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Combinational and Sequential Circuits - Key Differences

05 Jan 2019 10:05 am || 0

When it comes to an understanding of the essential concepts of digital electronics, then combinational and Sequential circuits are the two concepts that have to be understood in entirety.

Combinational and Sequential Circuits

Comparison Chart

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 input. 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 fundamental and universal logic gates. Useful for data storage and therefore used in RAM.
Elementary building blocks The primary 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 a clock; therefore, triggering is not essential 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 information.]
Application These circuits are used in association with encoders, adders, decoders, subtractors, multiplexers, etc. Sequential circuits are used in the 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 A 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. A 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 based on current inputs as well as the previous states.

Combinational Circuit

Combinational logic circuits referred to as time-independent logic, which linked to the set of different gates that generate the output of current inputs belonging to that instant. The fundamental building units of combinational circuits are AND, OR, NOT, and the universal gates are NAND and NOR.

In most cases of combinational logic circuit diagrams, the output lines 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.

Combinational Circuits Flow Chart

In the 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, seven segments, etc. In general, combination circuits contain m number of binary outputs and n number of binary inputs. They are useful for implementing all the crucial functions of digital computers.

Features of Combinational Circuits

  • 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.

Sequential Circuit

Sequential logic circuits are a particular 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 series of data inserted. In other words, sequential logic circuits comprise of memory space for storing immediate results. For 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.

Sequential Circuits Flow Chart

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 two states as needed. There are two types of sequential circuits – asynchronous and synchronous circuits. A sequential logic circuit is marked as simultaneous 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

  • Combinational circuits require the latest inputs for generating their output. On the other hand, the inputs related to sequential circuits decide upon the production (of provided inputs) after considering all previous output along with the current inputs.
  • The 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 adders, subtractors, decoders, encoders, etc. The sequential circuits models are best defined as flip-flops, latches, and registers.
  • A clock is not used in case of combinational circuits, whereas sequential circuits have a clock for triggering their functions.
  • While the combinational circuit does 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 the 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 for combinational circuits vs. sequential circuits. Please drop us a line in the Comments section below. We shall wait to hear from you.