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Difference Between Combinational and Sequential Circuits

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Difference Between Combinational and Sequential Circuits
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When it comes to understanding the essential concepts of digital electronics, then combinational and Sequential circuits are the two concepts that have to be understood in entirety. In this article, we aim to explain what is combinational circuit, what is sequential logic circuit, the difference between combinational and sequential circuits, examples of combinational and sequential circuits and more. Read on to get a better understanding of the difference between sequential and combinational circuit in the form of a comparative chart before anything else.

Combinational and sequential circuits Comparison Chart

The main differences between difference between sequential and combinational circuit can be tabulated as below:

Basis of difference

Combinational Logic Circuits

Sequential Logic Circuits

Output

The output is in the form of functions of the current inputs or time independent logic.

The output is in the form of functions related to clock, current inputs as well as the previous states of a given system.

Storage of data and memory space allocation

Combinational circuits are not capable of storing data (state). They do not contain any memory element.

Sequential circuits have the memory space needed for storing the current states sent as control inputs (enable) for the next set of operations. They contain memory elements that store data in digital circuits.

Requirement of feedback

They do not need any feedback. Combinational circuits generally output the input as per the logic designed.

These circuits need feedback from the output to save inputs in the memory space for the next set of operations.

Main usage

Mainly used or the purpose of Boolean and Arithmetic operations. The circuits are represented with the help of Boolean algebra; they are further simplified with the usage of basic and universal logic gates.

Useful for data storage and therefore used in RAM.

Elementary building blocks

The elementary building blocks of combinational circuits are logic gates.

The elementary building units of sequential logic circuits are Flip flops (binary storage devices).

Dependency on clocks

Combinational circuits are independent of clock, therefore triggering is not important for their operations.

Sequential logic circuits are clocked and so they are triggered for operations with the help of electronic pulses. The flip-flops, if triggered, are termed as synchronous sequential circuits. The circuits that are not triggered are termed as asynchronous sequential circuits.

Example of circuits

Adder [1+0=1; the dependency is only on current inputs i.e., 1 and 0].

Counter [Previous O/P +1=Current O/P; the dependency is on current inputs and the previous state of inputs.]

Application

These circuits are used in association with encoders, adders, decoders, subtractors, multiplexers, etc.

Sequential circuits are used in case of flip-flops and latches. In these circuits, latches are considered to be the simplest elements used for retaining the earlier state. The latches are also known as flip-flops.

Definition

Combinational logic circuit is a digital logic circuit wherein the output is capable of being determined with the help of logic functions related to the current state inputs.

Sequential logic circuit is a digital logic circuit wherein the output is capable of being determined with the usage of the logic function related to current state inputs as well as the previous state inputs.

Storage of state within the circuit

Combinational circuits are not capable of storing any state within them.

Sequential circuit are capable of retaining their earlier state on the basis of current inputs as well as the earlier states.

What is Combinational Logic Circuit?

Combinational logic circuits, sometimes referred to as time-independent logic, are a linked set of different gates that generate output that’s specific to current inputs belonging to that instant. The fundamental building units of combinational circuits are AND, OR and NOT as well as the universal gates NAND and NOR. In most cases of combinational logic circuit diagrams, the output lines are found to immediately follow the current lines. These circuits are implemented via Boolean circuits wherein the output happens to be a pure function of current inputs only. A decoder is a typical example of a combinational circuit; it is used for transforming binary code data into decimal code data.

Difference Between Combinational and Sequential Circuits

In case of combinational logic circuits, the generated output at any given time depends on the current inputs at any specific time. There are three types of combinational logic circuits – data transmission, arithmetic and logical functions and code converters. The data transmission circuit examples are multiplexers, encoders, demultiplexers, decoders, etc. Adders, comparators, PLDs, subtractors, etc. are instances of arithmetic and logic circuits. The examples of code converter circuits are BCD, 7 segments, etc. In general, combination circuits contain m number of binary outputs and n number of binary inputs. They are useful for implementing all crucial functions of digital computers.

The main features of combinational circuits are:

  • Graphical symbols: They depict the linked layout of logic gates.
  • Boolean equations: In these equations, the output signals are depicted as boolean functions of various input signals.
  • Truth table: This table produces the ‘binary output signals required for the set of 2nd input signals.
  • These time independent circuits are easy to design, fast and non-dependent on previous inputs for the sake of generating output.

What is Sequential Logic Circuits?

Sequential logic circuits are a certain category of circuits wherein the output is dependent on the value of the present inputs and the sequence of the past outputs alike. In sequential circuits, the output changes in line with the sequence of inputs inserted. In other words, sequential logic circuits comprise of memory space for storing immediate results. To cite an example, sequential circuits keep a record of whether logic level 0 or 1 has been connected to a given input; the same fact is employed to the output thus generated. The memory devices are generally made up of simple OR gates.

Difference Between Combinational and Sequential Circuits

Sequential circuits are implemented via different devices such as latches, flip-flops, registers, etc. The inputs required by sequential log circuits change from one of the 2 states as required. There ae two types of sequential circuits – asynchronous and synchronous circuits. A sequential logic circuit is marked as synchronous when the internal state of the device changes at specific times and as driven by a clock. Sequential logic circuits are designed to be tough to handle in comparison to combinational circuits. These time dependent, slow circuits need a feedback path that exists between their inputs and output. The sequential circuits are mainly used for the sake of data storage and are capable of storing any state or retaining their earlier state. These circuits require triggering as they are dependent on the clock.

Key differences between Sequential and Combinational Circuits

1. Combinational circuits require the latest inputs for generating their output. On the other hand, the inputs related to sequential circuits decide upon the output (of provided inputs) after considering all previous output along with the current inputs.

2. Combinational circuit is incapable of storing data whereas sequential circuits are capable of storing a given amount of data in memory spaces allocated for the purpose. On the contrary, Combinational circuits examples comprise of adders, subtractors, decoders, encoders, etc. The sequential circuits examples are best defined as flip-flops, latches and registers.

3. A clock is not used in case of combinational circuits whereas sequential circuits have a clock for triggering their functions.

4. While combinational circuit do not need any feedback for their functioning, sequential circuits require the same at all times.

Conclusion

In a nutshell, the digital devices comprising of combinational circuits do not need any previous output for their successful operations. In case of sequential logic circuits, previous output, as well as current inputs, are both required for procuring accurate results. We would like to hear from you in case you have any further inputs with respect to combinational circuits vs. sequential circuits. Please drop us a line in the Comments section below, we shall wait to hear from you.