Programmable Logic Devices:Advantages and Disadvantages of PLAs.

Advantages and Disadvantages of PLAs

PLAs, like ROMs which are more general, have the following advantages over random-logic gate networks, where random-logic gate networks are those that are compactly laid out on an IC chip:

1. There is no neeed for the time-consuming logic design of random-logic gate networks and even more time-consuming layout.

2. Design checking is easy, and design change is also easy.

3. Layout is far simpler than that for random-logic gate networks, and thus is far less time-consuming.

4. When new IC fabrication technology is introduced, we can use previous design information with ease but without change, making adoption of the new technology quick and easy.

5. Only the connection mask needs to be custom-made.

6. Considering all these, PLA is a very inexpensive approach, greatly shortening desing time. PLAs have the following disadvantages compared with random-logic gate networks:

1. Random-logic gate networks have higher speed than PLAs or ROMs.

2. Random-logic gate networks occupy smaller chip areas than PLAs or ROMs, although the logic design and the layout of random-logic gate networks are far more tedious and time- consuming.

3. Also, with large production volumes, random-logic gate networks are cheaper than PLAs or ROMs.

PLAs have the following advantage and disadvantage, compared with ROMs:

• For storing the same functions or tasks, PLAs can be smaller than ROMs; generally, the size difference sharply increases as the number of input variables increases.

• The small size advantages of PLAs diminishes as the number of terms in a disjunctive form increases. Thus, PLAs cannot store complex functions, i.e., functions whose disjunctive forms consist of many product terms.

Applications of PLAs

Considering the above advantages and disadvantages, PLAs have numerous unique applications. A micro- processor chip uses many PLAs because of easy of design change and check. In particular, PLAs are used in its control logic, which is complex and requires many changes, even during its design. Also, PLAs are used for code conversions, microprogram address conversions, decision tables, bus priority resolvers, and memory overlay.

When a new product is to be manufactured in small volume or test-marketed, PLAs is a choice. When the new product is well received in the market and does not need further changes, PLAs can be replaced by random-logic gate networks for low cost for high volume production and high speed. Also, a full- custom design approach is very time-consuming, probably taking months or years, but if PLAs are used in the control logic, a number of different custom-design chips with high performance can be made quickly by changing only one connection mask for the PLAs, although these chips cannot have drastically different performance and functions.