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8.3. Steps in VHDL design

The process of design may be divided into front-end and backend. Where:
 The Front-end section includes all the decision are made and the design is documented.
 The Back-end section includes the implementation and testing and the product.

Although this process is iterative by its nature, as the distance between the step that an error is
discovered and the step that the correction is made increases, the cost (time & resource) increases
exponentially.

 Program Structure

 VHDL was designed with principles of structured programming in mind and borrowed ideas from

Pascal and Ada Software Languages.

 VHDL Code has two parts (entity & architecture)

 Entity

A declaration of a module’s inputs and outputs. Entity is viewed as a wrapper for the

architecture, hiding what’s inside, while providing access for another module to use the

functionality.

 Architecture

A detailed description of the module’s internal structure or behavior.

 VHDL is hierarchical, meaning that a higher-level entity may use other entities while hiding lower

level entities from the higher level ones as shown by the following diagram:

Entity Declaration

Architecture

Definition

-- Eight-bit comparator

entity ent_compare is

port( A, B: in bit_vector(0 to 7);

EQ: out bit);

end ent_compare;

architecture arc_compare of ent_compare is

begin

EQ <= ‘1’ when (A = B) else ‘0’;

end arc_compare1;

hierarchy/

block diagram

(Step 1)

coding

(Step 2)

compilation

(Step 3)

simulation/

verification

(Step 4)

timing

verification

(Step 7)

fiting/Place+

route

(Step 6)

Synthesis

(Step 5)

Design

Requirements

Back-end

Steps

Front-end

Steps