TABLE OF CONTENTS A model of an analog system consists of the circuit nodes, analog unknowns to be calculated and the characteristic equations that specify analog behavior. In VHDL-AMS, we use terminals to represent the circuit nodes, quantities for the analog unknowns and simultaneous statements for the characteristic equations. In this chapter, we show how we use these features of VHDL-AMS to model analog, mixed-signal and mixed-technology systems.
In previous chapters we have discussed the use of entities and architectures to represent the elements of the system we are modeling. We have also introduced the types and natures we use to describe the allowable values that objects may hold and have discussed the sequential statements we use to describe behavior within processes. In Chapter 5 we focused on the use of entities and architectures for describing digital behavior and structure. In this chapter we turn our attention to the VHDL-AMS features used in entities and architectures to describe behavior and structure of analog, mixed-signal and mixed-technology systems.
We introduced quantities for analog modeling in Chapter 1. There are three kinds of quantity in VHDL-AMS: free, branch and source quantities. A free quantity is an analog-valued object that can be used in signal-flow modeling. A branch quantity is similar, but is specifically used to model conservative energy systems. We return to branch quantities and their association with terminals in Section 6.2. A source quantity is used for frequency and noise modeling, which we discuss in Chapter 13.
A free quantity can...
