Develop a testbench for the Half Adder that verifies the structural model. The testbench will have no ports. Your testbench should exhaustively stimulate the circuit and print output demonstrating that the model is correct. Text output can be generated using the $monitor and $display tasks.

Answer and Explanation:

--        Here we define the AND gate that we need for

-- the Half Adder

library ieee;

use ieee.std_logic_1164.all;

entity andGate is        

  port( A, B : in std_logic;

           F : out std_logic);

end andGate;

architecture func of andGate is

begin

  F <= A and B;

end func;

--        Here we define the XOR gate that we need for

-- the Half Adder

library ieee;

use ieee.std_logic_1164.all;

entity xorGate is

  port( A, B : in std_logic;

           F : out std_logic);

end xorGate;

architecture func of xorGate is

begin

  F <= A xor B;

end func;

-- At this point we construct the half adder using

-- the AND and XOR gates

library ieee;

use ieee.std_logic_1164.all;

entity halfAdder is

  port( A, B : in std_logic;

   sum, Cout : out std_logic);

end halfAdder;

architecture halfAdder of halfAdder is

component andGate is -- import AND Gate

     port( A, B : in std_logic;

              F : out std_logic);

  end component;

component xorGate is -- import XOR Gate

    port( A, B : in std_logic;

             F : out std_logic);

  end component;

begin

G1 : xorGate port map(A, B, sum);

G2 : andGate port map(A, B, Cout);

end halfAdder;

END

END

Test Bench:

--import std_logic from the IEEE library

library ieee;

use ieee.std_logic_1164.all;

entity halfAdder_tb is

end halfAdder_tb;

architecture tb of halfAdder_tb is

component halfAdder is

    port( A, B : in std_logic;

      sum, Cout : out std_logic);

  end component;

signal A, B, sum, Cout: std_logic;

begin

  mapping: halfAdder port map(A, B, sum, Cout);

  process

  variable errCnt : integer := 0;

  begin

--TEST 1

  A <= '0';

    B <= '1';

    wait for 10 ns;

    assert(sum = '1') report "sum error 1" severity error;

    assert(Cout = '0') report "Cout error 1" severity error;

    if(sum /= '1' or Cout /= '0') then

       errCnt := errCnt + 1;

    end if;

--TEST 2

  A <= '1';

    B <= '1';

    wait for 10 ns;

    assert(sum = '0') report "sum error 2" severity error;

    assert(Cout = '1') report "Cout error 2" severity error;

    if(sum /= '0' or Cout /= '1') then

       errCnt := errCnt + 1;

    end if;

--TEST 3

  A <= '1';

    B <= '0';

    wait for 10 ns;

    assert(sum = '1') report "sum error 3" severity error;

    assert(Cout = '0') report "Cout error 3" severity error;

    if(sum /= '1' or Cout /= '0') then

        errCnt := errCnt + 1;

    end if;

SUMMARY

    if(errCnt = 0) then

      assert false report "Success!" severity note;

    else

       assert false report "Faillure!" severity note;

    end if;

end process;

end tb;

configuration cfg_tb of halfAdder_tb is

  for tb

  end for;

end cfg_tb;

END

END

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