unit and actual values for generics and ports. case the ports have to be explicitly referenced (Example 2).

defined earlier as a component (see that is different from the one declared for the component. elsewhere, either as a component or as an entity/architecture pair

entity, the last compiled architecture associated with the entity This means that This circuit, called a full adder (FA), has the following truth table:Based on the truth table of a full adder, we get the following expressions:$$C_{i+1}=\big ( A_i \oplus B_i \big ) C_i + A_iB_i$$We can repeat Lines 9 and 10 to extend the VHDL code of a full adder into the VHDL code of a four-bit adder:An ISE simulation for this code is shown in Figure 3.However, there is an alternative approach that can lead to cleaner code. When using component instantiation in VHDL, we must define a component before it is used. VHDL allows the designer to parametrize the entity during the component instantiation. subsystem, which can be VHDL Component Instantiation. However, there is one circumstance which still requires using the component method.

This gives us a sum and a carry for each bit position. Have the same interface in terms of signal but different access time address and BUS width. subcomponent, and associates values with generics of that subcomponent.

Moreover, when the circuit has a pattern similar to that of Figure 7, we can use the “for-generate” statement to dramatically simplify the code.To understand the advantages of these VHDL statements, just imagine how unwieldy the code would be if we designed a 64-bit adder without using the component and “for-generate” statements!To see a complete list of my articles, please visit How to Use VHDL Components to Create a Neat Hierarchical Design

When writing VHDL, we declare a component … appears, associates signals or values with the ports of that It also touches on the "for-generate" statement and its uses.This article will discuss use of VHDL components. For most cases, this made the component instantiation method obsolete. In such a Component instantiation is like plugging a hardware component into a socket in a board (Fig. Named association allows to list the generics and ports in an order Component instantiation specifies the interconnection of the component with other parts of the system.For example, consider the rightmost FA in Figure 1.

direct instantiation, the component instantiation statement contains That’s when instantiating black-box modules in your design. 1 in Example 1). will be taken. statement defines a subcomponent of the design entity in which it This is called component declaration. There are three forms The instantiated component is called with the actual parameters for generics and ports. The association list can be either positional or named. entity/architecture pair can be instantiated directly. Imagine you need to write 2 RAM modules.

In this case, as shown in Figure 5, A0, B0, C0, S0 , and C1 of the top-level circuit should be connected to a, b, c_in, s, and c_out of the rightmost FA, respectively. In order to write the VHDL for this circuit, we need to cover two new concepts: component instantiation (placing the INV and AOI inside another higher-level design, MUX2I) and port mapping (connecting up the two components to each other and to the primary ports of MUX2I). It specifies a A component represents an entity/architecture pair. The label for component instantiation is obligatory.A component instantiation In VHDL, this is how we can model PCBs assembled from individual chips, for example. Edit, save, simulate, synthesize SystemVerilog, Verilog, VHDL and other HDLs from your web browser. The component instantiation contains a reference to the instantiated Instantiation of a component introduces a relationship to a unit defined earlier as a component (see Component). This article defines VHDL components, describes component declaration, and gives examples of how to use VHDL components in your code. The reserved word To declare the FA component in our main code, we need to add the following lines:This tells the synthesis software that FA is a component with three inputs (As you can see, these pieces of information are exactly the same as those provided by the “entity” part of the VHDL description of the FA (lines 3 to 6 of Listing 1).In essence, the component declaration describes the interface of a component with its environment (see Figure 4).Now that the FA component is declared, we can use (or instantiate) it. Also, we’ll look at the “for-generate” statement, which can sometimes dramatically simplify the code.To discuss the advantages of using VHDL components, let’s consider writing the VHDL code for a four-bit adder.Before we move on, if you'd like a refresher on VHDL and the purpose of hardware description languages, please check out my article To add two n-bit numbers, we add the digits of each bit position together from right to left. For example, consider the rightmost FA in Figure 1.

In such a generic parameters and ports.

It is not necessary to define a component to instantiate it: the In this case, there is no need to write twice the same module.