Programmable logic devices

In the development of integrated circuits the programmable logic devices are positioned between the standard logic gates and custom or semi-custom designed circuits. All programmable logic devices have a similar architecture consisting of an array of AND gates and an array of OR gates as described in chapter 1. However, in order to enhance the performance of programmable logic devices, a number of additional features are added to the basic structure. The use of registers and feedback circuits in programmable logic devices enables them to be used in the designs of state machines and sequential logic circuits. Additionally, the inclusion of macrocells in programmable logic devices provides the opportunity to emulate the output structure of many existing devices. The internal configuration between the inputs and outputs create the different types of PLDs which will be described in this chapter. Generally, the programmable logic devices provide cost-effective solutions to digital problems as they replace a large number of combinational logic families.