What are Sequential Circuits?

In Sequential Circuits, the present output depends on the present input as well as past output(s). It consists of a combinational circuit and memory elements.

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A block diagram of a sequential circuit is shown in Fig.1. It consists of a combinational circuit to which storage or memory elements are connected to form a feedback path.

block diagram of sequential circuit — Fig. 1 Block diagram of a sequential circuit

The storage/memory element is capable of storing binary information which is basically the past output. The binary information stored in these elements at any given time defines the state of the sequential circuits at that time. It simply means, in sequential circuits, the present output depends on the present input as well as on the past output(s).

Difference between Combinational and Sequential circuits

S.No.	Combinational circuit	Sequential circuit
1.	Output depends only on the present input.	Output depends on the present input and past output.
2.	The circuit does not have a memory element.	The circuit has a memory element
3.	Time independent.	Time-dependent.
4.	There is no feedback path between input and output.	There exists a feedback path between input and output.
5.	Speed is fast.	Speed is slow.
6.	A clock signal is not required.	A clock signal is required.
7.	Elementary building blocks are logic gates.	Elementary building blocks are flip flops.
8.	It is easy to use and handle.	Not easy to use and handle.
9.	Examples: Adder, Encoder, Decoder, Multiplexer, etc.	Examples: Flip flops, Counter, Register, etc.

Types of Sequential circuits

There are two types of sequential circuits, they are named as synchronous and asynchronous sequential circuits.

1. Synchronous sequential circuits

In these circuits, a clock signal is used to determine/control the exact time at which any output can change its states. These are also called as clocked sequential circuits. Flip flops are used as memory elements in these circuits. These circuits are easy to design and they are slower. The block diagram of this circuit is shown in Fig.2.

block diagram of synchronous sequential circuit — Fig. 2 block diagram of a synchronous sequential circuit

2. Asynchronous sequential circuits

In these circuits, the output of the circuit can change state at any time, as soon as any input changes its state. Latches are used as memory elements in these circuits. These circuits are difficult to design and are faster as compared to the synchronous circuits as the clock is not present. The block diagram of this circuit is shown in Fig.3.

block diagram of synchronous asequential circuit — Fig. 3 block diagram of an asynchronous sequential circuit

Applications of sequential circuits

The major applications are

1. Flip flops
2. Counters
3. Shift registers
4. Multiplexers
5. Analog to digital converters and digital to analog converters
6. Programming logic devices such as PLA, PAL, etc.

Author:

Shivani Gupta

Institute of Engineering and Technology
Lucknow, U.P.