GALs- what are they?

Jim Pieronek jvp%fuelrod at juliet.ll.mit.edu
Fri Nov 4 22:48:01 GMT 1994


BigBrother at MAIL.UTEXAS.EDU writes:
 > I was just looking over John's 68hc000 board and I noticed there are 3
 > GALs on the board.  Type: 16V8.
 > 
 > What exactly do PALs and GALs do?  They are some kind of programmable
 > logic device aren't they?  I checked my Jameco catalog and saw they
 > are only $3, but the programmer is $700.00.  Wow!  That is pretty
 > expensive.  Do alternative programmers exist for less money?
 > 

Generic Array Logic was conjured up by Lattice to replace a whole
schmeer of different Programmable Array Logic devices made by all
sorts of vendors.  

A 16V8 has 18 pins.  Two pins are for VCC and ground.  8 pins are
programmable as input or output pins, and the remaining 8 are
dedicated inputs.  The 16V8 refers to a maximum of 16 inputs and 8
outputs, but not all at the same time.

Each output consists of a D flip-flop that has a big (8-16 inputs) OR
gate connected to the D input.  Each of the OR inputs is connected to
an AND gate that is hooked into the logic array.  The array has all of
the input pins, all of the output pins, and the inverted form of each
in it.  You can connect the AND inputs up to as many inputs in the
array as you want, so you can generate sum-of-product equations with
arbitrary products (AND terms) and up to 16 such terms ORed together.
You can also bypass the D flip-flop to provide a purely combinatorial
output.  All of the flip-flops are clocked by pin 1 if you use the
flip-flops.  There are also open-collector and tri-state output
control options.

As far as costs, you need two things: a software package to compile
the equations down to a fuse blowing file, and you need a programmer.
Some companies offer free or nearly free software to work with their
parts.  Lattice is not such a company to my knowledge.  Programmers
can be had for around a hundred dollars - they may be limited to only
a few devices.

I use 22V10 and 26CV12 GALS (and the usual 7400 register parts -
74373, 374, 244, 245) for almost everything that I design.  I suspect
most other folks would do this too if they could afford the tools.
Perhaps its time to get some heads together out there and build a
public domain software tool and a real simple programmer that would
hook up to a printer port to program just a few parts so everyone
could use this technology.

- Jim
===================================================================
J. V. Pieronek                            Phone: (505) 243-5822
M.I.T. Lincoln Laboratory                 FAX:   (505) 243-5823
Terminal Radar Development Facility       Internet: jvp at ll.mit.edu
Albuquerque, New Mexico



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