Difference Between Combinational and Sequential Circuits for JEE Main 2024

Digital logic circuits are divided into "combinational" and "sequential" types. A combinational logic circuit has an output that is solely determined by its current inputs. Combinational circuits and memory elements known as "flip-flops" are used to build sequential circuits. These circuits produce output based on the current and previous states. A digital logic (or switching) circuit has voltage levels that can be switched from one value to another, but it only has a finite number of distinct values (typically 0 for false and 1 for true). Because these circuits follow a specific set of logic rules, they are also known as logic circuits. These are the basic circuits found in mobile phones, calculators, computers, and other electronic devices. 

A combinational circuit is made up of logic gates whose outputs are determined at any given time by only the current combination of inputs and have no memory. A sequential circuit is made up of logic gates, the outputs of which are determined at any given time by both the current input combination and the previous output.

A combinational logic circuit, in other words, is one whose output is solely determined by its current inputs. Combinational circuits and memory elements known as "flip-flops" are used to build sequential circuits. These circuits produce output based on their current and previous states.

Digital circuits are classified into two types based on their output and memory capacity:

1. Combinational circuits

2. Sequential circuits

Explain Combinational and Sequential Circuit?

Combinational Circuits:

These circuits are built with logic gates such as AND, OR, NOT, NAND, and NOR. These logic gates serve as the foundation for combinational circuits. A combinational circuit is made up of input and output variables. Combinational logic circuits are similar to these circuits in that they do not rely on previous input to generate any output. A combinational circuit can have as many as n inputs and as many as m outputs. The output of a combinational circuit is always a direct function of the applied external inputs. 

A combinational circuit is a circuit in which the circuit's various gates, such as the encoder, decoder, multiplexer, and demultiplexer, are combined. The following are some traits of combinational circuits:

At any given moment, the output of a combinational circuit is solely dependent on the levels at the input terminals.

There is no memory used by the combinational circuit. The current state of the circuit is unaffected by the previous state of the input.An n number of inputs and n equal number of outputs are possible for a combinational circuit.

Sequential Circuits:

A time sequence of inputs, outputs, and internal states defines a sequential circuit. A sequential circuit's output is determined not only by the combination of current inputs, but also by the previous outputs. Sequential circuits, as opposed to combinational circuits, combine memory elements with combinational circuits. Counters and shift registers are two examples.

The memory elements are circuits that can store binary data.At any given time, the binary information stored in these memory elements defines the state of the sequential circuit. A sequential circuit's external output is determined by both the current input and the previous output state.The memory elements' next state is also determined by the external input and the current state of the external output.

Difference Between Combinational and Sequential Circuit?

Characteristics of Combinational and Sequential circuit:

Combinational Circuit:

• Because the output is solely dependent on the current input and there is no need for feedback for input and output, a memory element is not required

• It is simpler to design, use, and manage.

• It does not require clock signals and is not time dependent

• Basic building blocks are simply logic gates

• These are more powerful logic circuits.

• These circuits are costly.

Sequential Circuit:

•  Because the output is dependent on both the current input and the current state (previous output), a memory element is required to save the feedback state.

• It is not easier to design, use, or manage.

• Clock signals are required, and it is time and clock dependent, so it requires triggering.

• Flip-flops are basic building blocks.

• Combinational circuits are faster than these circuits.

• These circuits are comparatively less costly.

Conclusion

Combinational circuits excel in terms of speed and performance, as they only require the current state as input. They are versatile, capable of handling both Boolean and Arithmetic operations. On the other hand, sequential circuits are slower due to their reliance on previous inputs, making the process more complex. They are primarily used for data storage. In the design of combinational circuits, steps involve determining the inputs and outputs, creating a truth table, simplifying Boolean functions, and designing a logic circuit diagram. In contrast, sequential circuit design entails creating a state table and equations, developing a state diagram, reducing the number of states, selecting the appropriate flip-flop type, deriving excitation equations from the excitation table, determining output and flip-flop input functions, and finally creating the necessary logic diagram.