Verilog Coding Tips and Tricks: Verilog code for Carry select adder with Testbench

Friday, November 3, 2017

Verilog code for Carry select adder with Testbench

A carry select adder is a special way of implementing a binary adder. Its simple yet fast adder. The block diagram below shows how you can implement a carry select adder.

                                                                                             

The '+' blocks are full adders and mux's used are 2:1 size.

The Verilog codes are given below:

fulladder.v

module fulladder
        (   input a,b,cin,
            output sum,carry
            );

assign sum = a ^ b ^ cin;
assign carry = (& b) | (cin & b) | (& cin);

endmodule

multiplexer2.v

module multiplexer2
        (   input i0,i1,sel,
            output reg bitout
            );

always@(i0,i1,sel)
begin
if(sel == 0)
    bitout = i0;
else
    bitout = i1; 
end

endmodule

carry_select_adder.v

module carry_select_adder
        (   input [3:0] A,B,
            input cin,
            output [3:0] S,
            output cout
            );
        

wire [3:0] temp0,temp1,carry0,carry1;

//for carry 0
fulladder fa00(A[0],B[0],1'b0,temp0[0],carry0[0]);
fulladder fa01(A[1],B[1],carry0[0],temp0[1],carry0[1]);
fulladder fa02(A[2],B[2],carry0[1],temp0[2],carry0[2]);
fulladder fa03(A[3],B[3],carry0[2],temp0[3],carry0[3]);

//for carry 1
fulladder fa10(A[0],B[0],1'b1,temp1[0],carry1[0]);
fulladder fa11(A[1],B[1],carry1[0],temp1[1],carry1[1]);
fulladder fa12(A[2],B[2],carry1[1],temp1[2],carry1[2]);
fulladder fa13(A[3],B[3],carry1[2],temp1[3],carry1[3]);

//mux for carry
multiplexer2 mux_carry(carry0[3],carry1[3],cin,cout);
//mux's for sum
multiplexer2 mux_sum0(temp0[0],temp1[0],cin,S[0]);
multiplexer2 mux_sum1(temp0[1],temp1[1],cin,S[1]);
multiplexer2 mux_sum2(temp0[2],temp1[2],cin,S[2]);
multiplexer2 mux_sum3(temp0[3],temp1[3],cin,S[3]);

endmodule 

tb_adder.v  (testbench code)

module tb_adder;

    // Inputs
    reg [3:0] A;
    reg [3:0] B;
    reg cin;
    // Outputs
    wire [3:0] S;
    wire cout;  
    integer i,j,error;

    // Instantiate the Unit Under Test (UUT)
    carry_select_adder uut (
        .A(A), 
        .B(B), 
        .cin(cin), 
        .S(S), 
        .cout(cout)
    );

//Stimulus block - all the input combinations are tested here.
//the number of errors are recorded in the signal named "error".
    initial begin
        // Initialize Inputs
        A = 0;
        B = 0;
        error = 0;
        //for carry in =0
        cin = 0;
      for(i=0;i<16;i=i+1) begin
            for(j=0;j<16;j=j+1) begin
                 A = i;
                 B = j;
                 #10;
                 if({cout,S} != (i+j)) 
                      error <= error + 1;
            end  
      end
        //for carry in =1
      cin = 1; 
      for(i=0;i<16;i=i+1) begin
            for(j=0;j<16;j=j+1) begin
                 A = i;
                 B = j;
                 #10;
                 if({cout,S} != (i+j+1)) 
                      error <= error + 1;
            end  
      end 
   end
    
endmodule


Waveform after functional simulation:

The code was synthesised and simulated using Xilinx ISE 14.6. The waveform is too long to show on a single screenshot. So I have selected a relevant section to show here.


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