Full Adder:-

• Full adder is a combinational logic circuit which can add three bits and produces sum and carry as output. Full adder performs binary addition on input A, input B and carry input Cin and produces output sum S and output carry Cout. The block diagram and truth table of full adder is given below.

• Inputs	Outputs
  A	B	Cin	Cout	Sum
  0	0	0	0	0
  0	0	1	0	1
  0	1	0	0	1
  0	1	1	1	0
  1	0	0	0	1
  1	0	1	1	0
  1	1	0	1	0
  1	1	1	1	1

  A full adder can also be implemented with 3:8 decoder is as follows 

Operation:-

• Decoder is a circuit which converts binary input into decimal equivalent. That is when the input decoder is 000 then its decimal equivalent is 0 so the Y0 goes high while other remaining outputs are low. Similarly when input is 101 then its decimal equivalent is 0 so the Y0 goes high while other remaining outputs are low.
• From the truth table of Full Adder we see that sum is high when inputs are 001,010,100, 111. These inputs when given to a 3-8 decoder then their decimal equivalent is 1, 2, 4, 7 so the Y1 ,Y2, Y4,  Y7  are high on their corresponding inputs while other remaining outputs are low. These outputs Y1 ,Y2, Y4, Y7 are given to OR gate.When any of these inputs to OR gate is high then the sum will be high.
• From the truth table of Full Adder we see that carry is high when inputs are 011,101,110, 111. These inputs when given to a 3-8 decoder then their decimal equivalent is 3, 5, 6, 7 so the Y3 ,Y5, Y6, Y7</sub</li>