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

100

101

102

103

104

105

106

107

108

109

110

111

112

113

114

115

116

117

118

119

120

121

122

123

124

125

126

127

128

129

130

131

132

133

134

135

136

137

138

139

140

141

142

143

144

145

146

147

148

149

150

151

152

153

154

155

156

157

158

`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:

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: