Component declaration

Phytopharmaca—Phytopharmaca are defined as those whose components declared as active substances are plants, of plants or plant preparations. Not included in this definition are ... 

In VHDL, a component declaration is required before instantiating a component. This defines the template of the component being instantiated and includes information such as, port mode or direction, port names and the data type of the ports. These templates or component declarations are bound to sub-design entities using configuration declaration statements. Hence, when reading in a VHDL netlist you... 

Example 2.6 shows a Verilog description which instantiates the same buffer cell from the technology libraiy. In Verilog, there is no concept of component declarations. The direction and datatypes of the ports of the component being instantiated are derived from the module to which it is linked. 

Your VHDL design instantiates a component "SUB" from the technology libraiy. The component declaration of the component "SUB" is in a package "comps which is made visible by the use clause. I get this error during read. 

SUB" is an enumeration literal defined in type "CHARACTER" in the package "STANDARD" which is implicitly visible in all VHDL designs. Since this definition as well as the component declaration for "SUB" are both visible in the code, a homograph is reported. You can explicitly avoid this ambiguity as follows... 

Just as a Process statement identifies a behavioural style, component declarations and instantiations identify a structural style. Unlike the behavioural style, which specifies what the circuit has to do, a VHDL description in the structural style determines the operation of the circuit by the manner in which any number of functional blocks - components - are interconnected. These may contain anything from simple two-input logic gates to 16 x 16 bit multipliers. 

In each case, the ccnnponents that are used must be declared and instantiated. A component declaration informs the architecture that a particular component is being used and lists its inputs and outputs. The declaration may be located in the architecture s declarative part or a package declaration. It is possible to omit the declaration and instantiate the entity and architecture of the component directly (Box 4.6). 

A component declaration provides an architecture with the interface d racteristics of a component that it may then instantiate. The s)mtax is as follows ... 

As the comments in Figure 4.8 explain, because the components are compiled and stored in the working library, they are automatically visible to this architecture. STRUCTURAL demonstrates the use of both component declarations and indirect instantiations. 

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... 

Component declarations in the architecture declarative part component NAND 2... 

Include the package containing the component declarations use WORK.SIMPLE COMPS.all ... 

The architecture body illustrates another way to instantiate components in a design. Again, there are no component declaration statements in the architecture s declarative part. However, this time these declarations do not even exist in a package. Instead, the components are directly instantiated by specifying the name of the entity and architecture (in parentheses) in the component instantiation statement. The port list follows in the normal way. 

The component is compiled into the working directory in this case and therefore no explicit references to libraries are required in the CONVERTER design xmit. Component instantiation has been made indirectly and so a component declaration is required. This has been placed in the declarative part of the architecture but could have appeared in a referenced package. 

A direct component instantiation does not require a Component declaration statement. 

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