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VHDL Tutorial |
VHDL Modelling Concepts | |||||||||||||||
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Introduction Fundamental concepts Modelling concepts Elements of behaviour Elements of structure Analysis elaboration Lexical elements Identifiers Numbers Characters and strings Syntax descriptions Constants and variables Scalar type Integer types Floating point types Time type Enumeration types Character types Boolean type Bits type Standard logic Sequential statements Case statements Loop and exit statements Assertion statements Array types & array operations Architecture bodies Entity declarations Behavioral descriptions Wait statements Delta delays Process statements Conditional signal assignment Selected signal assigment Structural descriptions Library and library clauses Procedures Procedure parameters Signal parameters Default values Unconstrained array parameter Functions Package declarations and bodies Subprograms in package Use clauses Resolved signals and subtypes Resolved signals and ports Parameterizing behavior Parameterizing structure
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. VHDL Modeling Concepts
In this section, we look at the basic VHDL concepts for behavioral and structural mod- eling. This will provide a feel for VHDL and a basis from which to work in later chap- ters. As an example, we look at ways of describing a four-bit register, shown in Figure 2-1. Using VHDL terminology, we call the module reg4 a design entity, and the inputs and outputs are ports. Figure 2-2 shows a VHDL description of the interface to this entity. This is an example of an entity declaration. It introduces a name for the entity and lists the input and output ports, specifying that they carry bit values (‘0’ or ‘1’) into and out of the entity. From this we see that an entity declaration describes the external view of the entity.
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four outputs, q0, q1, q2 and q3.
entity reg4 is port ( d0, d1, d2, d3, en, clk : in bit; q0, q1, q2, q3 : out bit ); end entity reg4;
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