# What Are The Different Types of Sequential Circuits?

A sequential circuit is a logical circuit, where the output depends on the present value of the input signal as well as the sequence of past inputs. While a combinational circuit is a function of present input only. A sequential circuit is a combination of combinational circuit and a storage element. the sequential circuits use current input variables and previous input variables which are stored and provides the data to the circuit on the next clock cycle.

## Types of Sequential Circuits

The sequential circuits are classified into two types

• Synchronous Circuit
• Asynchronous Circuit

In synchronous sequential circuits, the state of device changes at discrete times in response to a clock signal. In asynchronous circuits, the state of the device changes in response to changing inputs.

### Synchronous Circuits

In synchronous circuits, the inputs are pulses with certain restrictions on pulse width and propagation delay. Thus synchronous circuits can be divided into clocked and un-clocked or pulsed sequential circuits.

#### Clocked Sequential Circuit

The clocked sequential circuits have flip-flops or gated latches for its memory elements. There is a periodic clock connected to the clock inputs of all the memory elements of the circuit to synchronize all the internal changes of state. Hence the operation of the circuit is controlled and synchronized by the periodic pulse of the clock.

#### Unclocked Sequential Circuit

In an unclocked sequential circuit requires two consecutive transitions between 0 and 1 to alternate the state of the circuit. An unclocked mode circuit is designed to respond to pulses of certain durations which do not affect the circuit’s behaviour.

The synchronous logic circuit is very simple. The logic gates which perform the operations on the data, require a finite amount of time to respond to the changes in the input.

### Asynchronous Circuits

An asynchronous circuit does not have a clock signal to synchronize its internal changes of the state. Hence the state change occurs in direct response to changes that occur in primary input lines. An asynchronous circuit does not require the precise timing control from flip-flops.

Asynchronous logic is more difficult to design and it has some problems compared to synchronous logic. The main problem is that the digital memory is sensitive to the order that their input signals arrive them, like, if two signals arrive at a flip-flop at the same time, which state the circuit goes into can depend on which signal gets to the logic gate first.

Asynchronous circuits are used in critical parts of synchronous systems where the speed of the system is a priority, like as in microprocessors and digital signal processing circuits.

#### Flip Flop Circuit

A flip-flop is a sequential circuit which samples the input and changes the output at a particular instance of time. It has two stable states and can be used to store the state information. Signals are applied to one or more control inputs to change the state of the circuit and will have one or two outputs.

It is the basic storage element in sequential logic and fundamental building blocks of digital electronic systems. They can be used to keep a record of the value of a variable. Flip-flop is also used to control the functionality of a circuit.

#### RS Flip Flop

The R-S flip-flop is the simplest flip-flop. It has two outputs, one output is the reverse of the other, and two inputs. The two inputs are Set and Reset. The flip-flop basically uses NAND gates with an additional enable pin. The circuit gives output only when the enable pin is high.

Block Diagram

Circuit Diagram

#### JK Flip Flop

JK flip-flop is one of the important flip-flops. If the J and K inputs are one and when the clock is applied, the output changes regardless of past condition. If the J and K inputs are 0 and when the clock is applied, there will be no change in the output. There is no indeterminate condition in the JK flip-flop.

Circuit Diagram

#### D Flip Flop

D flip-flop has a single data line and a clock input.The D flip-flop is the simplification of an SR flip-flop. The input of the D flip-flop goes directly to the input S and the compliment goes to input R. D input is sampled throughout the clock pulse.

Circuit Diagram

#### T Flip Flop

It is a method of avoiding indeterminate state found in the process of an RS flip-flop. It is to provide only one input, i.e. T input. This flip-flop acts as a Toggle switch. Toggle means to change to another state. T flip-flop is designed from clocked RS flip-flop.

Circuit Diagram

#### Electronic Oscillator

An electronic oscillator is an electronic circuit that produces periodic, oscillating signals. An oscillator converts direct current from a power supply to an alternating current signal.

An oscillator is an amplifier which provides feedback with an input signal. It is a non-rotating device to produce alternating current. Enough power must be fed back to the input circuit for the oscillator to drive itself. The feedback signal in the oscillator is regenerative.

Electronic oscillators are classified into two categories

• Sinusoidal or Harmonic Oscillator
• Non-sinusoidal or Relaxation Oscillator

#### Sinusoidal or Harmonic Oscillator

The oscillators that give an output as a sine wave are called as sinusoidal oscillators. These oscillators can provide the output at frequencies ranging from 20Hz to GHz. Depending on the material or components used in oscillator, Sinusoidal oscillators are further classified into four types

• Tuned Circuit Oscillator
• RC Oscillator
• Crystal Oscillator
• Negative Resistance Oscillator

#### Non-Sinusoidal or Relaxation Oscillator

Non-sinusoidal oscillators provide output in the form of a square, rectangular or sawtooth waveform. These oscillators can provide an output at frequencies ranging from 0 to 20MHz.

#### Applications of Sequential Logic Circuits

The major applications of a Sequential Logic Circuits are,

This is all about the sequential circuits. The sequential circuits are the circuits, where the immediate value of outputs depends on the immediate values of inputs and also on states they were in previously. They contain memory blocks for storing the previous state of the circuit.

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