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VHDL Tutorial – 4: design, simulate and verify all digital GATE (AND, OR, NOT, NAND, NOR, XOR & XNOR) in VHDL

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In previous tutorial VHDL tutorial  3, we have learned how to design, simulate, and verify any digital circuit in VHDL using Altera’s MAX+II VHDL simulator software.

(If you are not following this VHDL tutorial series one by one, you are requested to go through all previous tutorials of these series before going ahead in this tutorial)

In this tutorial,

  • We shall write a VHDL program to build all digital gates
  • Simulate the program to design digital circuit for all gates
  • Verify the output waveform of program (digital circuit) with truth table of these logic GATES

Let us start with the digital circuit for which we shall write a VHDL program

Digital Circuit

Here is the truth table for the above circuit.
Truth Table

Now we shall write a VHDL program, compile it, simulate it, and get the output in the form of the waveform. Finely, we shall verify those output waveforms with the given truth table.

(Please go through step by step procedure given in previous tutorial (VHDL Tutorial 2) to create a project, edit and compile a program, create waveform file, simulate the program and generate output waveforms.)

VHDL Program
library IEEE;
use IEEE.STD_LOGIC_1164.ALL;

entity logic_gate is
     Port ( A,B : in std_logic;
               y_and,y_or,y_nand,y_nor,y_not,y_xor,y_xnor : out std_logic);
end logic_gate;

architecture all_gates of logic_gate is
begin
          y_and <= a and b;
          y_or <= a or b;
          y_nand <= a nand b;
          y_nor <= a nor b;
          y_not <= not a ;
          y_xor <= a xor b;
          y_xnor <= a xnor b;
end all_gates;

“entity” describes input-output connections of digital circuit. As per our circuit given above, we have only two inputs ‘A’ and ‘B’ and 7 outputs for 7 gates.

“architecture” describes the operation of the circuit – means how the output is generated from a given input.

(To know more and get more details about VHDL program(s), please go through the first two tutorials VHDL tutorial 1 and VHDL tutorial 2 of these series.)

Next, compile above program – create a waveform file with all inputs and outputs listed – simulate the project, and you will get the following result:
Simulation Waveform

Now verify these output waveforms with the truth table of GATEs. For example, here, one case is highlighted with inputs A=1 and B=1. You can verify the other three cases also.

This is how you can build a simple logic GATE circuit in VHDL and verify its output with their truth table.

In the next tutorial, we shall see how to build NAND, NOR, XOR, and XNOR gates using three basic gates AND, OR, and NOT using VHDL.

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