Introduction to Sequential Logic Circuits Tutorial

A Sequential logic circuits is a form of the binary circuit; its design employs one or more inputs and one or more outputs, whose states are related to some definite rules that depend on previous states. Both the inputs and outputs can reach either of the two states: logic 0 (low) or logic 1 (high). In these circuits, their output depends, not only on the combination of the logic states at its inputs but moreover on the logic states that existed previously. In other words, their output depends on a SEQUENCE of the events occurring at the circuit inputs. Examples of such circuits include clocks, flip-flops, bi-stables, counters, memories, and registers. The actions of the circuits depend on the range of basic sub-circuits.

What is a Sequential Logic Circuit?

Dissimilar Combinational Logic circuits can change state depending on the real signals that are applied to their inputs, at the same time,Sequential Logic Circuits include some form of inherent “Memory” build into them as they are capable of taking into account their previous input state as well as the individuals really present, a sort of “before” and “after” effect is involved with the sequential logic circuits.A very simple sequential circuit with no inputs can be created using an inverter to form a feedback loop

sequential Logic Circuit Block Diagram
Sequential Logic Circuit Block Diagram

Design Procedure of Sequential Logic Circuits

  1. This procedure involves the following steps
  2. First, derive the state diagram
  3. Take as the state table or an equivalence representation, such as a state diagram.
  4. The number of states may be reduced by the state reduction technique
  5. Verify the number of flip-flops needed
  6. Choose the type of flip-flops to be used
  7. Derive excitation equations
  8. Using the map or some other simplification method, derive the output function and the flip-flop input functions.
  9. Draw a logic diagram or a list of Boolean functions from which a logic diagram can be obtained.

Types of Sequential Logic Circuits

There are three types of sequential circuits:

  • Event-Driven
  • Clock Driven
  • Pulse Driven
Types of Sequential Logic Circuits
Types of Sequential Logic Circuits

Event-Driven: – Asynchronous circuits that can change the state immediately when enabled. Asynchronous (fundamental mode) sequential circuit: The behavior is dependent on the arrangement of the input signal that changes continuously over time, and the output can be a change at any time (clockless).

Clock Driven: Synchronous circuits that are synchronized to a specific clock signal. Synchronous (latch mode) sequential circuit: The behavior can be defined from the knowledge of circuits that achieve synchronization by using a timing signal called the clock.

Pulse Driven: This is a mixture of the two that responds to the triggering pulses.

Examples of Sequential Logic Circuits


State changes of most sequential circuits occur at times specified by free-running clock signals. As the name implies, sequential logic circuits require a means by which events can be sequenced.

Clock Sequential Circuit
Clock Sequential Circuit

The state changes are controlled by the clocks. A “clock” is a special circuit that sends pulses with accurate pulse width and an accurate interval between the consecutive pulses. The interval between consecutive pulses is called the clock cycle time. The Clock speed is normally measured in Megahertz or Gigahertz.


The basic building block of the combinational circuit has logic gates, while indeed the basic building block of a sequential circuit is a flip-flop. Flip-flop has a better and greater usage in shift register, counters and memory devices. It is a storage device capable of storing one bit of data. Flip flop has two inputs and two outputs labeled as Q and Q’. It is normal and complements.

Flip Flops
Flip Flops


In most cases, the bi-stables are indicated by a box or circle. Lines in or around bi-stables not only mark them as bi-stables but also indicate how they function. Bi-stables are of two types latch and flip flop. The bi-stables have two stable states one is SET and the other one is RESET. They can retain either of these stages indefinitely, which makes them useful for storage purposes. Latches and flip-flops are different in the way they change from one state to another.

Bistable input and output waveforms
Bi-stable input and output Waveforms


A counter is a register that goes throughout a predetermined sequence of states upon the application of clock pulses. From another viewpoint, a counter is some sort of sequential circuit whose state diagram is a single cycle. In other words, counters are a particular case of a finite state machine. The output is generally a  state value.

Basic Counter Circuit
Basic Counter Circuit

There are two types of counters: Asynchronous counters (Ripple counter) and the other one is Synchronous counters. The asynchronous counter is the clock signal (CLK), which is simply used to clock the first FF.  Each FF (except the first FF) is clocked by the preceding FF. The synchronous counter is the clock signal (CLK) that is functional to all FF, which means that all FF shares the same clock signal.  Thus, the output changes at the same time.


Registers are clocked sequential circuits. A register is a collection of flip-flops; each flip-flop is capable of storing one bit of information. An n-bit register consists of n flip-flops and is capable of storing n bits of information. Besides flip-flops, a register usually contains a combinational logic to perform some simple tasks. The flip-flops hold binary information. The gates to determine how the information is shifted into the register. Counters are a special type of register. A counter goes through a predetermined sequence of states.

Register Circuit
Register Circuit


Memory elements can be anything that creates a past value available at some future time-devices that can behold a binary value. Memory elements are typically flip-flops. Memory output which is considered as a circuit’s “current state” is a numerical label. The state embodies all the information about the past needed to define the current output.

Differences between the Combinational and Sequential Logic Circuits

Combinational circuits Sequential circuits
The circuit whose output, at any immediate time, depends only on the input present at that instant only is known as a combinational circuit. The circuit whose output at any immediate time depends not only on the input present but also on the past output,  is known as sequential circuit
These types of circuits have no memory unit. These types of circuits have a memory unit to store the past output.
It is Faster. It is Slower.
These are easy to design. These are difficult to design.
Examples of combinational circuits are a half adder, full adder, magnitude comparator, multiplexer, demultiplexer, etc. Examples of sequential circuits are flip-flop, register, counter, clocks, etc.

Computer circuits consist of combinational logic circuits and sequential logic circuits. Combinational circuits produce outputs immediately when their input changes. Sequential circuits require clocks to control their changes of state. The fundamental sequential circuit unit is the flip-flop and the behavior of the SR, JK, and D flip-flops are the most important to know. Furthermore, any queries regarding this circuit or electrical and electronics projects, please give your feedback by commenting in the comment section below. Here is a question for you, what is the function of a sequential logic circuit?

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