In this introductory chapter, we describe what we mean by digital, analog and mixed-signal system modeling and see why modeling and simulation are an important part of the design process. We see how the hardware description language VHDL-AMS can be used to model digital, analog and mixed-signal systems and introduce some of the basic concepts underlying the language. We complete this chapter with a description of the basic lexical and syntactic elements of the language, to form a basis for the detailed descriptions of language features that follow in later chapters.

1.1 Modeling Systems

If we are to discuss the topic of modeling digital, analog and mixed-signal systems, we first need to agree on what we mean by each of these terms. Different engineers would come up with different definitions, depending on their background and the field in which they were working. The primary difference between digital and analog systems is their representation of values and behavior with respect to time.

First, we focus on what we mean by a digital system. Some designers may consider a single VLSI circuit to be a self-contained digital system. Others might take a larger view and think of a complete computer, packaged in a cabinet with peripheral controllers and other interfaces, as a system. In both of these cases, it is not the size or complexity of the system that makes it digital; rather, it is how data values are represented. For the purposes of this book, we consider any digital circuit that...