Last Updated: Mar 27, 2024

# Introduction to Sequential Circuits

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## Introduction

A digital logic circuit is defined as one in which voltages are assumed to have a finite number of distinct values. There are mainly two types of digital logic circuits, namely:

1. Combinational Logic Circuits
2. Sequential Logic Circuits

These circuits are used in most digital electronic devices such as computers, calculators, mobile phones, etc. Digital logic circuits are also known as switching circuits, as in digital circuits, the voltage levels are switched from one value to another. These circuits are known as logic circuits, as their operation follows a definite set of logic rules. In this blog, we will discuss the sequential circuits and its types along with a brief introduction to the flip-flops and latches.

## Sequential Circuits

In a sequential circuit, the output of the logic device depends not only on the present inputs of the device but also on the past inputs. The output of a sequential logic circuit depends on the present input and the present state of a given circuit.

The sequential circuits also have memory devices to store the past outputs. Sequential digital logic circuits are only combinational circuits with memory. These types of digital logic circuits are designed using a finite-state machine. Examples of sequential logic circuits are flip-flops counters constructed using digital logic gates and memory.

Sequential circuits are further divided into two types:

1. Asynchronous sequential circuits
2. Synchronous sequential circuits

## Important Terminologies

Before diving further into the sequential circuits, let’s understand some standard terms used widely in digital circuits.

1. Clock Signal: A periodic signal where the ON and OFF need not be the same. When the clock signal's ON time and OFF time are the same, a square wave represents the clock signal. Consider the following diagram, which illustrates the clock signal

2. Triggering: Triggering is divided into two subcategories:

• Level Triggering: The Low and High are the two levels in a clock signal. The circuit is activated when the clock pulse is at a particular level in the level triggering. The level triggering is further divided into two categories:

Positive Level Triggering: In this triggering, the circuit is operated with a High logic type of clock signal.

Negative Level Triggering: In this triggering, the circuit is operated with Low logic type of clock signal.

• Edge Triggering: In the clock signal of edge triggering, two types of transitions occur, i.e., either from logic Low to logic High or from logic High to logic Low. The edge triggering is further divided into two categories:

Positive Edge Triggering: In positive edge triggering, the transition from logic low to logic High occurs in the clock signal.

Negative Edge Triggering: In positive edge triggering, the transition from logic High to logic Low occurs in the clock signal.

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## Types of Sequential Circuits

### Asynchronous Sequential Circuits

The Asynchronous sequential circuits do not use the clock signal. These circuits are operated through the pulses. As a result, the change in the input can affect the state of the circuit. The asynchronous circuits do not use clock pulses. Hence, they are faster than synchronous sequential circuits. The internal state is changed with the change in the input variable.

### Synchronous Sequential Circuits

The Synchronous sequential circuits use a clock signal to synchronize the memory element's state. The output is stored either in the flip-flops or the latches. The synchronization of the outputs is done with either the negative edges of the clock signal or with the positive edges.

#### Clocked Sequential Circuit

These circuits have gated latches or flip-flops for their memory elements. A periodic clock is connected to the clock inputs of all the memory elements to synchronize the internal changes. Hence, the operation of a circuit is controlled and synchronized by the periodic pulse of the clock.

#### Unclocked Sequential Circuit

These circuits require two consecutive transitions between 0 and 1 to alternate the state of a circuit. Such circuits are designed to respond to pulses of certain durations which do not affect the circuit’s behavior.

## Classification of Sequential Logic

Bi-stable latches and flip-flops are the building blocks of sequential logic circuits. Sequential logic circuits can be constructed to produce either simple edge-triggered flip-flops or complex sequential circuits such as shift registers, memory devices, storage registers, or counters. The sequential logic circuits are divided into the following three main categories:

1. Event-Driven - An asynchronous circuit that can change the state immediately when enabled. The behavior depends upon the arrangement of the input signal that varies continuously, and the output can be changed at any time.

2. Clock Driven -  synchronous circuit synchronized to a specific clock signal. The behavior is dependent on the arrangement of circuits that achieve synchronization by using a timing signal (clock).

3. Pulse Driven - This combines the two responses to the triggering pulses.

## Latches

A latch is a memory element consisting of a pair of logic gates with their inputs and outputs inter-connected in a feedback arrangement that allows storing a single bit. Latches operate with level-sensitive enable signals, whereas flip-flops are edge sensitive. There are commonly two types of latches as follows:

1. SR Latch
2. D Latch

### SR Latch

This  Latch is also known as the Set Reset Latch. The SR latch provides an enable line that must be driven high before data can be latched. A control line is required, the SR latch is not synchronous it affects the outputs as long as the enable E is maintained at 1. When the enable input is low, the output from the AND gates is also set to LOW, hence the Q and Q’ outputs remain latched to the previous data. However, when the enable input is high the state of the latch is changed, as shown in the truth table below.

Circuit Diagram:

Truth Table:

### D Latch

The next state value can’t be predicted in the SR latch when inputs S & R are 1. We can overcome this difficulty by using D Latch. It is also known as a Data latch. This latch is an extension of the SR latch that removes the possibility of invalid input states. As the SR latch allows to latch the output without using the S and R inputs, we can now remove one of the inputs by driving both the Set and Reset inputs with a complementary driver: we remove one input and automatically make it the inverse of the remaining input.

The D latch outputs the input whenever the enable line is high, else the output is the result of the last enabled high D input. This is why it is also known as a transparent latch

Circuit Diagram:

Truth Table:

## Flip-Flop Circuits

The building block of the combinational circuit is logic gates, whereas the basic building block of a sequential circuit is flip-flops. Flip-flops have more significant usage in a shift register,  memory devices, frequency dividers, storage registers, and counters. It is a storage device capable of storing one bit of data. Flip-flops have two inputs and outputs labeled as Y and Y'.

A flip-flop is a sequential circuit that samples the input and manipulates the output instantly. It has two stable states and can be used to store state information. The stored data can be changed by applying the varying inputs. It is the primary storage element in the sequential logic circuit and a fundamental building block of digital electronic systems. They are used to record the value of a variable. Flip-flops are also used to control the functionality of a circuit.

There are commonly four types of flip-flops:

## Difference between Latches and Flip-Flops

The differences between latches and flip-flops are listed below:

## FAQs

1. What do you understand by the race-around problem?
The clock pulse that remains in the high (1) state while both J and K are equal to 1 causes the output to repeat complementing until the pulse goes back to 0, this situation is known as the race-around problem. That’s why the clock pulse must have a time duration shorter than the propagation delay time of the flip-flop.

2. List the types of shift register counters.
There are two types of shift register counters which are named as
Ring counter
Johnson counter

## Key Takeaways

This blog discussed sequential logic circuits and their types. We also discussed some common terminologies used in digital logic circuits and the concept of latches and flip-flops.

If you are a beginner interested in learning other fields related to computer subjects such as web development, or competitive programming, you can follow our guided path to get a good grip on such concepts.

Topics covered
1.
Introduction
2.
Sequential Circuits
3.
Important Terminologies
4.
Types of Sequential Circuits
4.1.
Asynchronous Sequential Circuits
4.2.
Synchronous Sequential Circuits
4.2.1.
Clocked Sequential Circuit
4.2.2.
Unclocked Sequential Circuit
5.
Classification of Sequential Logic
6.
Latches
6.1.
SR Latch
6.2.
D Latch
7.
Flip-Flop Circuits
8.
Difference between Latches and Flip-Flops
9.
FAQs
10.
Key Takeaways