Combinational circuits can be defined as a circuit whose output is dependent only on the combination of the inputs at the same instant of time.
In combinational circuits past values of inputs or outputs is not required in the present output that is why combinational circuits have no memory, timing or feedback loops. In combinational circuits any changes to the signals being applied to their inputs have direct effects at the output.
Applications of combinational circuits are –
- Perform Arithmetic, logic, shift operations.
- Perform data transfer functions (like multiplexing, demultiplexing etc.)
- Perform Conversions (like gray to binary, Excess-3 to gray etc.)
A half adder is a logical circuits that performs an additions operation on two binary digits. It produces two outputs, S(sum) and c(carry). So you can say that half adder is a combinational circuits that takes two inputs and produces two outputs.
A full adder is a combinational circuits that performs an addations an addition on three binary digits. Full adder consists three data inputs and two outputs (sum and carry).
4-bit binary incrementer
A 4-bit increamenter adds a single bit 1 to 4-bits number say A3, A2, A1, A0 Increment microoperation is implemented with the help of combinational circuits half adder. The diagram of 4-bit combinational circuit incrementer is shown in fig.3.1.