1 Half Adder
Half adder is a circuit that will add two bits & produce a sum & a carry bit. It needs two input bits & two output bits.Fig.4.1 shows the block diagram of a half adder.
Ex-OR gate will only produce an output "1" when "EITHER" input is at logic "1", so we need an additional output to produce a carry output, "1" when "BOTH" inputs "A" and "B" are at logic "1" and a standard AND Gate fits the bill nicely. By combining the Ex-OR gate with the AND gate results in a simple digital binary adder circuit known commonly as the "Half Adder" circuit.
2 Full Adder
A half adder has only two inputs &there is no provision to add a carry coming from the lower order bits when multi addition is performed. For this purpose, a full adder is designed.
The 1-bit Full Adder circuit is basically two half adders connected together and consists of three Ex-OR gates, two AND gates and an OR gate, six logic gates in total. The truth table for the full adder includes an additional column to take into account the Carry-in input as well as the summed output and carry-output.
Figure: Logic diagram of a Full adder using two Half Adders
Table: Truth Table for Full Adder