In digital electronics, logic circuits are classified into two categories namely, combinational logic circuits and sequential logic circuits. The main difference between combination and sequential logic circuits is that a combinational logic circuit does not contain any memory element, while a sequential logic circuit necessarily contains a memory element to store past history of the circuit operation.
In this article, we will discuss combinational logic circuits and sequential logic circuits along with their comparative study.
What is a Combinational Logic Circuit?
A type of digital time-independent logic circuit in which the output depends on the present inputs only and not on the past operation of the circuit is called a combinational logic circuit. A combinational logic circuit is designed by using different types of logic gates like AND, NAND, NOR, OR, etc. The block diagram representation of a combination logic circuit is depicted in the following figure.
A combinational logic circuit has only a processing circuit and does not have any storing circuit. Therefore, the output of the combinational logic circuit is the function of the inputs at that time.
Combinational logic circuits are widely used in various digital systems to perform various operations like arithmetic operations, data transmission, code conversion, and more. Examples of combinational logic circuits include binary adders, binary subtractors, multiplexers, data comparators, etc.
Thus, combinational circuits do not have a memory element, whereas sequential circuits have a memory element which is the main difference between combinational and sequential circuits.
What is a Sequential Logic Circuit?
A sequential logic circuit is another type of time-dependent digital circuit having a memory element to store the history of the past operation of the circuit. Therefore, a sequential logic circuit is one whose output depends on the present inputs as well as past outputs.
Sequential logic circuits can be further classified into two types namely, synchronous sequential logic circuits and asynchronous sequential logic circuits. This type of logic circuit is mainly used in high-end applications like processing units, memory units, etc. Examples of sequential logic circuits are counters, finite state machines, registers, etc.
Thus, the sequential circuit has a memory element, and combinational circuits do not have a memory element, which is the main difference between combinational and sequential circuits.
Difference between Combinational and Sequential Logic Circuits
The important differences between combinational and sequential logic circuits are given in the following table:
Combinational Vs Sequential Circuits
| Parameter | Combinational Logic Circuit | Sequential Logic Circuit |
| Definition | A sequential logic circuit is another type of digital logic circuit whose output depends on the present input as well as previous outputs and is dependent on time. | Combinational logic circuits have faster response times. |
| Response time | Sequential logic circuits have comparatively slower response times than combinational circuits. | Combinational logic circuits do not require a clock signal to operate. |
| Output depends | The output of a combinational logic circuit is the function of only present inputs. | The output of a sequential circuit is the function of both present inputs and past outputs. |
| Time dependency | The operation of a combinational logic circuit is time-independent. | The operation of a sequential logic circuit is time-dependent. |
| Feedback path | Combinational logic circuits do not have any feedback path between input and output. | Sequential logic circuits have a feedback path between input and output. |
| Main components | The logic gates are the main components of a combinational logic circuit. | The logic gates and flip-flops are the main components of a sequential logic circuit. |
| Memory element | Combinational logic circuits do not have any memory element. | Sequential circuits have a memory element to store past outputs. |
| Clock signal | The typical examples of combinational logic circuits are binary adders, subtractors, decoders, multiplexers, demultiplexer, etc. | Sequential logic circuits require a clock signal to operate. |
| Complexity | Combinational logic circuits are simpler to design and operate. | Sequential logic circuits are more complex to design and operate. |
| Applications | Combinational logic circuits are used to perform arithmetic and other logical operations. | Sequential logic circuits are used to perform complex operations like data storage, signal processing, data conversion, and more. |
| Examples | Examples of combinational logic circuits are binary adders, subtractors, decoders, multiplexers, demultiplexers, etc. | Examples of sequential logic circuits are binary counters, registers, finite state machine, etc. |
Conclusion
In conclusion, this is all about combinational and sequential logic circuits and the important differences between them. The combinational logic circuits are fundamental building blocks of all digital circuits while sequential logic circuits are used where complex digital signals processing is required.