Pros And Cons Of Replacing Discrete Logic With Programmable Logic In Introductory Digital Logic Courses.

Abstract

Digital circuit construction with small-scale integrated (SSI) and medium-scale integrated (MSI) logic has long been a cornerstone of introductory digital logic design laboratories. Recently, in- structors have begun replacing these projects with designs using complex programmable logic such as programmable array logic (PLA) chips and field programmable gate arrays (FPGAs). This paper investigates the trend of replacing the “traditional” SSI/MSI breadboarded digital logic design projects with design projects utilizing more complex programmable integrated cir- cuits. Without a doubt, each style has its own strengths and weaknesses. Utilizing complex programmable integrated circuits (ICs) such as PLAs and FPGAs, more interesting and involved projects can be implemented. Modern programming tools allow the spec- ification of quite complex circuits from a graphical schematic or procedural hardware description. These specifications can be downloaded into the IC, which then functions as specified. Since much of the design is in the programming of the IC, very involved projects can be implemented without significantly increasing the wiring complexity of the project. Students don’t spend hours trying to find an upside-down IC or a broken connection, and can concentrate on digital design. The design complexity and innovation in these projects is unarguably increased over SSI/MSI projects, but this does not come without a pedagogic cost. Some of the arguments for having a laboratory course in the first place are to appeal to the sensor learning style, to expose the students to more “hands-on” and less theoretical projects, and to introduce the practical aspects of designing and implementing digital circuits. These objectives may not be met as well when moving from the SSI/MSI projects to more software-oriented projects. Both styles of digital design projects have pedagogic strengths and appeal to particular learning styles. It is important to study the course objectives and the student mix when deciding to move projects from the traditional style of physically constructing circuits from SSI and MSI compo- nents to a new style of simulating and programming complex chips as a means of verifying digital logic designs. By doing this, we can combine the two methodologies to arrive at a course that appeals to a broad range of students, provides the “hands-on” experience some students need, and utilizes modern technologies to increase the innovation, design complexity, and interest value of implemented projects.

1 Introduction

The construction of combinational and sequential digital logic circuits from discrete components, usually utilizing TTL (Transistor Transistor Logic) DIP (Dual In-Line Packaging) chips, will be familiar to anyone who has seen undergraduate electrical and computer engineering labs in the past few decades. In this type of lab, a desired behavior is often given in terms of a problem description