carry_look_ahead_adder.v

module carry_look_ahead_16bit(a,b, cin, sum,cout);
input [15:0] a,b;
input cin;
output [15:0] sum;
output cout;
wire c1,c2,c3;
 
carry_look_ahead_4bit cla1 (.a(a[3:0]), .b(b[3:0]), .cin(cin), .sum(sum[3:0]), .cout(c1));
carry_look_ahead_4bit cla2 (.a(a[7:4]), .b(b[7:4]), .cin(c1), .sum(sum[7:4]), .cout(c2));
carry_look_ahead_4bit cla3(.a(a[11:8]), .b(b[11:8]), .cin(c2), .sum(sum[11:8]), .cout(c3));
carry_look_ahead_4bit cla4(.a(a[15:12]), .b(b[15:12]), .cin(c3), .sum(sum[15:12]), .cout(cout));
 
endmodule

module carry_look_ahead_4bit(a,b, cin, sum,cout);
input [3:0] a,b;
input cin;
output [3:0] sum;
output cout;
 
wire [3:0] p,g,c;
 
assign p=a^b;
assign g=a&b;

assign c[0]=cin;
assign c[1]= g[0]|(p[0]&c[0]);
assign c[2]= g[1] | (p[1]&g[0]) | p[1]&p[0]&c[0];
assign c[3]= g[2] | (p[2]&g[1]) | p[2]&p[1]&g[0] | p[2]&p[1]&p[0]&c[0];
assign cout= g[3] | (p[3]&g[2]) | p[3]&p[2]&g[1] | p[3]&p[2]&p[1]&g[0] | p[3]&p[2]&p[1]&p[0]&c[0];
assign sum=p^c;
 
endmodule

module carry_look_ahead_16bit_tb;
reg [15:0] a,b;
reg cin;
wire [15:0] sum;
wire cout;
 
  carry_look_ahead_16bit uut(.a(a), .b(b),.cin(cin),.sum(sum),.cout(cout));
 
initial begin
  a=0; b=0; cin=0;
  #20 a=16'd2; b=16'd2; cin=1'd1;
  #20 a=16'd2; b=16'd3; cin=1'd1;
  #20 a=16'd18; b=16'd18; cin=1'd0;
  #20 a=16'd100; 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