29-12-2012, 05:05 PM
A Programmable Logic Array (PLA)
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ABSTRACT
With the advant of large scale integration technology, it has become feasible to fabricate large circuits within a single chip. one such consequence of this technology are the programmable logic devices.Programmable logic array is a class of PLDs. Which is programmed by the user. It consists AND array and OR array which is used to implement sum of product expressions.
This paper presents the basic introduction of programmable logic array, types of PLDs, general structure of PLA, implementation of logic expression using PLA, applications, advantages and disadvantages
Introduction
PLAs belong to a class of components called programmable logic devices or PLDs , a term applied to ICs containing many gates or other general - purpose cells whose interconnections can be configured or "programmed" to implement any desired combinational or sequential function. PLDs are relatively easy to design and inexpensive to manufacture. They constitute a key technology for building application-specific integrated circuits (ASICs).
A programmable logic array (PLA) is a kind of programmable logic device used to implement combinational logic circuits. The PLA has a set of programmable AND gate planes, which link to a set of programmable OR gate planes, which can then be conditionally complemented to produce an output. This layout allows for a large number of logic functions to be synthesized in the sum of products (and sometimes product of sums) canonical forms.
Types of Programmable Logic
Programmable logic devices are available in many different types. The current range of devices span from small devices capable of implementing only a handful of logic equations to huge FPGAs that can hold an entire processor core and peripherals. In addition to this incredible difference in size there is also much variation in architecture.
Programmable logic devices can be divided into three distinct architectural groups.
Complex Programmable Logic Devices - CPLDs
This group covers the middle ground in terms of complexity and density between SPLDs and FPGAs. CPLDs can handle significantly larger designs than SPLDs, but provide less logic than field programmable gate arrays (FPGAs). CPLDs contain several logic blocks, each of which includes eight to 500 macrocells. For most practical purposes, CPLDs can be thought of as multiple SPLDs (plus some programmable interconnect) in a single chip. The typical structure of a CPLD is shown in Fig 5. Each of the 16 logic array blocks shown is the equivalent of one SPLD. However, in an actual CPLD there may be more (or less) than 16 logic array blocks. Also each of these logic array blocks are themselves comprised of macrocells and interconnect wiring, just like an ordinary SPLD.
Field Programmable Gate Arrays – FPGAs
Field Programmable Gate Arrays are two dimensional array of logic blocks and flip-flops with a electrically programmable interconnections between logic blocks. The interconnections consist of electrically programmable switches which is why FPGA differs from Custom ICs, as Custom IC is programmed using integrated circuit fabrication technology to form metal interconnections between logic blocks. FPGAs can be used to implement just about any hardware design. One common use is to prototype a system that will eventually find its way into an ASIC. However, especially if the product has to be available as soon as possible then there is no reason why the FPGA can't be in the final product. Whether it does or not depends on the balance between development time and costs, and cost of the final device (number of required parts).