Subject: Digital System Design

Topic: Programmable Logic Devices

Difficulty: High

PLA — a Programmable Logic Array (PLA) is a relatively small FPD that contains two levels of logic, an AND-plane and an OR-plane, where both levels are programmable (note: although PLA structures are sometimes embedded into full-custom chips, we refer here only to those PLAs that are provided as separate integrated circuits and are user-programmable).

A PLA consists of two levels of logic gates: a programmable “wired” AND-plane followed by a programmable “wired” OR-plane.

A PLA is structured so that any of its inputs (or their complements) can be AND’ed together in the AND-plane; each AND-plane output can thus correspond to any product term of the inputs. Similarly, each OR plane output can be configured to produce the logical sum of any of the AND-plane outputs. With this structure, PLAs are well-suited for implementing logic functions in sum-of-products form. They are also quite versatile, since both the AND terms and OR terms can have many inputs (this feature is often referred to as wide AND and OR gates).

When PLAs were introduced in the early 1970s, by Philips, their main drawbacks were that they were expensive to manufacture and offered somewhat poor speed-performance. Both disadvantages were due to the two levels of configurable logic, because programmable logic planes were difficult to manufacture and introduced significant propagation delays. To overcome these weaknesses, Programmable Array Logic (PAL) devices were developed.

PAL* — a Programmable Array Logic (PAL) is a relatively small FPD that has a programmable AND-plane followed by a fixed OR-plane

As Figure illustrates, PALs feature only a single level of programmability, consisting of a programmable “wired” AND plane that feeds fixed OR-gates. To compensate for lack of generality incurred because the OR Outputs Plane AND Inputs & Flip−flop feedbacks D. Structure of a PAL plane is fixed, several variants of PALs are produced, with different numbers of inputs and outputs, and various sizes of OR-gates. PALs usually contain flip-flops connected to the OR-gate outputs so that sequential circuits can be realized.

PAL devices are important because when introduced they had a profound effect on digital hardware design, and also they are the basis for some of the newer, more sophisticated architectures that will be described shortly. Variants of the basic PAL architecture are featured in several other products known by different acronyms. All small PLDs, including PLAs, PALs, and PAL-like devices are grouped into a single category called Simple PLDs (SPLDs), whose most important characteristics are low cost and very high pin-to-pin speed-performance.