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VHDL-AMS is a set of analog extensions to standard digital VHDL to allow mixed signal modeling of systems. The VHDL-AMS language was approved as IEEE standard 1076.1 in 1999; however, it is important to note that IEEE 1076.1-1999 encompasses the complete digital VHDL 1076 standard and is not a subset.
The standard does not specify any libraries for analog disciplines, for example, electrical, mechanical, etc. This is a separate exercise and is covered by a subset working group IEEE 1076.1.1, which was released as IEEE Standard 1076.1.1 in 2004.
In order to put the extensions into context it is useful to show the scope of VHDL and then VHDL-AMS alongside it and this is shown in Figure 18.1:
The key extension of VHDL-AMS over VHDL is the ability to look upward to transfer functions (behavioral and in the Laplace domain) and downward to differential equations at the circuit level. This gives the possibility for the designer to think about design from a systems perspective (in terms of high level transfer function models) and also a “real world” systems view (linking to different domains).
The specific extensions of VHDL for VHDL-AMS can be summarized as follows:
• A new type of port called TERMINALS—basically analog pins.
• A new type of TYPE called a NATURE, which defines the relationship between analog pins and variables.
• A new type of variable called a QUANTITY, which is an analog variable.
• A new type of variable assignment that is used to define analog equations that are solved simultaneously.
• Differential equation operators for derivative (’DOT) and integration (’INTEG) with respect to time.
• IF statements for equations (IF USE).
• Break statement to initialize the nonlinear solver.
• STEP LIMIT Control for limiting the analog time step in the solver.
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