Experiment 12 - The Full Adder

How can two 4-bit numbers be added in a digital circuit? A complete truth table would be too great. The solution is to find first a partial solution for each position of the binary number.

Consider the second digit of the binary number. First the inputs A and B are added. Furthermore the transfered information from the first digit, the Carry Bit, has to be added. Hence the circuit gets a third input, the Carry-IN CI. Such a logical unit is called a full adder.

CI A B Q CO 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 Truth Table Full Adder

Circuit Diagram Full Adder.

There are many ways to build a full adder using logic gates. The circuit used here consists basically of two half adders. As a result the logic can be traced through the circuit more easily.

Circuit Full Adder. (Enlarge)

Now back to the problem of two 4-bit numbers to be added.