An Introduction to VHDL

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AN INTRODUCTION TO VHDL

Technically Speaking Designing Hardware with Software

by James Antonakos

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

EXAMPLES

Letıs take a look at several examples of applying VHDL to digital design. The first example involves a 2-bit greater-than comparator (see Figure 7). The interface for the comparator can be seen in Listing 6.

Figure 7ıIn this 2-bit greater-than comparator, the output is high if A > B.

entity GTCOMP is

port

(A,B : in bit_vector(1 downto 0);

AGTB : out bit);

end GTCOMP;

Listing 6ıThe interface for the comparator is shown here.

One way to implement the 2-bit greater-than comparator is to define several internal signals, write equations for the output of each AND gate, and a final equation for the OR gate. The design entity body for this method is shown in Listing 7.

Listing 7ıThis is one way to specify the comparator design.

Compare this implementation style with another design entity body that does the same thing (see Listing 8). It makes you wonder if thereıs any advantage to the first architecture body, doesnıt it?

Listing 8ıThis is a second way of specifying the comparator design.

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