VHDL Code Structure( Library, Entity, Architecture)

Library Declaration:

A library is a collection of codes which can be used by several other functions.

Structure of library declaration is given below:

---

Library ieee;

Use ieee.std_logic_1164.all;

Use ieee.std_logic_arith.all;

Use ieee.std_logic_unsigned.all;

---

The above code can be used for all VHDL code.

Entity:

Entity defines all the input and output ports of the circuit.

Example:
Suppose a full adder circuit which has three inputs and two outputs. So ENTITY declaration of  fulladder can be written in the following way.

---

Entity full_adder is                            --full_adder is the name of the entity

Port( Ain: in  std_logic;            --Port Ain to Port Cout declaration

Bin: in std_logic;

Cin: in std_logic;

Sum: out std_logic;

Cout: out std_logic);

End full_adder;

---

Introduction to VHDL

VHDL stands for Very High Speed Integrated Circuit Hardware Description Language. It is a type of behavioral language from which the physical structure of any logical system can be implemented. VHDL language is basically invented to perform circuit synthesis as well as circuit simulation. It a vender independent language i.e. its language does not vary from vendor to vendor. The application fields of VHDL codes are PLD, ASIC, CPLD. Now a day microcontrollers are also designed using VHDL code.

  VHDL is not programmings as sequential execution of statements are not performed every time. Using VHDL we can design ADDER, ALU, Comparators, Decoders etc.

VHDL Code Structure:

VHDL code consists of three fundamental sections

                  (1) Library Declaration

                  (2) Entity

                  (3) Architecture