Combinational circuit Vs Sequential circuits

Digital circuits are basically divided into two types, viz. combinational circuits and sequential circuits. Almost all the digital systems use both of these circuits. Both these classes of circuits have properties different from eachother while there are some similarities also. To design a digital system efficiently, one should understand these classes properly. It is compulsory for a digital design engineer to have the basic understanding and only then he will be able to design a complete system for a given specifications.

In this article I will explain these two categories of digital circuits in detail comparing their similarities and differences by giving relevant examples. Once you understand the advantages of each of these categories it will be easy to decide when to use each of them.

Combinational Circuit:

"Combinational circuit is a digital circuit whose output at any given time depends only on one or more inputs present at that particular time and nothing else."

The following are the properties of combinational circuits,

• It is represented by a truth table of output for all possible inputs.
• Normally the output function is represented in either SOP(sum of products) form or POS(product of sums) form.
• It is a time independent logic and hence we don't need a clock for the proper operation of a combinational circuit.
• There is no feedback from output to input.
• Combinational circuits form the DataPath of a digital system.
• Some of the combinational circuits are Multiplexres,Encoders,Decoders,Adder,Subtractor,code converters etc.
• The only memory that can be designed using combinational logic is ROM(Read Only Memory).
• Combinational circuits are designed by K-Map minimization of output function and are implemented using logic gates..

Sequential Circuit:

"Sequential circuit is a digital circuit whose output at any given time depends on inputs present at that particular time and previous outputs"

The following are the properties of sequential circuits,

• It is represented by a truth table of present output for all combinations of input and previous outputs.
• Normally the output function is represented in either SOP(sum of products) form or POS(product of sums) form.
• It is a time dependent logic and a clock input is required for identifying and differentiating the present and previous outputs.
• Sequential circuits need memory to store the previous outputs and even a memory is a sequential circuit.(May be a bit confusing but will be clear once we discuss sequential circuits.)
• Some examples of sequential circuits are Latches,Flipflops,Counters,Registers.
• Almost all the memories are made of sequential elements(flipflops).
• Sequential circuits form an integral part of the ControlPath of a digital system.
• Sequential circuits are designed using K-Map minimization of output function and FSM(Finite State Machine). Implementation of sequential circuits is done using logic gates.