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Difference between combinational and sequential circuit
In digital electronics, both combinational and sequential circuits are the most widely used circuits. These are two broad categories of circuits defined in the digital electronics where one type of circuit is independent of time and other is dependent on time.
Read this article to find out more about combinational circuits and sequential circuits and how these two circuits are different from each other. Let's start with a basic overview of combinational and sequential circuits so that it becomes easier to understand how they are different from each other.
What is a Combinational Circuit?
A combinational circuit is one whose output at any time is based only on the present combination of inputs at that point of time. A combinational logic circuit consists of NAND, NOR, and NOT logic gates. These logic gates are the basic building blocks of the circuitry of a combinational circuit. A combinational circuit has a range of operations including the arithmetic operation of two operands, sharing of data, conversion of code, etc.
In combinational circuits, the output produced at a time would be based on the input at that time. There are three primary applications of the combinational logic circuits that are the arithmetic and logical functions, data transmission, and code conversion. The examples of the combination logic circuits are adders, subtractors, comparators, etc.
What is a Sequential Circuit?
A sequential circuit is a type of digital logic circuit whose output not only relies on the current input but also depends on the previous output. Hence, a sequential circuit is basically a set of combinational circuits and memory elements connected in a feedback path.
Memory elements are devices that can save binary data within them. The binary data saved in the memory elements at any given time denotes the state of the sequential circuit. Therefore, the output of a sequential circuit is determined by a time sequence of inputs and previous outputs.
Sequential circuits are used as the central unit of digital circuits. A common example of a sequential circuit is the finite state machine. The sequential circuits can be further classified into two types namely, Synchronous Sequential Circuits and Asynchronous Sequential Circuits.
Difference between Combinational and Sequential Circuit
The following table highlights how a combinational circuit is different from a sequential circuit −
| Key | Combinational Circuit | Sequential Circuit |
|---|---|---|
| Definition | A Combinational Circuit is a type of circuit in which the output is independent of time and only relies on the input present at that particular instant. | A Sequential circuit is a type of circuit where output not only relies on the current input but also depends on the previous output. |
| Feedback | Since output does not depend on the time instant, no feedback is required for its next output generation. | The output relies on its previous feedback so output of previous input is being transferred as feedback used with input for next output generation. |
| Performance | As the input of current instant is only required in case of Combinational circuit, it is faster and better in performance as compared to that of Sequential circuit. | Sequential circuits are comparatively slower and has low performance as compared to that of Combinational circuit. |
| Complexity | No implementation of feedback makes the combinational circuit less complex as compared to sequential circuit. | The implementation of feedback makes sequential circuit more complex as compared to combinational circuit. |
| Elementary Blocks | The elementary building blocks of a combinational circuit are its logic gates. | The building blocks of a sequential circuit are the logic gates along with flip flops. |
| Operation | Combinational circuits are mainly used for arithmetic as well as Boolean operations. | Sequential circuits are mainly used for storing data. |
Conclusion
The most significant difference between combinational circuits and sequential circuits is that a combinational circuit is one whose output is independent of time and only relies on the input present at that particular instant, whereas a sequential circuit is one whose output does not only rely on the current input but also depends on the previous output.
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