Types of Sequential Circuit

A sequential circuit is widely used for storing and processing information such as retaining and manipulating data which makes them an essential component in various electronic devices in the realm of digital electronics. Sequential circuits process data in a sequential manner which makes them essential building blocks of many applications computer processors, memory systems, and communication networks by enabling memory retention and feedback capabilities.

Definition of Sequential Circuit

Unlike a combinational circuit which operates solely based on the present, a sequential stores and processes digital information in a sequential manner i.e. it has an additional element called memory which enables it to consider not only the present but also the past inputs or states. This memory element plays a vital role as it enables the processing of streams of data.

As shown in the above figure, there are two types of input to the combinational logic : 

• External inputs which are not controlled by the circuit.
• Internal inputs, which are a function of a previous output state.

Examples of Sequential Circuits:

Sequential circuits are used to develop various building units which are discussed below:

• Flip-Flops: A flipflop is used as a memory element to store bitwise binary information(0 and 1 only). Flipflops are the main form of sequential circuit where two stable states(set and reset) are working sequentially based on trigger condition(known as clock pulse).
• Counters: Counters are specially designed sequential circuits which are simply used for counting pulses or events or various sequences like binary, decimal or Gray code etc. Counters can be found in digital clocks, frequency dividers etc.
• Shift Registers: Shift registers are nothing but chain of multiple flipflops which are connected to each other serially to store and shift data serially. It listens to the clock pulses to decide to perform shifting operation. Shift registers are used in data storage and communication system and parallel-to-serial(P2S) or serial-to-parallel(S2P) conversion systems.

Types of Sequential Circuit

There are two types of sequential circuits available:

1. Synchronous Sequential Circuit

Synchronous circuits synchronize with either positive edge or negative edge of the clock signal, that means, the outputs of synchronous sequential circuits change or affect at the same time. These circuits use clock signal and level input (or pulsed with restrictions on pulse width and circuit propagation). Since they wait for the next clock pulse to arrive to perform the next operation, so these circuits are a bit slower compared to asynchronous. Level output changes state at the start of an input pulse and remains in that until the next input or clock pulse. The synchronous sequential circuit can be locked or unlocked (or pulsed).

For more detail you can refer Synchronous Sequential Circuits in Digital Logic article.

2. Asynchronous Sequential Circuit

Asynchronous circuits do not synchronize with positive edge or negative edge of the clock signal, that means, the outputs of asynchronous sequential circuits do not change or affect at the same time and change their state immediately when there is a change in the input signal. So, these circuits are faster and independent of the internal clock pulses. But these circuits have uncertainty in the outputs and are difficult to design.

For more detail you can refer Asynchronous Sequential Circuits article.

Applications of Sequential Circuit

Many applications have use of sequential circuits like:

• Computer Processors: Most of the modern CPUs contain sequential circuits to fetch instructions, decode data and execute processes. The reason of sequential circuits within CPU is to maintain the flow of work or operations of CPU in sequential manner(one after another to avoid overlapping).
• Memory Systems: RAM and ROM are the backbone of memory of computer. And both of these contains sequential circuits for better utilization during storing and retrieving data.
• Communication Systems: All communication system consists of sequential circuit for data encoding, decoding and synchronization and ensures secure data transmission and better quality error detection.

Advantages of Sequential Circuit

Modern technology depends upon sequential circuits for its wide range of advantages which are listed below:

• Memory Retention: Sequential circuits are capable of retaining information even if the output changes by storing past states.
• Feedback Capabilities: Not only it saves the previous input states but also it can store past output states which enables feedback response system, necessary for data control and communication systems.
• Complex Functionality: Developers can design complex logical units by connecting multiple Flipflops, counters or shift registers i.e. sequential circuits which can perform high end logical tasks.

Disadvantages of Sequential Circuit

Though sequential circuits are widely used, still it faces some drawbacks which are listed below:

• Increased Complexity: Sequential circuits are comparatively complex in nature than combinational circuits as it has memory elements and feedback paths. So, developer need to give special care during designing complex circuits with it.
• Higher Power Consumption: As sequential circuits contains memory elements it consumes high power during data retaining. Moreover, when it is in idle state still it listens for any data changes and make synchronizations. It constantly consumes high power.
• Synchronization Challenges: All synchronization is depending upon the clock pulses. So, proper design is required for correct data storing after data alterations especially where the data changes are more often.