7 Dec
2006
7 Dec
'06
5:04 p.m.
On 12/8/06, Chris M <chrism3667 at yahoo.com> wrote:
--- Lee Davison <lee at geekdot.com> wrote:
> Two 8 bit buffers, two 8 bit latches and one GAL20V8
> or similar.
> The GAL isn't essential but saves four or more
> packages to do the address decoding.
Note the "or more" here... it's not hard to imagine reasonable
situations where one GAL saves 6 or 8 popcorn TTL parts, especially
where you start using latched outputs.
So 10 discrete ic's. Why all the fuss then? And
why
bother with the GAL at all if it only saves 4 chips?
I really have to wonder why these developers go to
all the trouble they do if that's all it takes (I
would have guessed at least 20).
There are two major reasons why a hardware developer would use a GAL...
1) board space/interconnections are "expensive"
When one is hand-wiring a project, replacing 4 16-20 pin parts with a
single 18 or 20 pin part, that's a significant savings of effort.
Equally if one is trying to fit in a small space, 4 DIPs is a huge
amount of room these days (and there are PLCC GALs, allowing a lot of
DIP circuity to fit in one cm^2).
2) GALs are easy to change
Practically speaking, folks don't want to rewire a board to change an
address setting, so you end up with address comparators, jumpers, etc.
With a GAL, for infrequent changes (once or twice over the life of
the product, not once or twice per year), you can arrange the logic so
that there are no jumpers, but the GAL still does internal address
compares, etc. The Spare Time Gizmos Elf2000 does this - if you don't
like what devices are selected by which combination of N-lines, etc.,
take the (provided) GAL source, change it, recompile it with WinCUPL,
then burn a GAL to your liking. They are re-usable, so you don't even
have to buy a spare unless you'd like to do a quick swap back.
The downside, of course, is that as the hobbyist-end-user, you are
somewhat out in the cold if you don't own a GAL programmer. They can
easily run to hundreds of dollars for basic ones, and, unlike an old
4K EPROM, they are not trivial to make programmers for from scratch.
I have an older GAL programmer, so I don't mind GAL-based designs.
The only thing that's eluded me lately is programming "C rev" Lattice
GAL22V10s. I've had no problems with burning older Lattice GALs or
other sizes of new Lattice GALs, so it's been an inconvenience, but
not a show stopper. OTOH, I'm willing to look around for a newer
programmer and consider an upgrade. I'm not going to give up GALs
entirely, so I'll find some solution when the need arises. Others
(those not already posessing GAL programmers) might find the jump to
be too expensive to justify.
-ethan