Entity declaration

Adding or deleting element, attribute, or entity declarations (change classes 3 and 4). 

Update attribute or entity facets such as changing a default value of an attribute or updating the replacement text of an entity declaration (change classes 5 and 12). 

Entity declaration defines an entity and the interface between entity and the external environment. It is defined in a specific format as shown in Fig. APV/4.0.2-1B. 

A design entity contains an entity declaration and an architecture body. These are described in detail below. Figure 3.1 shows the organization of the VHDL design hierarchy and illustrates the main elements of an architecture body. The port connections, indicated by arrowheads, enable information to be passed between blocks and, at the top level, allow the design entity to communicate with the external environment. A design entity may contain any or all of the elements shown in the diagram. It will always have an entity declaration and architecture body at the top level. 

The entity declaration is the interface between the external environment such as a top-level schematic and flie design. It is at the top of ttie design tree for every external block. Hence, it will usually contain a description of the inputs to and outputs from the block in the form of a port statement. An entity declaration without a port statement does not have any external connections. 

The entity declaration statement defines the interface to a design unit and the external environment. Apply the following syntax ... 

The entity declarative part can include declarations for any of the following items ... 

The architecture declarative part serves the same function as the entity declarative part. However, all declarations are local to the architecture and are not visible to any other architecture blocks. 

All package and entity declarations in the working library, WORK, and the STANDARD package are implicitly visible when the current design unit is being compiled. Any package or library that is visible to a primary unit is implicitly visible to the associated secondary unit. 

The entity declaration and architecture body are a pair of design units that enable the function and structure of a circuit to be described. 

The port list should match the list given in the components entity declaration. Constructing a port list is discussed in Qiapter 5. 

The entity declaration and architecture body of each component is shown in Figure 4.9. Again these descriptions are compiled and stored in the working library where the multiplexer architecture, STRUCTURAL2, can access them. Unlike STRUCTURAL , however, this architecture does not declare the components in its declarative part. Instead it uses a package declaration to store the component declarations. A Use clause is then required to... 

Construct the entity declaration for the top-level block of the circuit shown below. Use ihe signal types encoimtered in this chapter. 

This occurs because the S3mthesizer does not see a JK flip flop in the process function but knows, from the clock condition in the Wait statement, that a sequential circuit is being inferred. As discussed in section 5.1.3, the choice of flip flop in an inferred sequential section is left entirely up to the synthesizer. It will therefore tend to select a simple one rather than a complex one, and so not risk the overspedfication of the circuit. Observe the feedback of S, which is required to determine the next state of the circuit. Because this signal is not only updated and output by the process but also read, the connection to the top level of the drcuit is defined in the schematic as bidirectional. This does not mean that it can be used as an input to the circuit as the entity declaration for this component states that the port Q, to which it is directly connected, is of mode output. 

A function is flexible. As the package body of ARITH TYPES shows, the parameters passed and returned from a function do not have to be constrained. If die function is designed without constraints it can be used in many different designs or several times in the same one, performing operations on n-bit objects. To achieve this flexibility, attributes are used to provide information about each object passed to the function. The RIPPLE functions use the Left and Reverse.ramge attributes discussed further in Box 6.3. A component, on the other hand, is constructed from an entity declaration and an architectural body. This means that the size of each I/O path must be determined at tfie component s compile time, reducing its flexibility. 

This behavioural description requires the buffers contained within the BUFFERS block to enable and disable the input and output ports connecting die random sequence generator to die external bus. The code demonstrates another way of declaring and defining a procedure - bodi the procedure declaration and body are contained within the entity declarative part. 

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