Carry Select Adder Verilog Code

Verilog Implementation of Carry Select Adder

A carry select adder is a particular way to implement an adder, which is a logic element that computes the (n+1)-bit sum of two n-bit numbers. The carry select adder is simple but rather fast. For more information about carry select adder you can refer to this wikipedia article. Also, you can go through Ripple Carry Adder, Carry Skip Adder, Carry Look-ahead Adder etc. posts to get insight about different adders.

Basic structure of 4-bit Carry Select Adder is shown below.

carry select adder 4 bit

The Verilog Code for 16-bit Carry Select Adder is given below-

`timescale 1ns / 1ns

module carry_select_adder_16bit(a, b, cin, sum, cout);
input [15:0] a,b;
input cin;
output [15:0] sum;
output cout;

wire [2:0] c;

ripple_carry_4_bit rca1(
.a(a[3:0]),
.b(b[3:0]),
.cin(cin),
.sum(sum[3:0]),
.cout(c[0]));

// first 4-bit by ripple_carry_adder
carry_select_adder_4bit_slice csa_slice1(
.a(a[7:4]),
.b(b[7:4]),
.cin(c[0]),
.sum(sum[7:4]),
.cout(c[1]));

carry_select_adder_4bit_slice csa_slice2(
.a(a[11:8]),
.b(b[11:8]),
.cin(c[1]),
.sum(sum[11:8]), 
.cout(c[2]));

carry_select_adder_4bit_slice csa_slice3(
.a(a[15:12]),
.b(b[15:12]),
.cin(c[2]),
.sum(sum[15:12]),
.cout(cout));
endmodule

//////////////////////////////////////
//4-bit Carry Select Adder Slice
//////////////////////////////////////

module carry_select_adder_4bit_slice(a, b, cin, sum, cout);
input [3:0] a,b;
input cin;
output [3:0] sum;
output cout;

wire [3:0] s0,s1;
wire c0,c1;

ripple_carry_4_bit rca1(
.a(a[3:0]),
.b(b[3:0]),
.cin(1'b0),
.sum(s0[3:0]),
.cout(c0));

ripple_carry_4_bit rca2(
.a(a[3:0]),
.b(b[3:0]),
.cin(1'b1),
.sum(s1[3:0]),
.cout(c1));

mux2X1 #(4) ms0(
.in0(s0[3:0]),
.in1(s1[3:0]),
.sel(cin),
.out(sum[3:0]));

mux2X1 #(1) mc0(
.in0(c0),
.in1(c1),
.sel(cin),
.out(cout));
endmodule

/////////////////////
//2X1 Mux
/////////////////////

module mux2X1( in0,in1,sel,out);
parameter width=16; 
input [width-1:0] in0,in1;
input sel;
output [width-1:0] out;
assign out=(sel)?in1:in0;
endmodule


/////////////////////////////////
//4-bit Ripple Carry Adder
/////////////////////////////////
module ripple_carry_4_bit(a, b, cin, sum, cout);
input [3:0] a,b;
input cin;
output [3:0] sum;
output cout;

wire c1,c2,c3;

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));
endmodule

/////////////////////
//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));
or or_1(cout,z,y);
endmodule

//////////////////////
// 1 bit Half Adder
//////////////////////

module half_adder( a,b, sum, cout );
input a,b;
output sum, cout;
xor xor_1 (sum,a,b);
and and_1 (cout,a,b);
endmodule

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`timescale 1ns / 1ns

module carry_select_adder_16bit(a, b, cin, sum, cout);

input [15:0] a,b;

input cin;

output [15:0] sum;

output cout;

wire [2:0] c;

ripple_carry_4_bit rca1(

.a(a[3:0]),

.b(b[3:0]),

.cin(cin),

.sum(sum[3:0]),

.cout(c[0]));

// first 4-bit by ripple_carry_adder

carry_select_adder_4bit_slice csa_slice1(

.a(a[7:4]),

.b(b[7:4]),

.cin(c[0]),

.sum(sum[7:4]),

.cout(c[1]));

carry_select_adder_4bit_slice csa_slice2(

.a(a[11:8]),

.b(b[11:8]),

.cin(c[1]),

.sum(sum[11:8]),

.cout(c[2]));

carry_select_adder_4bit_slice csa_slice3(

.a(a[15:12]),

.b(b[15:12]),

.cin(c[2]),

.sum(sum[15:12]),

.cout(cout));

endmodule

//////////////////////////////////////

//4-bit Carry Select Adder Slice

//////////////////////////////////////

module carry_select_adder_4bit_slice(a, b, cin, sum, cout);

input [3:0] a,b;

input cin;

output [3:0] sum;

output cout;

wire [3:0] s0,s1;

wire c0,c1;

ripple_carry_4_bit rca1(

.a(a[3:0]),

.b(b[3:0]),

.cin(1'b0),

.sum(s0[3:0]),

.cout(c0));

ripple_carry_4_bit rca2(

.a(a[3:0]),

.b(b[3:0]),

.cin(1'b1),

.sum(s1[3:0]),

.cout(c1));

mux2X1 #(4) ms0(

.in0(s0[3:0]),

.in1(s1[3:0]),

.sel(cin),

.out(sum[3:0]));

mux2X1 #(1) mc0(

.in0(c0),

.in1(c1),

.sel(cin),

.out(cout));

endmodule

/////////////////////

//2X1 Mux

/////////////////////

module mux2X1( in0,in1,sel,out);

parameter width=16;

input [width-1:0] in0,in1;

input sel;

output [width-1:0] out;

assign out=(sel)?in1:in0;

endmodule

/////////////////////////////////

//4-bit Ripple Carry Adder

/////////////////////////////////

module ripple_carry_4_bit(a, b, cin, sum, cout);

input [3:0] a,b;

input cin;

output [3:0] sum;

output cout;

wire c1,c2,c3;

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));

endmodule

/////////////////////

//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));

or or_1(cout,z,y);

endmodule

//////////////////////

// 1 bit Half Adder

//////////////////////

module half_adder( a,b, sum, cout );

input a,b;

output sum, cout;

xor xor_1 (sum,a,b);

and and_1 (cout,a,b);

endmodule

Carry Select Adder Simulation Result is as follows:

16 bit Carry Select Adder Testbench

////Testbench
module carry_select_adder_16bit_tb;
reg [15:0] a,b;
reg cin;
wire [15:0] sum;
wire cout;

  carry_select_adder_16bit uut(.a(a), .b(b),.cin(cin),.sum(sum),.cout(cout));

initial begin
  a=0; b=0; cin=0;
  #10 a=16'd0; b=16'd0; cin=1'd1;
  #10 a=16'd14; b=16'd1; cin=1'd1;
  #10 a=16'd5; b=16'd0; cin=1'd0;
  #10 a=16'd999; b=16'd0; cin=1'd1;
end

initial
  $monitor( "A=%d, B=%d, Cin= %d, Sum=%d, Cout=%d", a,b,cin,sum,cout);
endmodule

////Testbench

module carry_select_adder_16bit_tb;

reg [15:0] a,b;

reg cin;

wire [15:0] sum;

wire cout;

carry_select_adder_16bit uut(.a(a), .b(b),.cin(cin),.sum(sum),.cout(cout));

initial begin

a=0; b=0; cin=0;

#10 a=16'd0; b=16'd0; cin=1'd1;

#10 a=16'd14; b=16'd1; cin=1'd1;

#10 a=16'd5; b=16'd0; cin=1'd0;

#10 a=16'd999; b=16'd0; cin=1'd1;

end

initial

$monitor( "A=%d, B=%d, Cin= %d, Sum=%d, Cout=%d", a,b,cin,sum,cout);

endmodule

Cadence RTL compiler is used to synthesize the verilog code with Uofu standard library. We got the following schematic after mapping the hdl code to the standard library. This is basically the synthesized view.

16 bit Carry Select Adder Module: