Carry Save Adder Verilog Code | Verilog Implementation of Carry Save Adder

Carry Save Adder Verilog Code

Verilog Implementation of Carry Save Adder


In this we are going to share the Verilog code of carry save adder.  We have already shared Verilog code of Ripple Carry Adder, Carry Skip AdderCarry Look-ahead Adder etc.  

We have implemented 4 bit carry save adder in Verilog with 3 inputs A, B, C of 4-bits and one Carry Input D of 4bits.. The following diagram shows the block level implementation of carry save adder. The Verilog code of carry save adder is written as per the blocks.

Carry Save Adder Verilog Code

module carry_save_adder(a,b,c,d, sum,cout);
  input [3:0] a, b,c,d;
output [4:0] sum;
output cout;
wire [3:0] s0,s1;
  wire [3:0] c0, c1;

//1st Statge
full_adder fa0( .a(a[0]), .b(b[0]), .cin(c[0]), .sum(s0[0]), .cout(c0[0]));
full_adder fa1( .a(a[1]), .b(b[1]), .cin(c[1]), .sum(s0[1]), .cout(c0[1]));
full_adder fa2( .a(a[2]), .b(b[2]), .cin(c[2]), .sum(s0[2]), .cout(c0[2]));
full_adder fa3( .a(a[3]), .b(b[3]), .cin(c[3]), .sum(s0[3]), .cout(c0[3]));

//2nd Stage
full_adder fa4( .a(d[0]), .b(s0[0]), .cin(1'b0), .sum(sum[0]), .cout(c1[0]));
full_adder fa5( .a(d[1]), .b(s0[1]), .cin(c0[0]), .sum(s1[0]), .cout(c1[1]));
full_adder fa6( .a(d[2]), .b(s0[2]), .cin(c0[1]), .sum(s1[1]), .cout(c1[2]));
full_adder fa7( .a(d[3]), .b(s0[3]), .cin(c0[2]), .sum(s1[2]), .cout(c1[3]));

 ripple_carry_4_bit rca1 (.a(c1[3:0]),.b({c0[3],s1[2:0]}), .cin(1'b0),.sum(sum[4:1]), .cout(cout));


//4-bit Ripple Carry Adder
module ripple_carry_4_bit(a, b, cin, sum, cout);
input [3:0] a,b;
input cin;
wire c1,c2,c3;
output [3:0] sum;
output cout;
full_adder fa0(.a(a[0]), .b(b[0]),.cin(cin), .sum(sum[0]),.cout(c1));
full_adder fa1(.a(a[1]), .b(b[1]), .cin(c1), .sum(sum[1]),.cout(c2));
full_adder fa2(.a(a[2]), .b(b[2]), .cin(c2), .sum(sum[2]),.cout(c3));
full_adder fa3(.a(a[3]), .b(b[3]), .cin(c3), .sum(sum[3]),.cout(cout));

//1bit Full Adder
module full_adder(a,b,cin,sum, cout);
input a,b,cin;
output sum, cout;
wire x,y,z;
half_adder  h1(.a(a), .b(b), .sum(x), .cout(y));
half_adder  h2(.a(x), .b(cin), .sum(sum), .cout(z));
assign cout= y|z;
// 1 bit Half Adder
module half_adder( a,b, sum, cout );
input a,b;
output sum,  cout;
assign sum= a^b;
assign cout= a & b;


Carry Save Adder Test Bench Code

module carry_save_tb;
wire [4:0] sum;//output
wire cout;//output
reg [3:0] a,b,c,d;//input
 carry_save_adder uut(
initial begin
$display($time, " << Starting the Simulation >>");
     a=0; b=0; c=0; d=0;
  #100 a= 4'd10; b=4'd0; c=4'd0; d=4'd0;
  #100 a= 4'd10; b=4'd10; c=4'd0; d=4'd0;
  #100 a= 4'd4; b=4'd6; c=4'd12; d=4'd0;
  #100 a= 4'd11; b=4'd2; c=4'd4; d=4'd7;
  #100 a= 4'd20; b=4'd0; c=4'd20; d=4'd0;
  #100 a= 4'd12; b=4'd5; c=4'd10; d=4'd10;
  #100 a= 4'd7; b=4'd6; c=4'd12; d=4'd8;
  #100 a= 4'd15; b=4'd15; c=4'd15; d=4'd15;
  $monitor("A=%d, B=%d, C=%d,D=%d,Sum= %d, Cout=%d",a,b,c,d,sum,cout);

Carry Save Adder Simulation Result


If you have any query/suggestion please feel free to comment below the post.

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2 years ago

Hi, I want to ask, on the beginning you said that 4 bit carry save adder in Verilog with 3 inputs.
But on the code and the block diagram, it seems to be 4 inputs with 4 bit. Could you explain that part ?

And also how to do it if I want to expand it maybe into 8bit or 16 bit? Thanks