This VHDL tutorial shows you how to write VHDL code for N-bit Ripple Carry Adder using for loop. This technique is requires less code than technique that uses one-bit full adder component instantiation.

A ripple carry adder is an digital adder that consist of N full adder cascaded in sequence. Each Full adder(FA)(see Full Adder VHDL design) has input bits a and b, previous carry input(cin) and produces sum(s) and carry output(cout).

The block diagram is shown below,

It is possible create single full adder entity and then create a N bit adder by using component instantiation. This way of implementing the full adder will make the code large. Instead the following VHDL code shows how a N-bit full adder can be constructed using for loop for each bit of inputs and carry in to produce sum and carry out for each bit.

The following is VHDL code for 8-bit Ripple Carry Adder,

**VHDL 8-bit Ripple Carry Adder Code**

library IEEE;

use IEEE.STD_LOGIC_1164.ALL;

entity carry_ripple_adder is

generic (N: integer:=8);

Port ( a : in STD_LOGIC_VECTOR(N-1 downto 0);

b : in STD_LOGIC_VECTOR(N-1 downto 0);

cin : in STD_LOGIC;

s : out STD_LOGIC_VECTOR(N-1 downto 0);

cout : out STD_LOGIC);

end carry_ripple_adder;

architecture Behavioral of carry_ripple_adder is

begin

process(a, b, cin)

variable carry: STD_LOGIC_VECTOR(N downto 0);

begin

carry(0) := cin;

for i in 0 to N-1 loop

s(i) <= a(i) xor b(i) xor carry(i);

carry(i+1) := (a(i) and b(i)) or (a(i) and carry(i)) or (b(i) and carry(i));

end loop;

cout <= carry(N);

end process;

end Behavioral;

**Testbench for 8-bit Ripple Carry Adder**

The following is the testbench for the above 8 bit ripple carry adder

LIBRARY ieee;

USE ieee.std_logic_1164.ALL;

USE ieee.numeric_std.ALL;

ENTITY testbench IS

END testbench;

ARCHITECTURE behavior OF testbench IS

COMPONENT carry_ripple_adder

generic (N: integer:=8);

Port (

a : in STD_LOGIC_VECTOR(N-1 downto 0);

b : in STD_LOGIC_VECTOR(N-1 downto 0);

cin : in STD_LOGIC;

s : out STD_LOGIC_VECTOR(N-1 downto 0);

cout : out STD_LOGIC

);

END COMPONENT;

signal test_input1, test_input2, test_s : std_logic_vector(7 downto 0):=(others => '0');

signal test_cin: std_logic:='0';

signal test_cout : std_logic:='0';

BEGIN

uut: carry_ripple_adder PORT MAP(

a => test_input1,

b => test_input2,

cin => test_cin,

s => test_s,

cout => test_cout

);

tb : PROCESS

BEGIN

test_input1 <= "00110001"; --1

test_input2 <= "00110010"; -- 2

wait for 4 ns;

test_input1 <= "00111001"; --9

test_input2 <= "00111000"; -- 8

wait for 4 ns;

wait;

END PROCESS tb;

END;

The digital waveform is shown below,

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