Digital logic circuits are classified into two major categories. They are, (a) Combinational Circuits and (b) Sequential Circuits. In practice, both combinational and sequential circuits are together used in a variety of applications. However, there exist contrasting differences between the two and it is presented here.
A combinational circuit is a logic circuit whose output at any instant depends upon the present combination of inputs.
A sequential circuit is a logic circuit whose output at any instant depends upon the present inputs and also the past inputs and outputs.
A combinational circuit consists of inputs, logic gates, and outputs.
Block Diagram of Combinational Circuit
A sequential circuit can be considered a combinational circuit with storage elements.
Block Diagram of Sequential Circuit
There is no need to store previous states and outputs.
There is a need for storage of past outputs. So, sequential circuits employ storage elements called memory.
There is no feedback in combinational circuits.
The past outputs are stored in memory and fed back to the logic circuit to determine the present output.
The combinational circuits are independent of clock signals.
Sequential circuits are classified into two types based on the timing of feedback signals. They are, Synchronous sequential circuits that use clock pulses and Asynchronous sequential circuits that do not use clock pulses. Also read: Synchronous Sequential Circuit vs Asynchronous Sequential Circuit
The combinational circuit needs no triggering.
Synchronous sequential circuits need to be triggered for their operation.
The behavior of the combinational circuit can be defined by either a Boolean expression or a Truth table.
The behavior of the sequential circuit can be defined using the State equation (also known as the Transition equation) or the State table.
Example of Combinational Circuit: Half Adder.
Example of a Combinational Circuit
Example of Synchronous Sequential circuit: Sequential circuit using D flip flop.
Example of Sequential Circuit
The application of combinational logic circuits are adders, subtractors, comparators, multiplexers, demultiplexers, encoders, decoders, and code converters.
The application of sequential circuits is shift registers, flip flops, counters, memory devices, etc.