4 Bit Carry Look Ahead Adder

4-Bit Carry Look Ahead Adder:

A carry look-ahead adder reduces the propagation delay by introducing more complex hardware

//structural for four-bit carry lookahead adder using

//built-in primitives and conditional assignment

module cla_4bit (a, b, cin, sum, cout);

input [3:0] a, b; //define inputs and outputs

input cin;

output [3:0] sum;

output cout;

//design the logic for the generate functions

and (g0, a[0], b[0]),

(g1, a[1], b[1]),

(g2, a[2], b[2]),

(g3, a[3], b[3]);

//design the logic for the propagate functions

xor (p0, a[0], b[0]),

(p1, a[1], b[1]),

(p2, a[2], b[2]),

(p3, a[3], b[3]);

//design the logic for the sum equations

xor (sum[0], p0, cin),

(sum[1], p1, c0),

(sum[2], p2, c1),

(sum[3], p3, c2);//design the logic for the carry equations

//using the continuous assign statement

assign c0 = g0 | (p0 & cin),

c1 = g1 | (p1 & g0) | (p1 & p0 & cin),

c2 = g2 | (p2 & g1) | (p2 & p1 & g0)

 | (p2 & p1 & p0 & cin),

c3 = g3 | (p3 & g2) | (p3 & p2 & g1)

 | (p3 & p2 & p1 & g0)

 |(p3 & p2 & p1 & p0 & cin);

//design the logic for cout using assign

assign cout = c3;

endmodule

Test bench code:

//test bench for the four-bit carry lookahead adder

module cla_4bit_tb;

reg [3:0] a, b; //inputs are reg for test bench

reg cin;

wire [3:0] sum; //outputs are wire for test bench

wire cout;

cla_4bit dut(a, b, cin, sum, cout);

//define input sequence

initial

begin

#0 a = 4'b0000; b = 4'b0000; cin = 1'b0;

#10 a = 4'b0001; b = 4'b0010; cin = 1'b0;

#10 a = 4'b0010; b = 4'b0110; cin = 1'b0;

#10 a = 4'b0111; b = 4'b0111; cin = 1'b0;

#10 a = 4'b1001; b = 4'b0110; cin = 1'b0;

#10 a = 4'b1100; b = 4'b1100; cin = 1'b0;

#10 a = 4'b1111; b = 4'b1110; cin = 1'b0;

#10 a = 4'b1110; b = 4'b1110; cin = 1'b1;

#10 a = 4'b1111; b = 4'b1111; cin = 1'b1;

#10 a = 4'b1010; b = 4'b1010; cin = 1'b1;

#10 a = 4'b1000; b = 4'b1000; cin = 1'b0;

#10 a = 4'b1101; b = 4'b1000; cin = 1'b1;

end

initial

begin

//display variables

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

#100 $finish;

end 

endmodule

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