Testbench and Simulation

Having entity declaration and body architecture only one thing is still to do before compilation. I have to inform compiler which libraries are needed to analyze the design. Therefore, before entity declaration I inserted name of the library and package which will be used. This is ieee library and package std_logic_1164 (package, which contains declarations of basic types used in the design).

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Architecture body

In previous post I presented entity declaration and gave simple example. Now it is time for second part – architecture body. Without this part, you won’t be able to create any module in VHDL.

Entity declaration gives information how module is connected with external world. Architecture body gives information what is inside of the module. It specifies relationship between input and output ports. In other words it specifies behavior of the module.

You should know there are three different modeling styles which allow to describe architecture body:

  • structural description
  • behavioral description
  • dataflow description

Sometimes some people add the fourth style – mixed, which is combination of three mentioned styles.

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Entity declaration

As mentioned in previous post, systems are created using one or more entities. In case of many entities in the project, the uppermost level of it, is top-level entity. To connect modules with other modules or external world, ports and signals are used (Fig. 1). Main role of entity declaration is to define these ports – their names, types, width and direction. Basically entity declaration shows how module is seen by other modules. It describes the external view of the module with no information what is inside. Additionally entity declaration includes name of the entity and other parameters (constants, types, asserts, function etc).

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About VHDL…

All about history, advantages and disadvantages of VHDL you can find on wiki or other similar sites, so there is no point in copying this information. Here I would like only to list main features of this language:

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