Sequential circuits refer to the combinational logic circuits that consist of input variables (X) and logic gates (or Computational circuits) along with the output variable (Z). For example, flip-flops, counter, register, clocks, etc.
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Table of Contents
- What are Sequential Circuits?
- Types of Sequential Circuits
- Clock Signal and Triggering
- Practice Questions on Sequential Circuits
- FAQs
What are Sequential Circuits?
A sequential circuit refers to a special type of circuit. It consists of a series of various inputs and outputs. Here, the outputs depend on a combination of both the present inputs as well as the previous outputs. This previous output gets treated in the form of the present state. Thus, the sequential circuit consists of the combinational circuit along with its memory storage elements. It doesn’t need to consist of a combinational circuit all the time. A sequential circuit can basically contain only the memory element.
Types of Sequential Circuits
There are basically two types of sequential circuits:
1. Asynchronous sequential circuits
2. Synchronous sequential circuits
1. Asynchronous Sequential Circuits
The asynchronous sequential circuits don’t make use of the clock signals. This type of circuit is operated through various pulses. Thus, the changes in our input can easily make a change in the state of our circuit. Clock pulses are not used in the case of asynchronous circuits. Here, the internal state gets altered whenever there is a change in the input variable. The time-delayed or the unclocked flip-flops form the memory elements of the asynchronous sequential circuits. This type of circuit is very similar to that of the combinational circuits along with the feedback.
2. Synchronous Sequential Circuits
The clock signal performs the synchronization of the state of memory elements in the case of synchronous sequential circuits. The output, in this case, is either stored in latches (memory devices) or flip-flops. The output gets synchronized with the clock’s only positive edges or only the negative edges.
Clock Signal and Triggering
The clock signal refers to a periodic signal where the ON and OFF times do not need to be the same. Thus, whenever the ON and OFF times of the clock signal happen to be the same, we use a square wave to represent the clock signal. Here is a diagram that represents a typical clock signal:
Remember that a clock signal is often considered to be the square wave. The signal here sometimes stays at logic. It’s either 5V (high) or 0V (low), to an equal share of time. It basically gets repeated with a certain time, which is equal to twice the ‘OFF time’ or ‘ON time’. Let us now know more about triggering.
Types of Triggering
A sequential circuit has two types of triggering:
1. Level triggering
2. Edge Triggering
Let us discuss these two types of triggering along with their segmentation in sequential circuits:
1. Level Triggering
There are two levels present in the clock signal – the logic Low and the logic High. The circuit is only activated in the case of a level triggering whenever the clock pulse happens to be at any particular level. Thus, there are the following types of level triggering in a sequential circuit:
1.1. Positive Level Triggering
In the case of a positive level triggering, the signals with Logic High would occur. Thus, in this type of triggering, the circuit gets operated with such a type of clock signal. Here is a diagrammatic representation of positive level triggering:
1.2. Negative Level Triggering
In the case of a negative level triggering, a signal occurs with Logic Low. Thus, in this type of triggering, the available circuit gets operated with such a type of clock signal. Here is a diagrammatic representation of Negative level triggering:
2. Edge Triggering
Two major types of transitions occur in the case of edge triggering clock signals. This transition either occurs from Logic High to Logic Low or from Logic Low to Logic High. Now, on the basis of the transitions of a given clock signal, the edge triggering can be of the following types:
2.1. Positive Edge Triggering
In the clock signal of the positive edge triggering, a transition occurs from the Logic Low to the Logic High. Thus, the circuit of a positive edge triggering is operated using such a type of clock signal. Below is the diagram of the positive edge triggering:
2.2. Negative Edge Triggering
In the clock signal of the negative edge triggering, a transition occurs from the Logic High to the Logic Low. Thus, the circuit of a negative edge triggering is operated using such a type of clock signal. Below is the diagram of the negative edge triggering:
Practice Questions on Sequential Circuits
1. Which of these sequential circuits would generate the feedback path as a result of the cross-coupled connection from the output of one of the given gates to the input of some other gate?
A. Asynchronous
B. Synchronous
C. Both of these
D. None of these
Answer – A. Asynchronous
2. How do we specify sequential circuits in terms of the available time sequence?
A. By Internal States
B. By Inputs
C. By Outputs
D. All of these
Answer – D. All of these
3. We can predict the behaviour of a synchronous sequential circuit whenever we define the signals at ________ instants of time.
A. Continuous
B. Discrete
C. Any
D. Sampling
Answer: B. Discrete
4. What is the paramount function of memory elements that are used in any sequential circuit?
A. Storing binary information
B. Specifying state of sequential
C. Both of these
D. None of these
Answer – C. Both of these
Frequently Asked Questions on Sequential Circuits
What are sequential circuits?
Sequential circuits refer to the combinational logic circuits that consist of input variables (X) and logic gates (or Computational circuits) along with the output variable (Z).
What is a sequential circuit? Give some examples.
A sequential circuit refers to a special type of circuit with a series of various inputs and outputs. Here, the outputs depend on a combination of both the present inputs as well as the previous outputs. It doesn’t need to consist of a combinational circuit all the time. For example, flip-flops, counter, register, clocks, etc.
What are sequential logic circuits used for?
We use sequential logic circuits to construct various finite state machines. These are nothing but the basic building blocks of all digital circuitry, along with the memory circuits. All circuits that are used in practical digital devices around us are a mixture of combinational as well as sequential logic circuits.
What are clock pulses?
It refers to a signal that is used to synchronize all the operations of a given electronic system. The clock pulses are always continuous. These are precisely the spaced changes in the voltage.
What is a synchronous sequential circuit?
The synchronous sequential circuits are basically the digital sequential circuits where the clock signals govern the feedback of the input for the next output generation.
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Also Explore,
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- Boolean Algebra
- Laws of Boolean Algebra
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- Various Implicants in K-Map
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- Flip-Flop Types, Conversion and Applications
- The Base of Number System
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- Number System Notes
- Decimal to Binary Conversion
- Decimal to Hexadecimal Conversion
- Decimal to Octal Conversion
- Minimization of Boolean Functions
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