Full Adder

A full adder is an arithmetic circuit which adds three bits: A, B and Cin. The circuit outputs are the sum S and carry Cout. The difference between a half adder and a full adder is that the half adder does not have the Cin input bit.

A full adder can be used as the basic building block for adding two N-bit numbers giving a (N+1)-bit result. Such a circuit would contain N full adders. If we number the bits of our numbers starting at 0 for the least significant bit, then the Cout of bit k becomes the Cin of bit k+1. The adder for bit number 0 can either be a half adder, or, a full adder with Cin tied to 0. Such a configuration is called a ripple carry adder.

The following truth table gives the specification:

A	B	Cin	S	Cout
0	0	0	0	0
0	0	1	1	0
0	1	0	1	0
0	1	1	0	1
1	0	0	1	0
1	0	1	0	1
1	1	0	0	1
1	1	1	1	1

The simplified equations for the full adder are:

• S = A xor B xor Cin
• C = (A and B) or (A and Cin) or (B and Cin)

In schematic form this is:

Below is a Verilog structural model for the full adder. In the code, the first argument to xor, and, and or is the gate output, the other arguments are gate inputs.

	    
module full_adder(S, Cout, A, B, Cin);
   output S;
   output Cout;
   input  A;
   input  B;
   input  Cin;
   
   wire   w1;
   wire   w2;
   wire   w3;
   wire   w4;
   
   xor(w1, A, B);
   xor(S, Cin, w1);
   and(w2, A, B);   
   and(w3, A, Cin);
   and(w4, B, Cin);   
   or(Cout, w2, w3, w4);
endmodule
	    
	  

References

Mano, M. Morris, and Kime, Charles R. Logic and Computer Design Fundamentals. 2nd Edition. Prentice Hall, 2000.
Kleitz, W. Digital Microprocessor Fundamentals. 3rd Edition. Prentice Hall, 2000.