April 7, 2014

# VHDL Code for Full Adder

Contents

### Full Adder

The VHDL Code for full-adder circuit adds three one-bit binary numbers (A B Cin) and outputs two one-bit binary numbers, a sum (S) and a carry (Cout). Truth Table describes the functionality of full adder. sum(S) output is High when odd number of inputs are High. Cout is High, when two or more inputs are High. VHDL Code for full adder can also be constructed with 2 half adder Port mapping in to full adder.

### Full Adder Truth Table

**Full Adder Logic Circuit**

### VHDL Code for Full Adder

library IEEE; use IEEE.STD_LOGIC_1164.ALL; entity full_adder_vhdl_code is Port ( A : in STD_LOGIC; B : in STD_LOGIC; Cin : in STD_LOGIC; S : out STD_LOGIC; Cout : out STD_LOGIC); end full_adder_vhdl_code; architecture gate_level of full_adder_vhdl_code is begin S <= A XOR B XOR Cin ; Cout <= (A AND B) OR (Cin AND A) OR (Cin AND B) ; end gate_level;

### Testbench VHDL Code for Full Adder

LIBRARY ieee; USE ieee.std_logic_1164.ALL; ENTITY Testbench_full_adder IS END Testbench_full_adder; ARCHITECTURE behavior OF Testbench_full_adder IS -- Component Declaration for the Unit Under Test (UUT) COMPONENT full_adder_vhdl_code PORT( A : IN std_logic; B : IN std_logic; Cin : IN std_logic; S : OUT std_logic; Cout : OUT std_logic ); END COMPONENT; --Inputs signal A : std_logic := '0'; signal B : std_logic := '0'; signal Cin : std_logic := '0'; --Outputs signal S : std_logic; signal Cout : std_logic; BEGIN -- Instantiate the Unit Under Test (UUT) uut: full_adder_vhdl_code PORT MAP ( A => A, B => B, Cin => Cin, S => S, Cout => Cout ); -- Stimulus process stim_proc: process begin -- hold reset state for 100 ns. wait for 100 ns; -- insert stimulus here A <= '1'; B <= '0'; Cin <= '0'; wait for 10 ns; A <= '0'; B <= '1'; Cin <= '0'; wait for 10 ns; A <= '1'; B <= '1'; Cin <= '0'; wait for 10 ns; A <= '0'; B <= '0'; Cin <= '1'; wait for 10 ns; A <= '1'; B <= '0'; Cin <= '1'; wait for 10 ns; A <= '0'; B <= '1'; Cin <= '1'; wait for 10 ns; A <= '1'; B <= '1'; Cin <= '1'; wait for 10 ns; end process; END;

### Output Waveform for full adder VHDL Code

6 Comments

Your truth table is wrong! Please correct for others.

Thanks john, truth table is corrected now.

The truth table is still incorrect. Here is the correct truth table:

http://hyperphysics.phy-astr.gsu.edu/hbase/Electronic/fulladd.html

Fixed Again Thanks ðŸ™‚

This s not behavioral modeling ,This is dataflow modeling.

Yes correct. The xilinx tool automatically generate architecture name by default as behavioral.