GALs and PALs (was Re: [Free] Old Data Books (Australia))
ethan.dicks at gmail.com
Wed Sep 5 11:59:14 CDT 2007
On 9/4/07, Roy J. Tellason <rtellason at verizon.net> wrote:
> On Tuesday 04 September 2007 23:41, Ethan Dicks wrote:
> > Hope that illuminates more than obscures,
> Sure. I have a bunch of PALs around, mostly pulls from a bunch of XT-class
> machines that Zenith made, and it's debatable whether there will ever be a
> use for them.
Indeed. You might check for "PALCE" - those are CMOS PALs that _can_
be reprogrammed. Bipolar PALs by MMI and other vendors are strictly
You _could_ poke around the PAL with a signal generator and a logic
analyzer/logic probe/VOM/O-scope and divine the internal programming,
but, honestly, blown PALs are rarely useful out of their original
context. The only use _I_ have ever had for them is a pattern to be
able to make modifications to an existing device - I've started with
PALs for, say, an Amiga 3000, then burned a faster GAL to get rid of
video jitter - I was partially successful. I've never tried to take a
PAL out of a circuit and re-use it in a new circuit.
> GALs, on the other hand, sound like they'be be useful for
> all sorts of things.
Indeed they are. Bob Armstrong is a heavy user of GALs for the Spare
Time Gizmos line of hobby products. In the case of the Elf 2000, for
example, you can change the logic equations yourself (he provides the
source) and change the memory and/or I/O map - all of the select logic
goes through a GAL, so the memory map is mutable.
If you are designing your own circuits, a 18 or 24-pin GAL can replace
several square inches of TTL, with the benefit of being able to make
changes later that don't involve cutting traces and adding blue wires.
You lose the ability to poke around in the middle of select circuits,
and to make substantial changes, but it all depends on what your goals
for the project are if that's a worthwhile tradeoff or not. You do
have to start with a GAL programmer, though. Not all hobbyists have
them, so if you make a product to sell, as Bob does, you should
consider selling programmed parts for those that can't burn their own.
> I should probably seek out some data on these parts,
> or even a databook, somewhere. Got any pointers to specific numbers?
The two most common parts I've worked with are the 18-pin Lattice GAL
16V8 and the 24-pin Lattice GAL 22V10. There's also a 22-pin GAL
20V8, and I do have a few pull, but I've never run into a hobby
project that uses them. I would expect to pay $1.25 for small
quantities of 16V8s and up to $3.50 for small quantities of 22V10s.
Bargains can be had, but to be honest, when I see 22V10s for under
$2.00 each, I tend to buy a few. They aren't as easy to find cheaply
as 16V8s. One type of occasional bargain are surplussed
already-programmed parts. Just throw them in your programmer, erase
them, then you are ready to go with only a couple of burn cycles
ticked off their lifetime.
AMD also makes (made?) the PALCE line of PLDs. They are pin
compatible with GALs, and should program in the same programmer as a
GAL. I do not know if there are any substantial technical advantages
of PALCEs over GALs or not.
In general, for non-modern circuits, a GAL is going to be fast
compared to layer after layer of TTL. For example, 15ns GALs are not
particularly fast or expensive. 15ns would be a problem for a
multi-gate address select circuit for, say, a 6502. Fortunately, with
1MHz and 8Mhz designs and such, speed really isn't a factor, but it's
nice to know how much slack you have in your design.
You can start here for technical details...
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