how to solve 4 bit full adder verilog

Question

I am supposed to create 4 bit full adder verilog code in vivado.But when I try to test in the simulation.It give me z and x output.Which part of code I have to change to get an output in simulation

module my_full_adder( input A,
       input B,
       input CIN,
       output S,
       output COUT
       );
       assign S = A^B^CIN;
       assign COUT = (A&B) | (CIN&(A^B));

endmodule

This is the one bit full adder verilog code

I have check the schematic for this code and everything is correct.

module four_bit_adder( 
    input [3:0] A,
    input [3:0] B,
    input C0,
    output [3:0] S,
    output C4
    );
    wire C1,C2,C3;

    my_full_adder fa0 (A[0],B[0],C0,S[0],C1);
    my_full_adder fa1 (A[1],B[1],C1,S[1],C2);
    my_full_adder fa2 (A[2],B[2],C2,S[2],C3);
    my_full_adder fa3 (A[3],B[3],C3,S[3],C4);


endmodule

Test bench

module test_4_bit(

    );
     reg [3:0] A;
     reg [3:0] B;
     reg C0;
     wire [3:0] S;
     wire C4;
     four_bit_adder dut(A,B,C0,S,C4);    
     initial begin
     A = 4'b0011;B=4'b0011;C0 = 1'b0; #10;
     A = 4'b1011;B=4'b0111;C0 = 1'b1; #10;
     A = 4'b1111;B=4'b1111;C0 = 1'b1; #10;
     end

endmodule

Answer 1

enter code here
    module fulladder(
        input [3:0] a,
        input [3:0] b,
        input cin,
        output [3:0] sum,
        output cout
        );
    /*assign cin=0;*/
    wire c1,c2,c3;
        adder fa0 (a[0],b[0],cin,sum[0],c1);
        adder fa1 (a[1],b[1],c1,sum[1],c2);
        adder fa2 (a[2],b[2],c2,sum[2],c3);
        adder fa3 (a[3],b[3],c3,sum[3],cout);
    
    endmodule
    
    
    first design full adder to implemet the above 
    module adder(
        input [3:0] a,
        input [3:0] b,
        input cin,
        output [3:0] sum,
        output cout
        );
    assign sum = a ^ b ^ cin;
    
    assign cout = (a&b) | (cin&(a^b));
    
    
    endmodule


ucf file
NET "a[0]" LOC = "A10";
NET "a[1]" LOC = "D14";
NET "a[2]" LOC = "C14";
NET "a[3]" LOC = "P15";
NET "b[0]" LOC = "P12";
NET "b[1]" LOC = "R5";
NET "b[2]" LOC = "T5";
NET "b[3]" LOC = "E4";
NET "sum[0]" LOC = "U18";
NET "sum[1]" LOC = "M14";
NET "sum[2]" LOC = "N14";
NET "sum[3]" LOC = "L14";
NET "cout" LOC = "M13";

The above ucf file is for Spartan 6 board

Answer 2

I don't have any idea about your testbench code but I think you have a mistake in your main code.Please try again with this code:

module fourbit_fulladder(Sum ,Cout , Cin , X ,Y);

output [3:0]Sum;
output  Cout;
input  Cin;
input  [3:0]X,Y;
wire C1,C2,C3;

FullAdder_m FA1(Sum[0],C1,X[0],Y[0],Cin);
FullAdder_m FA2(Sum[1],C2,X[1],Y[1],C1);
FullAdder_m FA3(.S(Sum[2]),.Cout(C3),.A(X[2]),.B(Y[2]),.Cin(C2));
FullAdder_m FA4(.S(Sum[3]),.Cout(Cout),.A(X[3]),.B(Y[3]),.Cin(C3));

endmodule