Digital Electronics: Principles, Devices and Applications

9Programmable Logic DevicesLogic devices constitute one of the three important classes of devices used to build digital electronicssystems, memory devices and microprocessors being the other two. Memory devices such as ROMand RAM are used to store information such as the software instructions of a program or the contentsof a database, and microprocessors execute software instructions to perform a variety of functions,from running a word-processing program to carrying out far more complex tasks. Logic devicesimplement almost every other function that the system must perform, including device-to-deviceinterfacing, data timing, control and display operations and so on. So far, we have discussed thoselogic devices that perform fixed logic functions decided upon at the manufacturing stage. Logicgates, multiplexers, demultiplexers, arithmetic circuits, etc., are some examples. Sequential logicdevices such as flip-flops, counters, registers, etc., to be discussed in the following chapters, alsobelong to this category of logic devices. In the present chapter, we will discuss a new categoryof logic devices called programmable logic devices (PLDs). The function to be performed by aprogrammable logic device is undefined at the time of its manufacture. These devices are programmedby the user to perform a range of functions depending upon the logic capacity and other featuresoffered by the device. We will begin with a comparison of fixed and programmable logic, andthen follow this up with a detailed description of different types of PLDs in terms of operationalfundamentals, salient features, architecture and typical applications. A brief introduction to thedevices offered by some of the major manufacturers of PLDs and PLD programming languages isgiven towards the end of the chapter.9.1 Fixed Logic Versus Programmable LogicAs outlined in the introduction, there are two broad categories of logic devices, namely fixed logicdevices and programmable logic devices. Whereas a fixed logic device such as a logic gate ora multiplexer or a flip-flop performs a given logic function that is known at the time of devicemanufacture, a programmable logic device can be configured by the user to perform a large variety ofDigital Electronics: Principles, Devices and Applications Anil K. Maini©2007 John Wiley &Sons, Ltd. ISBN: 978-0-470-03214-5