10 Nov
2011
10 Nov
'11
6:33 a.m.
On Nov 10, 2011, at 12:09 AM, Scott Kevill wrote:
So..
- a huge 64-pos (MC68000 and some microcontrollers) universal ZIF socket
Do they make those? The 64-pin DIPs are even wider than the 40-pin ones; I think it's
900mil (almost an inch).
- alternatively a 48-pos universal ZIF and a
regular 64-pos socket
That might be more doable; I see 40-pin ones, at least, on most ROM burners.
- a way to output 5V from all 64 I/Os
You probably don't actually need to output 5v. TTL (the logic family you're
looking to deal with) generally didn't actually *output* anywhere over 3v, so the
switching threshold is considerably lower than that (generally 2.0v for TTL). Thus 3.3v
CMOS logic (which does swing the full 3.3v) should be fine as long as your inputs can take
occasional excursions above 3.3v.
- additional 5V tolerance for 16 of the
non-5V-tolerant FPGA I/Os (48 are already 5V tolerant)
Bear in mind that for a lot of the giant DIPS (64 pins, etc) there are multiple power and
ground pins. So you may not need them all to be tolerant.
- either additional -5V to +15V tolerance for all I/Os
or a way to manually isolate per-I/O from the manually wired supply voltages above (eg.
SPDT dip switches?) The FPGA I/Os do support tri-stating, but I'm not sure that would
be safe.
Tristating a pin will not save it; all it does is turn off a pass transistor or two, and
if that transistor's breakdown voltage is exceeded, it will conduct (usually
catastrophically). You could use external switches (such as a CD4016 or similar, which
will go up to 15v), but this one will take some more thought.
- highest signal rate needed would probably be 8MHz
for the MC68000, although hmm.. if I were sampling or measuring IC timing, perhaps higher
for input signals (eg. 32 MHz)
Give yourself some margin and build for about 20MHz bandwidth. It'll save you a lot
of signal integrity headaches.
I would be creating a PCB for this, but through-hole
would be preferred as I have no experience (yet) with SMT.
If you can fit it, great! Through-hole is also generally a lot more mechanically stable
(I can't count the number of times I've ripped off an SMT pin header because
I've been clumsy or not thinking straight; usually takes the pads off, too).
Thoughts? Suggestions? Or is this crazy/na?ve? :)
It's not a bad idea; it sounds like what a lot of ROM burners/readers do, just
expanded. If you're looking to program ROMs, you might want to keep in mind that a lot
of older EPROMs and similar devices (even older PICs and such) use a high voltage
(typically 12v) on a pin to signify that you're ready to program. You might want a
way to switch that in.
Scott.
SIde note: Are you the Scott Kevill responsible for GameRanger? If so, thank you, it made
my high school years a lot more fun (back when it first came out, around the time of Quake
2 for Mac). If not, never mind.
- Dave