4 bit Ripple Carry Adder using Verilog

full_adder.v

module full_adder(in0, in1, cin, out, cout);
	input in0, in1, cin;
	output out, cout;

	assign out = in0 ^ in1 ^ cin;
	assign cout = ((in0 ^ in1) & cin) | (in0 & in1);
endmodule

ripple_carry_adder.v

module ripple_carry_adder(in0, in1, out, cout);
	input [3:0] in0;
	input [3:0] in1;
	output [3:0] out;
	output cout;

	wire c1, c2, c3;
	full_adder fa0(in0[0], in1[0], 0, out[0], c1);
	full_adder fa1(in0[1], in1[1], c1, out[1], c2);
	full_adder fa2(in0[2], in1[2], c2, out[2], c3);
	full_adder fa3(in0[3], in1[3], c3, out[3], cout);

endmodule

ripple_carry_adder_tb.v

module ripple_carry_adder_tb;

	reg [3:0] in0;
	reg [3:0] in1;
	wire [3:0] out;
	wire cout;

	ripple_carry_adder rca(.in0(in0), .in1(in1), .out(out), .cout(cout));

	initial begin
		in0 = 4'b0000; in1 = 4'b0000; #10
		in0 = 4'b0000; in1 = 4'b0001; #10
		in0 = 4'b0000; in1 = 4'b0010; #10
		in0 = 4'b0000; in1 = 4'b0011; #10
		in0 = 4'b0000; in1 = 4'b0100; #10
		in0 = 4'b0000; in1 = 4'b0101; #10
		in0 = 4'b0000; in1 = 4'b0110; #10
		in0 = 4'b0000; in1 = 4'b0111; #10
		in0 = 4'b0000; in1 = 4'b1000; #10
		in0 = 4'b0000; in1 = 4'b1001; #10
		in0 = 4'b0000; in1 = 4'b1010; #10
		in0 = 4'b0000; in1 = 4'b1011; #10
		in0 = 4'b0000; in1 = 4'b1100; #10
		in0 = 4'b0000; in1 = 4'b1101; #10
		in0 = 4'b0000; in1 = 4'b1110; #10
		in0 = 4'b0000; in1 = 4'b1111; #10
		in0 = 4'b0001; in1 = 4'b0000; #10
		in0 = 4'b0001; in1 = 4'b0001; #10
		in0 = 4'b0001; in1 = 4'b0010; #10
		in0 = 4'b0001; in1 = 4'b0011; #10
		in0 = 4'b0001; in1 = 4'b0100; #10
		in0 = 4'b0001; in1 = 4'b0101; #10
		in0 = 4'b0001; in1 = 4'b0110; #10
		in0 = 4'b0001; in1 = 4'b0111; #10
		in0 = 4'b0001; in1 = 4'b1000; #10
		in0 = 4'b0001; in1 = 4'b1001; #10
		in0 = 4'b0001; in1 = 4'b1010; #10
		in0 = 4'b0001; in1 = 4'b1011; #10
		in0 = 4'b0001; in1 = 4'b1100; #10
		in0 = 4'b0001; in1 = 4'b1101; #10
		in0 = 4'b0001; in1 = 4'b1110; #10
		in0 = 4'b0001; in1 = 4'b1111; #10
		in0 = 4'b0010; in1 = 4'b0000; #10
		in0 = 4'b0010; in1 = 4'b0001; #10
		in0 = 4'b0010; in1 = 4'b0010; #10
		in0 = 4'b0010; in1 = 4'b0011; #10
		in0 = 4'b0010; in1 = 4'b0100; #10
		in0 = 4'b0010; in1 = 4'b0101; #10
		in0 = 4'b0010; in1 = 4'b0110; #10
		in0 = 4'b0010; in1 = 4'b0111; #10
		in0 = 4'b0010; in1 = 4'b1000; #10
		in0 = 4'b0010; in1 = 4'b1001; #10
		in0 = 4'b0010; in1 = 4'b1010; #10
		in0 = 4'b0010; in1 = 4'b1011; #10
		in0 = 4'b0010; in1 = 4'b1100; #10
		in0 = 4'b0010; in1 = 4'b1101; #10
		in0 = 4'b0010; in1 = 4'b1110; #10
		in0 = 4'b0010; in1 = 4'b1111; #10
		in0 = 4'b0011; in1 = 4'b0000; #10
		in0 = 4'b0011; in1 = 4'b0001; #10
		in0 = 4'b0011; in1 = 4'b0010; #10
		in0 = 4'b0011; in1 = 4'b0011; #10
		in0 = 4'b0011; in1 = 4'b0100; #10
		in0 = 4'b0011; in1 = 4'b0101; #10
		in0 = 4'b0011; in1 = 4'b0110; #10
		in0 = 4'b0011; in1 = 4'b0111; #10
		in0 = 4'b0011; in1 = 4'b1000; #10
		in0 = 4'b0011; in1 = 4'b1001; #10
		in0 = 4'b0011; in1 = 4'b1010; #10
		in0 = 4'b0011; in1 = 4'b1011; #10
		in0 = 4'b0011; in1 = 4'b1100; #10
		in0 = 4'b0011; in1 = 4'b1101; #10
		in0 = 4'b0011; in1 = 4'b1110; #10
		in0 = 4'b0011; in1 = 4'b1111; #10
		in0 = 4'b0100; in1 = 4'b0000; #10
		in0 = 4'b0100; in1 = 4'b0001; #10
		in0 = 4'b0100; in1 = 4'b0010; #10
		in0 = 4'b0100; in1 = 4'b0011; #10
		in0 = 4'b0100; in1 = 4'b0100; #10
		in0 = 4'b0100; in1 = 4'b0101; #10
		in0 = 4'b0100; in1 = 4'b0110; #10
		in0 = 4'b0100; in1 = 4'b0111; #10
		in0 = 4'b0100; in1 = 4'b1000; #10
		in0 = 4'b0100; in1 = 4'b1001; #10
		in0 = 4'b0100; in1 = 4'b1010; #10
		in0 = 4'b0100; in1 = 4'b1011; #10
		in0 = 4'b0100; in1 = 4'b1100; #10
		in0 = 4'b0100; in1 = 4'b1101; #10
		in0 = 4'b0100; in1 = 4'b1110; #10
		in0 = 4'b0100; in1 = 4'b1111; #10
		in0 = 4'b0101; in1 = 4'b0000; #10
		in0 = 4'b0101; in1 = 4'b0001; #10
		in0 = 4'b0101; in1 = 4'b0010; #10
		in0 = 4'b0101; in1 = 4'b0011; #10
		in0 = 4'b0101; in1 = 4'b0100; #10
		in0 = 4'b0101; in1 = 4'b0101; #10
		in0 = 4'b0101; in1 = 4'b0110; #10
		in0 = 4'b0101; in1 = 4'b0111; #10
		in0 = 4'b0101; in1 = 4'b1000; #10
		in0 = 4'b0101; in1 = 4'b1001; #10
		in0 = 4'b0101; in1 = 4'b1010; #10
		in0 = 4'b0101; in1 = 4'b1011; #10
		in0 = 4'b0101; in1 = 4'b1100; #10
		in0 = 4'b0101; in1 = 4'b1101; #10
		in0 = 4'b0101; in1 = 4'b1110; #10
		in0 = 4'b0101; in1 = 4'b1111; #10
		in0 = 4'b0110; in1 = 4'b0000; #10
		in0 = 4'b0110; in1 = 4'b0001; #10
		in0 = 4'b0110; in1 = 4'b0010; #10
		in0 = 4'b0110; in1 = 4'b0011; #10
		in0 = 4'b0110; in1 = 4'b0100; #10
		in0 = 4'b0110; in1 = 4'b0101; #10
		in0 = 4'b0110; in1 = 4'b0110; #10
		in0 = 4'b0110; in1 = 4'b0111; #10
		in0 = 4'b0110; in1 = 4'b1000; #10
		in0 = 4'b0110; in1 = 4'b1001; #10
		in0 = 4'b0110; in1 = 4'b1010; #10
		in0 = 4'b0110; in1 = 4'b1011; #10
		in0 = 4'b0110; in1 = 4'b1100; #10
		in0 = 4'b0110; in1 = 4'b1101; #10
		in0 = 4'b0110; in1 = 4'b1110; #10
		in0 = 4'b0110; in1 = 4'b1111; #10
		in0 = 4'b0111; in1 = 4'b0000; #10
		in0 = 4'b0111; in1 = 4'b0001; #10
		in0 = 4'b0111; in1 = 4'b0010; #10
		in0 = 4'b0111; in1 = 4'b0011; #10
		in0 = 4'b0111; in1 = 4'b0100; #10
		in0 = 4'b0111; in1 = 4'b0101; #10
		in0 = 4'b0111; in1 = 4'b0110; #10
		in0 = 4'b0111; in1 = 4'b0111; #10
		in0 = 4'b0111; in1 = 4'b1000; #10
		in0 = 4'b0111; in1 = 4'b1001; #10
		in0 = 4'b0111; in1 = 4'b1010; #10
		in0 = 4'b0111; in1 = 4'b1011; #10
		in0 = 4'b0111; in1 = 4'b1100; #10
		in0 = 4'b0111; in1 = 4'b1101; #10
		in0 = 4'b0111; in1 = 4'b1110; #10
		in0 = 4'b0111; in1 = 4'b1111; #10
		in0 = 4'b1000; in1 = 4'b0000; #10
		in0 = 4'b1000; in1 = 4'b0001; #10
		in0 = 4'b1000; in1 = 4'b0010; #10
		in0 = 4'b1000; in1 = 4'b0011; #10
		in0 = 4'b1000; in1 = 4'b0100; #10
		in0 = 4'b1000; in1 = 4'b0101; #10
		in0 = 4'b1000; in1 = 4'b0110; #10
		in0 = 4'b1000; in1 = 4'b0111; #10
		in0 = 4'b1000; in1 = 4'b1000; #10
		in0 = 4'b1000; in1 = 4'b1001; #10
		in0 = 4'b1000; in1 = 4'b1010; #10
		in0 = 4'b1000; in1 = 4'b1011; #10
		in0 = 4'b1000; in1 = 4'b1100; #10
		in0 = 4'b1000; in1 = 4'b1101; #10
		in0 = 4'b1000; in1 = 4'b1110; #10
		in0 = 4'b1000; in1 = 4'b1111; #10
		in0 = 4'b1001; in1 = 4'b0000; #10
		in0 = 4'b1001; in1 = 4'b0001; #10
		in0 = 4'b1001; in1 = 4'b0010; #10
		in0 = 4'b1001; in1 = 4'b0011; #10
		in0 = 4'b1001; in1 = 4'b0100; #10
		in0 = 4'b1001; in1 = 4'b0101; #10
		in0 = 4'b1001; in1 = 4'b0110; #10
		in0 = 4'b1001; in1 = 4'b0111; #10
		in0 = 4'b1001; in1 = 4'b1000; #10
		in0 = 4'b1001; in1 = 4'b1001; #10
		in0 = 4'b1001; in1 = 4'b1010; #10
		in0 = 4'b1001; in1 = 4'b1011; #10
		in0 = 4'b1001; in1 = 4'b1100; #10
		in0 = 4'b1001; in1 = 4'b1101; #10
		in0 = 4'b1001; in1 = 4'b1110; #10
		in0 = 4'b1001; in1 = 4'b1111; #10
		in0 = 4'b1010; in1 = 4'b0000; #10
		in0 = 4'b1010; in1 = 4'b0001; #10
		in0 = 4'b1010; in1 = 4'b0010; #10
		in0 = 4'b1010; in1 = 4'b0011; #10
		in0 = 4'b1010; in1 = 4'b0100; #10
		in0 = 4'b1010; in1 = 4'b0101; #10
		in0 = 4'b1010; in1 = 4'b0110; #10
		in0 = 4'b1010; in1 = 4'b0111; #10
		in0 = 4'b1010; in1 = 4'b1000; #10
		in0 = 4'b1010; in1 = 4'b1001; #10
		in0 = 4'b1010; in1 = 4'b1010; #10
		in0 = 4'b1010; in1 = 4'b1011; #10
		in0 = 4'b1010; in1 = 4'b1100; #10
		in0 = 4'b1010; in1 = 4'b1101; #10
		in0 = 4'b1010; in1 = 4'b1110; #10
		in0 = 4'b1010; in1 = 4'b1111; #10
		in0 = 4'b1011; in1 = 4'b0000; #10
		in0 = 4'b1011; in1 = 4'b0001; #10
		in0 = 4'b1011; in1 = 4'b0010; #10
		in0 = 4'b1011; in1 = 4'b0011; #10
		in0 = 4'b1011; in1 = 4'b0100; #10
		in0 = 4'b1011; in1 = 4'b0101; #10
		in0 = 4'b1011; in1 = 4'b0110; #10
		in0 = 4'b1011; in1 = 4'b0111; #10
		in0 = 4'b1011; in1 = 4'b1000; #10
		in0 = 4'b1011; in1 = 4'b1001; #10
		in0 = 4'b1011; in1 = 4'b1010; #10
		in0 = 4'b1011; in1 = 4'b1011; #10
		in0 = 4'b1011; in1 = 4'b1100; #10
		in0 = 4'b1011; in1 = 4'b1101; #10
		in0 = 4'b1011; in1 = 4'b1110; #10
		in0 = 4'b1011; in1 = 4'b1111; #10
		in0 = 4'b1100; in1 = 4'b0000; #10
		in0 = 4'b1100; in1 = 4'b0001; #10
		in0 = 4'b1100; in1 = 4'b0010; #10
		in0 = 4'b1100; in1 = 4'b0011; #10
		in0 = 4'b1100; in1 = 4'b0100; #10
		in0 = 4'b1100; in1 = 4'b0101; #10
		in0 = 4'b1100; in1 = 4'b0110; #10
		in0 = 4'b1100; in1 = 4'b0111; #10
		in0 = 4'b1100; in1 = 4'b1000; #10
		in0 = 4'b1100; in1 = 4'b1001; #10
		in0 = 4'b1100; in1 = 4'b1010; #10
		in0 = 4'b1100; in1 = 4'b1011; #10
		in0 = 4'b1100; in1 = 4'b1100; #10
		in0 = 4'b1100; in1 = 4'b1101; #10
		in0 = 4'b1100; in1 = 4'b1110; #10
		in0 = 4'b1100; in1 = 4'b1111; #10
		in0 = 4'b1101; in1 = 4'b0000; #10
		in0 = 4'b1101; in1 = 4'b0001; #10
		in0 = 4'b1101; in1 = 4'b0010; #10
		in0 = 4'b1101; in1 = 4'b0011; #10
		in0 = 4'b1101; in1 = 4'b0100; #10
		in0 = 4'b1101; in1 = 4'b0101; #10
		in0 = 4'b1101; in1 = 4'b0110; #10
		in0 = 4'b1101; in1 = 4'b0111; #10
		in0 = 4'b1101; in1 = 4'b1000; #10
		in0 = 4'b1101; in1 = 4'b1001; #10
		in0 = 4'b1101; in1 = 4'b1010; #10
		in0 = 4'b1101; in1 = 4'b1011; #10
		in0 = 4'b1101; in1 = 4'b1100; #10
		in0 = 4'b1101; in1 = 4'b1101; #10
		in0 = 4'b1101; in1 = 4'b1110; #10
		in0 = 4'b1101; in1 = 4'b1111; #10
		in0 = 4'b1110; in1 = 4'b0000; #10
		in0 = 4'b1110; in1 = 4'b0001; #10
		in0 = 4'b1110; in1 = 4'b0010; #10
		in0 = 4'b1110; in1 = 4'b0011; #10
		in0 = 4'b1110; in1 = 4'b0100; #10
		in0 = 4'b1110; in1 = 4'b0101; #10
		in0 = 4'b1110; in1 = 4'b0110; #10
		in0 = 4'b1110; in1 = 4'b0111; #10
		in0 = 4'b1110; in1 = 4'b1000; #10
		in0 = 4'b1110; in1 = 4'b1001; #10
		in0 = 4'b1110; in1 = 4'b1010; #10
		in0 = 4'b1110; in1 = 4'b1011; #10
		in0 = 4'b1110; in1 = 4'b1100; #10
		in0 = 4'b1110; in1 = 4'b1101; #10
		in0 = 4'b1110; in1 = 4'b1110; #10
		in0 = 4'b1110; in1 = 4'b1111; #10
		in0 = 4'b1111; in1 = 4'b0000; #10
		in0 = 4'b1111; in1 = 4'b0001; #10
		in0 = 4'b1111; in1 = 4'b0010; #10
		in0 = 4'b1111; in1 = 4'b0011; #10
		in0 = 4'b1111; in1 = 4'b0100; #10
		in0 = 4'b1111; in1 = 4'b0101; #10
		in0 = 4'b1111; in1 = 4'b0110; #10
		in0 = 4'b1111; in1 = 4'b0111; #10
		in0 = 4'b1111; in1 = 4'b1000; #10
		in0 = 4'b1111; in1 = 4'b1001; #10
		in0 = 4'b1111; in1 = 4'b1010; #10
		in0 = 4'b1111; in1 = 4'b1011; #10
		in0 = 4'b1111; in1 = 4'b1100; #10
		in0 = 4'b1111; in1 = 4'b1101; #10
		in0 = 4'b1111; in1 = 4'b1110; #10
		in0 = 4'b1111; in1 = 4'b1111;
	end

	initial begin
		$dumpfile("ripple-carry-adder.vcd");
		$dumpvars(0, ripple_carry_adder_tb);
		$monitor($time, ": %b + %b = %b, %b", in0, in1, out, cout);
	end

endmodule

Cool
There is another method to implement more parametric RCA look at my implementation:

module RippleCarryAdder(a, b, cin, s, cout, N);
    parameter N;

    input [N-1:0] a;
    input [N-1:0] b;
    input cin;

    output [N-1:0] s;
    output cout;

    wire [N-1:0] carry;

    genvar i;
    generate
    for (i = 0; i < N; i = i + 1) begin
        if (i == 0)
            FullAdder f(a[0], b[0], cin, s[0], carry[0]);
        else
            FullAdder f(a[i], b[i], carry[i-1], s[i], carry[i]);
    end

    assign cout = carry[N-1];
    endgenerate
endmodule

Cool
There is another method to implement more parametric RCA look at my implementation:

module RippleCarryAdder(a, b, cin, s, cout, N);
    parameter N;

    input [N-1:0] a;
    input [N-1:0] b;
    input cin;

    output [N-1:0] s;
    output cout;

    wire [N-1:0] carry;

    genvar i;
    generate
    for (i = 0; i < N; i = i + 1) begin
        if (i == 0)
            FullAdder f(a[0], b[0], cin, s[0], carry[0]);
        else
            FullAdder f(a[i], b[i], carry[i-1], s[i], carry[i]);
    end

    assign cout = carry[N-1];
    endgenerate
endmodule

This one got some bug I think! The carry out is 1 for a=1, b=3, cin=0 [for 4 bit RCA] and this error took place in lots of other test cases

You can use for loop instead of writing all conditions

	initial begin

		A = 4'b0000;
		B = 4'b0000;
		count = 0;

		for (count = 0; count < 256; count = count + 1) begin
			#5 {A, B} = count;
		end
	end

	initial #1285 $finish;

Would be nice to change up the testbench in a more compact way. Nice example nevertheless.

module ripple_carry_adder_tb ();

reg [3:0] in0 = 4'b0, in1 = 4'b0;
wire [3:0] out;
wire cout;

ripple_carry_adder rca(.in0(in0), .in1(in1), .out(out), .cout(cout));

initial repeat(255) #10 {in0,in1} = {in0,in1} + 8'b1;

initial begin
    $dumpfile("ripple-carry-adder.vcd");
    $dumpvars(0, ripple_carry_adder_tb);
    $monitor($time, ": %b + %b = %b, %b", in0, in1, out, cout);
end

endmodule

Apologies for replying 5 years after the code was released, but an uni teacher of mine used this code as an example of a solution and I couldn't help but want to correct it.

Thanks for sharing your code by the way, its always helpful for beginners to see examples like this!