VHDL-Behavioural Modelling-Structure of VHDL-Syntax and example programs

1. Review:
 Structure of VHDL –explanation & Syntax
 Behavioral Modelling-with examples
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2. Structure of VHDL module
 The main components of a VHDL description consists of
following kinds of declarations:
 Package
 Entity
 Architecture
 Configuration
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3. Entity Declaration
 It gives the specification of input/output signals to external
circuitry.
 An entity is modeled using an entity declaration and at least
one architecture body.
 Entity gives interfacing between device and the other
peripherals.
Syntax:
entity entity_name is
port( signal_names: mode signal_type;
signal_names: mode signal_type;
.
.
.
signal_names: mode signal_type);
end entity_name;
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4. Architecture
 It specifies the behavior, functionality, interconnections or
relationship between inputs and outputs.
 It is the actual description of the design.
 An architecture consists of two portions: architecture
declaration and architecture body
 An architecture body specifies the internal details of the
entity.
 set of concurrent assignment statements(to represent dataflow)
 set of interconnected components(to represent structure)
 set of sequential assignment statements(to represent behavior)
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5. Contd…
Syntax:
Architecture architecture_name of entity_name is
Declarations
begin
concurrent statements;
sequential statements;
end architecture_name;
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6. Configuration declaration
 It may be used to associate particular design entities to
component instances in a hierarchical design, or to
associate a particular architecture to an entity.
Package declaration
 It is a convenient mechanism to store and share the
declarations that are common across many design units
 A package is represented by: Package declaration and
Package body
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7. Contd…
Syntax for package declaration:
PACKAGE package_name IS
Type declarations
Subtype declarations
Constant declarations
Signal declarations
Use clauses
END package_name;
Syntax for package body:
Package body package_name is
Subprogram bodies
Subprogram declarations
Type and subtype declarations
File and alias declarations
Use clauses
End package_name;
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8. Behavioral Modelling
 It is method of directly describe the behavior or the
functionality of a circuit.
 In VHDL, the behavior of the entity is expressed
using sequentially executed, procedural code.
 The key mechanism used to model the behavior of the
entity is a process statement.
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9. Examples of Behavioral modelling
 Write the VHDL code half adder in behavioral model.
entity half_add is
Port(A,B: in bit;
Sum,cout:out bit);
end half_add;
architecture adder of half_add is
begin
sum<= A XOR B;
cout<= A and B;
end adder;
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10. Contd…
 Write the VHDL code full adder in behavioral model.
entity full_add is
Port(A,B,Cin: in bit;
Sum,cout:out bit);
end full_add;
architecture adder of full_add is
begin
Process(A,B,Cin)
begin
sum<= A xor B xor Cin;
cout<= (A and B) or (Cin and A) or (Cin and B) ;
end process;
end adder;
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11. Contd…
 Write the VHDL code for a logical gate which gives
high output only when both the inputs are high.
Library ieee;
Use ieee.std_logic_1164.all;
Entity AND_G is
Port(A,B :in bit;
Y :out bit);
End AND_G;
Architecture AND of AND_G is
Begin
Q<=A and B;
End AND;
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12. Contd…
 Write VHDL behavioral model for D flip flop.
Library ieee;
Use ieee.std_logic_1164.all;
Entity DFF is
Port(D,Clock :in std_logic;
Q :out std_logic);
End DFF;
Architecture behavior of DFF is
Begin
If Clock’event and clock=’1’ then
Q<=D;
End if;
End process;
End behavior;
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13. Conclusion & References
 Through this presentation we have learned about the basic structure of
VHDL with the syntax of all its components
 Also the behavioral model of VHDL is discussed with examples.
References:
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1. Mandal, “Digital Electronics Principles & Application, McGraw Hill Edu, 2013.
2. William Keitz, Digital Electronics-A Practical Approach with VHDL, Pearson,
2013.
3. Thomas L.Floyd, ‘Digital Fundamentals’, 11th edition, Pearson Education, 2015.
4. Charles H.Roth, Jr, Lizy Lizy Kurian John, ‘Digital System Design using VHDL,
Cengage,2013.
5. D.P.Kothari,J.S.Dhillon, ‘Digital circuits and Design’,Pearson Education, 2016.
6. A.P.Godse., Dr.D.A.Godse, ‘Digital Logic Circuits’ , Technical Publications Third
Edition 2016
7. Other Web Sources