Interrupts vs Polling in Microcontrollers: When to Use Which?

Introduction

Microcontrollers are built to interface with the physical world through sensors, switches, communication interfaces, motors, displays, and other peripheral devices. It is also necessary for the embedded systems to have a method to detect events and respond to them efficiently to control all these interactions.

Two commonly used methods are interrupts and polling. Both are used to allow a microcontroller to detect hardware events and to respond when certain conditions arise. However, they have very different means for detection and dealing with events by the processors.

Poll emulates the same as made continuously checking a status of a device or condition but interrupts make a peripheral or external event tell the processor to time out to them6

The right instance for interrupt or polling is a vital choice in embedded software development which can enhance processor utilization, system responsiveness, power consumption, and overall application performance.

This post discusses the inner workings of interrupts and polling, their differences, practical applications, and also why and when to use one method over another when programming a microcontroller.

1. What Is Polling in a Microcontroller?

Polling is a way by which the microcontroller constantly checks whether a hardware peripheral or input has triggered an event. It involves the processor repeatedly asks a device whether it needs attention.

For instance, push-button input to a microcontroller. Below is a simple example, in which the processor constantly polling button status.

For example, a push-button is connected to a microcontroller. Then, the processor uses polling to check whether the button is pressed or not. When the input changes from LOW to HIGH, the program carried out the necessary operation.

How Polling Works

The polling mechanism generally follows these steps:

• The microcontroller initializes the required peripheral or input pin.
• The main program enters a continuous loop.
• The processor repeatedly reads the device status register or input pin.
• If the expected condition is detected, the program executes the corresponding operation.
• The processor continues checking for other events.

Example of Polling in Embedded C

#include <stdint.h>

#define BUTTON_PIN  (1U << 0)
#define LED_PIN     (1U << 1)

int main(void)
{
    while (1)
    {
        if (GPIO_INPUT & BUTTON_PIN)
        {
            GPIO_OUTPUT |= LED_PIN;
        }
        else
        {
            GPIO_OUTPUT &= ~LED_PIN;
        }
    }

    return 0;
}

Note: GPIO_INPUT and GPIO_OUTPUT are illustrative register names. Actual register definitions depend on the microcontroller.

In this example, the processor continuously monitors the button input. When the button is pressed, the LED turns ON. When the button is released, the LED turns OFF.

Advantages of Polling

• Simple implementation: Polling is relatively easy to understand, write, and debug.
• Predictable execution: The program follows a predefined sequence of instructions.
• No interrupt management: Developers do not need to configure interrupt priorities or interrupt service routines.
• Suitable for simple systems: It works effectively when monitoring a limited number of inputs or peripherals.

Disadvantages of Polling

• CPU wastage: The processor repeatedly checks device status even when no event occurs.
• Response delays: An event may not be detected until the next polling cycle.
• Increased power consumption: Continuous processor activity can increase energy usage.
• Limited scalability: Monitoring multiple peripherals can make the main loop complicated and inefficient.

2. What Are Interrupts in a Microcontroller?

An interrupt provides a way of interrupting its normal execution when a specific hardware or software event occurs. The processor can be notified of an event instead of continually checking if something has occurred. An external interrupt, for example, could inform a microcontroller when a push-button has been pressed, causing its processor to pause the current operation and execute a dedicated routine known as an Interrupt Service Routine (ISR). The processor continues its execution of the previous program when it completed the handler functions of the interrupt.

How Interrupts Work

The interrupt mechanism generally follows these steps:

• The microcontroller configures the required interrupt source.
• The main application executes its normal instructions.
• A hardware or software event triggers an interrupt.
• The processor temporarily pauses its current execution.
• The processor executes the Interrupt Service Routine.
• After the ISR finishes, the processor resumes the previous program.

Example of Interrupts in Embedded C

#include <stdint.h>

volatile uint8_t button_pressed = 0;

void GPIO_IRQHandler(void)
{
    button_pressed = 1;

    // Clear the interrupt flag
}

int main(void)
{
    while (1)
    {
        if (button_pressed)
        {
            button_pressed = 0;

            // Perform required operation
        }
    }

    return 0;
}

Note: GPIO_IRQHandler is an illustrative ISR. Actual interrupt names and flag-clearing procedures depend on the microcontroller.

In this example, the GPIO interrupt handler sets a flag whenever the button generates an interrupt. The main loop checks this flag and performs the required operation.

This approach allows the processor to respond to an event without continuously monitoring the button.

Advantages of Interrupts

• Better responsiveness: The processor can respond to events without waiting for the next polling cycle.
• Efficient CPU utilization: The processor does not need to continuously check peripheral status.
• Improved power efficiency: Interrupt-driven designs can allow the processor to remain in low-power sleep modes.
• Suitable for real-time applications: Interrupts help handle time-sensitive hardware events.
• Better peripheral management: Multiple interrupt sources can be handled through dedicated interrupt handlers.

Disadvantages of Interrupts

• Complex implementation: Interrupt configuration and handling require additional programming knowledge.
• Debugging challenges: Interrupt-related issues can be difficult to reproduce and troubleshoot.
• Context switching overhead: Interrupt entry and exit require processor resources.
• Race conditions: Shared variables between the main program and ISR must be managed carefully.
• Priority management: Multiple interrupts may require proper priority configuration to avoid delays or conflicts.

3. Interrupts vs Polling: Key Differences

Although both approaches help microcontrollers detect and handle events, their execution mechanisms are different.

The main difference is that polling follows an active monitoring approach, while interrupts follow an event-driven approach.

However, interrupts are not automatically faster in every situation. Their response time also depends on interrupt latency, priority, processor load, and whether interrupts are temporarily disabled.

4. When Should You Use Polling?

Polling is a suitable choice when the application has simple requirements and does not need immediate responses to hardware events.

It is particularly useful when the microcontroller needs to monitor devices at regular intervals.

Common Applications of Polling

1. Simple Button Monitoring

For basic applications where a button is checked periodically, polling can provide a straightforward solution.

2. Periodic Sensor Reading

If a temperature sensor needs to be checked every few seconds, polling may be sufficient.

3. Simple Embedded Projects

Applications involving basic LED control, switches, and simple input-output operations can use polling without unnecessary interrupt configuration.

4. Non-Critical Monitoring

When missing an event for a few milliseconds does not affect system functionality, polling can be an effective approach.

5. Controlled Communication Sequences

Polling can be useful when software needs to wait for a peripheral status flag within a short, bounded operation.

5. When Should You Use Interrupts?

Interrupts are preferable when an application must respond quickly to unpredictable or time-sensitive events.

They are especially useful when continuously checking hardware status would waste processor resources.

Common Applications of Interrupts

1. External Hardware Events

Push-buttons, emergency switches, and external trigger signals can generate interrupts when an event occurs.

2. UART Communication

UART receive interrupts allow the microcontroller to respond when incoming serial data becomes available.

3. Timer-Based Operations

Timer interrupts can execute periodic tasks, maintain timing intervals, and generate precise scheduling events.

4. Real-Time Embedded Systems

Automotive electronics, industrial automation, and control systems often use interrupts to handle critical events.

5. Low-Power Applications

Battery-powered devices can use interrupts to wake the microcontroller from sleep when a sensor or external signal requires attention.

6. Can You Use Interrupts and Polling Together?

Yes. In fact, many practical embedded systems combine interrupts and polling to achieve better performance and maintain simpler software architecture.

This approach is known as a hybrid interrupt-polling mechanism.

For example, consider an embedded system that receives data through UART communication.

Instead of continuously checking whether new data has arrived, the UART receive interrupt can notify the processor when data becomes available.

The ISR can then store the received byte in a buffer and update a flag. The main program periodically checks the buffer and processes the received data.

This provides several benefits:

• Interrupts handle time-sensitive events.
• Polling handles non-critical background tasks.
• The ISR remains short and efficient.
• The main application manages complex processing operations.

A hybrid approach is particularly useful in systems involving multiple sensors, communication interfaces, and control operations.

7. Best Practices for Using Interrupts and Polling

Choosing the correct mechanism is important, but implementing it properly is equally essential.

Here are some recommended practices for embedded software developers:

1. Keep ISRs Short

Avoid performing lengthy calculations, blocking delays, or complex communication operations inside an ISR. Set a flag or store the data and handle the remaining processing in the main loop.

2. Use Volatile for Shared Variables

Variables shared between an ISR and the main program should generally be declared volatile so the compiler does not optimize away necessary memory accesses.

However, volatile alone does not guarantee atomic access or prevent race conditions.

3. Avoid Unnecessary Polling

Do not continuously check hardware devices when the application does not require constant monitoring.

4. Configure Interrupt Priorities Carefully

Time-critical interrupts should receive appropriate priority based on system requirements.

5. Consider Debouncing

Mechanical switches can generate multiple electrical transitions during a single press. Use suitable hardware or software debouncing techniques.

6. Measure System Performance

Evaluate CPU utilization, interrupt latency, response time, and power consumption to determine whether the selected approach meets application requirements.

8. Conclusion

Press there oops or polling are used in microcontroller programming in order to know when one more element in the data device occurs.

Polling is easier to implement and works for simple applications, periodic sensor monitoring, and noncritical operations. Interrupts are better suited to event-driven systems, communication interfaces, time-sensitive operations, and low power applications.

There is no rule that one way will always be the best way. The answer to which one is better is derived from the application functionality, response time, processor resources, power consumption, software complexity.

For embedded developers, it is essential to master both of these techniques to design more reliable, efficient and responsive microcontroller-based systems.

No matter if you work with ARM, AVR, PIC, STM32, or ESP32 microcontrollers, knowing when to choose interrupts, polling, or a combination of both will assist your use of embedded C code.