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Vhdl for Logic Synthesis, Third Edition by Andrew Rushton

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Chapter 10

Hierarchy

Hierarchy should be used extensively in a design to divide-and-conquer the design problem. Furthermore, third-party components can be incorporated into a design more easily this way and this leads to a higher degree of confidence in the integrity of the design.

The natural form of hierarchy in VHDL, at least when it is used for RTL design, is the component. Do not be tempted to use subprograms as a form of hierarchical design! Any entity/architecture pair can be used as a component in a higher-level architecture. Thus, complex circuits can be built up in stages from lower-level components.

It is possible to design parameterised circuits using generics. The combination of generics with synthesis, which allows the design of technology-independent circuits, is a very powerful combination.

10.1 The Role of Components

There are a number of reasons for using hierarchy.

Each subcomponent can be designed and tested in isolation before being incorporated into the higher levels of the design. This testing of intermediate levels is much simpler than system testing and consequently is usually more thorough. Using components for hierarchy is compatible with the Test-Driven Design paradigm since it makes testing of subsystems easier. This means that the designer can have a high degree of confidence in the subcomponents and this also contributes to the overall integrity of the design.

It is good practice to collect useful subcomponents together into reusable libraries so ...

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