2.1 Fundamental VHDL Units

As depicted in figure 2.1, a standalone piece of VHDL code is composed of at least three fundamental sections:

• LIBRARY declarations: Contains a list of all libraries to be used in the design. For example: ieee, std, work, etc.

• ENTITY: Specifies the I/O pins of the circuit.

• ARCHITECTURE: Contains the VHDL code proper, which describes how the circuit should behave (function).

Figure 2.1: Fundamental sections of a basic VHDL code.

A LIBRARY is a collection of commonly used pieces of code. Placing such pieces inside a library allows them to be reused or shared by other designs.

The typical structure of a library is illustrated in figure 2.2. The code is usually written in the form of FUNCTIONS, PROCEDURES, or COMPONENTS, which are placed inside PACKAGES, and then compiled into the destination library.

Figure 2.2: Fundamental parts of a LIBRARY.

The fundamental units of VHDL (figure 2.1) will be studied in Part I of the book (up to chapter 9), whereas the library-related sections (figure 2.2) will be seen in Part II (chapters 10 12).

2.2 Library Declarations

To declare a LIBRARY (that is, to make it visible to the design) two lines of code are needed, one containing the name of the library, and the other a use clause, as shown in the syntax below.

LIBRARY library_name;USE library_name.package_name.package_parts;

At least three...