still horrendously expensive. Also, the version with a CMOS decode/drive went out of production several years back and we're stuck with the power-hungry TTL version. The newer types with similar function aren't much cheaper, and are less convenient in their mechanical arrangement. Anybody got suggestions for a cheap, low-consumption alternative? (For a 32 bit bus display, I've got a paper design using the multi-digit dot-matrix types plus a CPLD, but that's inelegant). This is probably more off-topic than Terry's original query, and I'd better take it away to the Chip Directory mailing list (if you don't know ChipDir, you should, check out http://www.xs4all.nl/~ganswijk/chipdir/chipdir.html ) Good luck. Tim Forcer tmf@ecs.soton.ac.uk Department of Electronics & Computer Science The University of Southampton, UK The University is not responsible for my opinions