VHDL code for full subtractor using behavioral method

In this post, we will take a look at implementing the VHDL code for full subtractor using behavioral method. We have seen the design for a full subtractor in our digital electronics course. Here, first, we will explain the logic and then the syntax. For the full code, scroll down.

Contents

Explanation of the VHDL code for full subtractor using behavioral method. How does the code work?

Logic circuit of the full subtractor

Truth table of the full subtractor

A	B	D	DIFFERENCE	BORROW
0	0	0	0	0
0	0	1	1	1
0	1	0	1	1
0	1	1	0	1
1	0	0	1	0
1	0	1	0	0
1	1	0	0	0
1	1	1	1	1

In the VHDL code for full adder post, we made the use of if-else commands. Here, we will make use of if-elsif commands.

VHDL code for full subtractor using behavioral method

library IEEE;

use IEEE.STD_LOGIC_1164.ALL;

use IEEE.STD_LOGIC_ARITH.ALL;

use IEEE.STD_LOGIC_UNSIGNED.ALL;


entity FULLSUBTRACTOR_BEHAVIORAL_SOURCE is

Port ( A : in  STD_LOGIC_VECTOR (2 downto 0);

Y : out  STD_LOGIC_VECTOR (1 downto 0));

end FULLSUBTRACTOR_BEHAVIORAL_SOURCE;


architecture Behavioral of FULLSUBTRACTOR_BEHAVIORAL_SOURCE is


begin


process (A)


begin


if (A = "001" or A = "010" or A = "111") then

Y <= "11";

elsif (A = "011") then

Y <= "01";

elsif (A = "100") then

Y <= "10";

else

Y <= "00";

end if;

end process;

end Behavioral;

Testbench

library IEEE;
use IEEE.STD_LOGIC_1164.ALL;
use IEEE.STD_LOGIC_ARITH.ALL;
use IEEE.STD_LOGIC_UNSIGNED.ALL;

entity full_sub_tb is
end entity;

architecture tb of full_sub_tb is
component FULLSUBTRACTOR_BEHAVIORAL_SOURCE is
Port ( A : in STD_LOGIC_VECTOR (2 downto 0);
Y : out STD_LOGIC_VECTOR (1 downto 0));
end component;

signal A: STD_LOGIC_VECTOR(2 downto 0);
signal Y: STD_LOGIC_VECTOR(1 downto 0);

begin

uut: FULLSUBTRACTOR_BEHAVIORAL_SOURCE port map(
A => A, Y => Y);

stim:process
begin

A <= "000";
wait for 20 ns;

A <= "001";
wait for 20 ns;

A <= "010";
wait for 20 ns;

A <= "011";
wait for 20 ns;

A <= "100";
wait for 20 ns;

A <= "101";
wait for 20 ns;

A <= "110";
wait for 20 ns;

A <= "111";
wait for 20 ns;
wait;

end process;

end tb;