Verilog Coding Tips and Tricks: Verilog code for Carry Save Adder with Testbench

Friday, November 3, 2017

Verilog code for Carry Save Adder with Testbench

Carry save adder is very useful when you have to add more than two numbers at a time. Normally if you have three numbers, the method would be to add the first two numbers together and then add the result to the third one. This causes so much delay.

In this post I have implemented a 4 bit carry save adder which adds three numbers at a time. You might want to see page 1 and 2 of this paper to get a better understanding of how this exactly works.


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

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



module CSA
        (   input [3:0] x,y,z,
            output [4:0] s,
            output cout
wire [3:0] c1,s1,c2;

fulladder fa_inst10(x[0],y[0],z[0],s1[0],c1[0]);
fulladder fa_inst11(x[1],y[1],z[1],s1[1],c1[1]);
fulladder fa_inst12(x[2],y[2],z[2],s1[2],c1[2]);
fulladder fa_inst13(x[3],y[3],z[3],s1[3],c1[3]); 

fulladder fa_inst20(s1[1],c1[0],1'b0,s[1],c2[1]);
fulladder fa_inst21(s1[2],c1[1],c2[1],s[2],c2[2]);
fulladder fa_inst22(s1[3],c1[2],c2[2],s[3],c2[3]);
fulladder fa_inst23(1'b0,c1[3],c2[3],s[4],cout); 

assign s[0] = s1[0];



module tb_adder;

    reg [3:0] x,y,z;
    wire [4:0] s;
    wire cout;  
    integer i,j,k,error;

    // Instantiate the Unit Under Test (UUT)
    CSA uut (x,y,z,s,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
        x = 0;
        y = 0;
        z = 0;
        error = 0;
        //three for loops to test all input combinations.
      for(i=0;i<16;i=i+1) begin
            for(j=0;j<16;j=j+1) begin
                for(k=0;k<16;k=k+1) begin
                     x = i;
                     y = j;
                     z = k;
                     if({cout,s} != (i+j+k)) 
                          error <= error + 1;

Simulation waveform:

The simulated waveform in Xilinx ISIM would look like this:

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