Re: what are GALS (related TWGS)

[dempson@actrix.gen.nz (David Empson)]

Joseph Turner <jturner@tartarus.uwa.edu.au> wrote:

> Hi I have been looking at the FAQ, and it mentions something on GALS. I
> don't actually know what they are. I guess they are chips, but what
> ACTUALLY are they? and where are they? ROMS?

"GAL" stands for "General Array Logic".  It is a programmable logic
chip, though I don't know how it differs from a typical "PAL"
(programmable array logic) or "PLD" (programmable logic device).
Probably just a different name used for marketing reasons.

A PAL or PLD (and presumably a GAL) accepts several input signals,
combines them using some logic formula involving ORs and ANDs, and
outputs the results.  They are commonly used in address decoding
circuitry.  Inside, the chip is a huge grid of crossed signal lines,
with programmable intersections.  One direction is used for combining
terms with AND operations, and the other direction does the OR
operations.

(There are more advanced chips that offer additional features such as
latching of data for implementing state machines.)

The whole thing is very fast (around 5 to 40 nanoseconds, depending on
the product family), though not usually as fast as separate logic chips.

The two major benefits of using a PAL/PLD are:

1. The logic can easily be changed by reprogramming or replacing one
chip.

2. A PAL/PLD will take up a lot less space on a circuit board than the
corresponding discrete logic gates.


By comparison, a ROM is a memory device with address inputs and data
outputs.  The address is used to look up a group of memory cells inside
the ROM, and the contents of those cells are made available on the data
outputs.

In some situations, a fast ROM can be used as part of an address
decoding circuit (e.g. if a complicated memory map is required), but
they are more commonly used to hold code or data for a microprocessor.


Some types of EPROM programmers are also able to support programming of
PALs/PLDs, etc., though you often need a special adaptor and software.

-- 
David Empson
dempson@actrix.gen.nz
Snail Mail: P.O. Box 27-103, Wellington, New Zealand