# Combinational Circuit vs. Sequential Circuit: What's the Difference?

Edited by Aimie Carlson || By Janet White || Updated on October 22, 2023
A combinational circuit's output relies solely on current inputs, whereas a sequential circuit's output depends on current inputs and past states.

## Key Differences

A combinational circuit computes its output based exclusively on its present input values. In contrast, a sequential circuit incorporates memory elements, making its output a function of both current inputs and previous states.
In the realm of digital electronics, combinational circuits are those without memory, meaning they cannot remember past input values. Sequential circuits, however, possess memory components, allowing them to "remember" and use past input values to determine their outputs.
Common examples of combinational circuits include adders, subtracters, and multiplexers. Sequential circuits encompass elements like flip-flops and registers, which can store information.
While combinational circuits produce outputs that change immediately as inputs change, sequential circuits might exhibit delays due to their dependence on specific clock pulses or prior inputs.

## Comparison Chart

### Output Dependency

Only on current inputs
On current inputs and past states

### Memory Components

None
Present (e.g., flip-flops)

### Examples

Flip-flops, registers

### Response Time

Immediate
Might have delays due to clock or prior inputs

### Functionality

Performs arithmetic/logic operations
Can store and recall information

## Combinational Circuit and Sequential Circuit Definitions

#### Combinational Circuit

Circuit whose output depends strictly on its current inputs.
In a calculator, the combinational circuit determines the result displayed.

#### Sequential Circuit

Incorporates feedback paths, enabling the storage of past information.
In digital watches, a sequential circuit helps keep track of elapsed time.

#### Combinational Circuit

Electronic configuration lacking storage elements.
The decoder is a simple combinational circuit that selects a specific output.

#### Sequential Circuit

Digital system whose outputs change based on present inputs and past states.
State machines, which move through predefined states, are implemented with sequential circuits.

#### Combinational Circuit

A digital circuit without memory components.
The arithmetic logic unit (ALU) in a CPU is a typical example of a combinational circuit.

#### Sequential Circuit

A digital circuit incorporating memory components.
Flip-flops, essential in digital electronics, are foundational elements of sequential circuits.

#### Combinational Circuit

Digital system that provides instant output changes in response to input changes.
Multiplexers are combinational circuits that select one of many inputs to produce an output.

#### Sequential Circuit

Electronic configuration possessing storage capability.
Counters, which track numbers in sequence, are built using sequential circuits.

#### Combinational Circuit

Implements basic logical or arithmetic functions directly via its inputs
. Boolean functions can be represented using combinational circuits like AND, OR, and NOT gates.

#### Sequential Circuit

Circuit whose output depends on current inputs and historical data.
Registers in computer systems are sequential circuits storing bits of data.

## FAQs

#### How do combinational circuits differ from sequential ones?

Combinational circuits output depends only on current inputs, while sequential circuits consider both current inputs and past states.

#### Can sequential circuits act like combinational circuits?

Yes, but their defining feature is the ability to also store and use past data.

#### Can a digital system have both combinational and sequential circuits?

Yes, many digital systems use both types for different functions.

#### Why are clock pulses important for sequential circuits?

Clock pulses help synchronize changes and transitions in the states of sequential circuits.

#### Can a combinational circuit be converted into a sequential circuit?

While you can add memory elements to a combinational circuit, it inherently won't rely on past states unless designed as a sequential circuit.

#### How does feedback play a role in sequential circuits?

Feedback paths in sequential circuits enable them to consider and "remember" previous states.

#### Why don't combinational circuits have memory components?

They're designed to give outputs based strictly on immediate inputs without considering past states.

#### What's a typical use for a sequential circuit?

They're often used in registers, counters, and memory units due to their storage capability.

#### How can sequential circuits "remember" past states?

They use components like flip-flops, which can retain a state until changed by an input or a clock pulse.

#### What kind of circuit would an AND gate be categorized as?

An AND gate is a basic element of a combinational circuit.

#### Why might engineers choose a combinational circuit over a sequential one for certain tasks?

For tasks needing instant outputs based on present inputs without storage needs, combinational circuits are more suitable.