The concepts of type and nature are very important when describing data in a VHDL-AMS model. The type or nature of a data object defines the set of values that the object can assume, as well as the set of operations that can be performed on those values. A scalar type consists of single, indivisible values. A scalar nature defines values that are associated with a single terminal. In this chapter we look at the basic scalar types and natures provided by VHDL-AMS and see how they can be used to define data objects that model the internal state of a module.

2.1 Constants and Variables

An object is a named item in a VHDL-AMS model that has a value of a specified type or, in the case of a terminal, has a specified nature. There are six classes of objects: constants, variables, signals, terminals, quantities and files. In this chapter, we look at constants and variables; signals are described fully in Chapter 5, terminals and quantities in Chapter 6, and files in Chapter 21. Constants and variables are objects in which data can be stored for use in a model. The difference between them is that the value of a constant cannot be changed after it is created, whereas a variable's value can be changed as many times as necessary using variable assignment statements.