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

IDENTIFIERS, DATA TYPES, AND OPERATORS

All three items discussed in this section can be found in the 4-bit ripple adder design. Identifiers are names that are chosen for the various signals and entities in the design. Identifiers have the following properties:

ı any length, however, avoid long identifiers

ı not case sensitive, so upper or lowercase may be used (unless some other style rules are in effect)

ı legal symbols are AıZ, aız, 0ı9, and the underscore

ı the first symbol must be a letter

ı the last symbol can not be an underscore

The data types used in the ripple adder were bit and bit_vector. Several other data types are available in VHDL, as indicated in Table 1.

Data type	Range of values
Boolean	True, False
Bit	0, 1
Bit_vector	Array of bits
Character	Single ASCII symbol
String	Array of characters
Integer	Depends on software
Real	Depends on software
Natural	Zero to the maximum integer value
Positive	One to the maximum integer value
Time	Depends on software
Table 1ıThere are several VHDL data types that may be used in a design.

Several operators are also used in the ripple adder, such as <= (assignment), -- (comment), and three logical operators: AND, OR, and XOR. Additional operators are arithmetic (+, -, *, /, mod, rem, abs, **), relational (>, >=, <, <=, =, and /=), and shift (sla, sra, sll, srl, rol, and ror).

You may have noticed that the <= operator has two different meanings. In one case, it means assignment and in the other it means less-than-or-equal-to. The VHDL compiler determines which interpretation is valid based on the context of where it is used. VHDL allows for other operators, and even certain identifiers, to be overloaded as well.

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