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AN INTRODUCTION TO VHDL

Designing Hardware with Software

by James Antonakos

Start ý Levels of Design ý The Interface ý The Body ý Full_Adder ý Half _Adder ý Identifiers, Data Types, and Operators ý Examples ý The Five-Input AND Gate ý The 2:4 Decoder ý Timing Examples ý Other Methods ý Sources and PDF

FULL_ADDER

As previously mentioned, more detail is added to the design as you proceed to the lower levels. Now that you have an idea of how the full adders are used inside the ripple adder, itýs time to work on the Full_Adder entity construction (see Listing 2). As usual, the entity interface indicates only the associated input and output signals. The implementation details are left for the body statements.

In Figure 5 you can see the internal structure of the full adder. Two half adders (HA) and an OR gate are required to implement a full adder. Three internal signals complete the wiring scheme. Compare the diagram in Figure 5 with the corresponding Full_Adder body statements.

Figure 5ýThe full adder implementation can be seen here.

The body of the Full_Adder entity can be seen in Listing 3. There are many similarities to the body statements of the ripple adder. However, notice the last statement before the end of the code snippet, where the CYO output is generated. This is called a signal assignment statement. Here only a single logic gate is needed to combine signals, so a simple Boolean expression can be used.

The logical operations available in VHDL are NOT, AND, NAND, OR, NOR, XOR, and XNOR (VHDL-93 only). All operations have the same precedence except NOT, which has the highest precedence. This means that parenthesis must be used to enforce a particular order of operations.

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