What is the Difference Between Combinational and Sequential Circuits

The main difference between combinational and sequential circuits is that the output of combinational circuits depend on the present input while the output of sequential circuits depends on the present input as well as past outputs.  

Digital circuits are circuits that operate on digital signals. These signals consist of two discrete values (1 and 0). Digital signals help to overcome the limitations of analogue signals such as attenuation and noise. There are two main types of digital circuits as combinational and sequential circuits. The outcome of a combinational circuit relies on the present input. On the other hand, the outcome of a sequential circuit depends on the present inputs and past outputs. Therefore, it contains memory to store data.

Key Areas Covered

1. What are Combinational Circuits
     – Definition, Functionality
2. What are Sequential Circuits
     – Definition, Functionality
3. What is the Difference Between Combinational and Sequential Circuits
     – Comparison of Key Differences

Key Terms

Combinational Circuits, Digital Circuits, Sequential Circuits

What are Combinational Circuits

A combinational circuit is a circuit whose output depends on the present input values. The basic gates such as AND, OR, NOT, NAND, and NOR are the building blocks to create combinational circuits. It is also possible to create more complex combinational circuits using them. These circuits do not maintain a memory unit.

Some common combinational circuits are as follows.

Half adder – There are two inputs and two outputs (carry, sum). It is capable of adding two single bit binary numbers.

Full adder – There are three inputs and two outputs. It is capable of adding two one bit numbers.

N bit parallel adder – It is a combination of full adders and is capable of adding two n bit binary numbers. Here, the carry output of previous full adder transfers to the carry input of the next full adder. e.g. 4-bit parallel adder.

Half subtractor – It is capable of finding the difference between two binary bits.

Multiplexer – It has several input lines and one output line.  Moreover, it selects one input and switches it to the output.

De multiplexer – It performs the opposite operation of the multiplexer and obtains one input and sends it to several outputs.

Encoder – It has 2^n input and n output lines. It encodes the 2^n input into n bit code.

Decoder – It performs the opposite operation of the encoder.  It has n input lines and 2^n output lines.  Moreover, it decodes n inputs to 2^n outputs.

What are Sequential Circuits

A sequential circuit is a type of circuit in which the output depends on the present input and the past outputs.  These circuits contain memory units to store previous outputs. They might also consist of a clock to change the state at discrete time intervals.

Some common sequential circuits are as follows.

Flip flops – It is a circuit that can be in one of two states. It maintains the state until receiving a trigger. There are various flip flop types such as SR flip flop, T flip flop, JK flip flop, etc.

Registers – They are fast memory locations, but located in the processor.

Difference Between Combinational and Sequential Circuits

Definition

A combinational circuit is a type of digital circuit where the output is only a pure function of the present input. A sequential circuit is a type of digital circuit whose output depends not only on the present value of its input signals but also on the sequence of past inputs.

Output

The output of a combinational circuit depends on the present input while the output of the sequential circuit depends on the present input and past outputs.  Thus, this is the main difference between combinational and sequential circuits.

Memory Unit

Another difference between combinational and sequential circuits is that there is no memory unit in combinational circuits. However, there is a memory unit in sequential circuits to store immediate results.

Clock

Also, there is no clock in a combinational circuit. However, there is a clock in a sequential circuit. Hence, this is one difference between combinational and sequential circuits.

Examples

Half adder, full adder, multiplexer, de-multiplexer, encoder, and, decoder are some examples for combinational circuits while Flip Flops and registers are some examples for sequential circuits.

Conclusion

Two types of digital circuits are combinational and sequential circuits. The main difference between combinational and sequential circuits is that the output of combinational circuits depend on the present input while the output of the sequential circuits depends on the present input as well as past outputs.  In brief, sequential circuits are more complex than combinational circuits.

Reference:

1. “Combinational Logic.” Wikipedia, Wikimedia Foundation, 26 Nov. 2018, Available here.
2. “Sequential Logic.” Wikipedia, Wikimedia Foundation, 25 July 2018, Available here.
3. “Combinational Logic Circuits Using Logic Gates.” Basic Electronics Tutorials, 9 Sept. 2018, Available here.
4. “Chapter 3. – Sequential Circuits.” Karnaugh Maps, Available here.

Image Courtesy:

1. “Get logic function from combinational circuit” By Dizquierdol – Own work (CC0) via Commons Wikimedia
2. “Sequential circuits” By Ashutosh nayak1992 – when i study in bca (CC BY-SA 3.0) via Commons Wikimedia