GALs and PALs (was Re: [Free] Old Data Books (Australia))

Roy J. Tellason rtellason at verizon.net
Thu Sep 6 00:21:19 CDT 2007


On Wednesday 05 September 2007 12:59, Ethan Dicks wrote:
> 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
> OTP.
>
> 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.

I really had little expectation of having any use for them,  though I do 
somewhere have manuals for that stuff -- used to be a ZDS service center back 
when -- and I could probably at least see what they are.  I'm guessing mostly 
address decoding glue or similar.

> > 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.

Hm,  how complicated would one of those programmers be?  Or expensive?  This 
may not be something I'd want to pursue after all...  :-)

> >  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.

Sounds good.  At least I can go for a better understanding of the parts.

> 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.

This is the first I can recall ever hearing of them.

> 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...
>
> http://www.latticesemi.com/products/cpldspld/gal.cfm

Thanks for the link,  I'll have a look...

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