Digital Electronics: Principles, Devices and Applications

9

Programmable Logic Devices

Logic devices constitute one of the three important classes of devices used to build digital electronics systems, memory devices and microprocessors being the other two. Memory devices such as ROM and RAM are used to store information such as the software instructions of a program or the contents of 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 devices implement almost every other function that the system must perform, including device-to-device interfacing, data timing, control and display operations and so on. So far, we have discussed those logic devices that perform fixed logic functions decided upon at the manufacturing stage. Logic gates, multiplexers, demultiplexers, arithmetic circuits, etc., are some examples. Sequential logic devices such as flip-flops, counters, registers, etc., to be discussed in the following chapters, also belong to this category of logic devices. In the present chapter, we will discuss a new category of logic devices called programmable logic devices (PLDs). The function to be performed by a programmable logic device is undefined at the time of its manufacture. These devices are programmed by the user to perform a range of functions depending upon the logic capacity and other features offered by the device. We will begin with a comparison of fixed and programmable logic, and then follow ...