Polling vs. Interrupts in Microcontrollers: Making the Right Choice in Embedded Systems

Table of Contents

1. Introduction
2. What is Polling?
3. Understanding Interrupts
4. Key Differences Between Polling and Interrupts
5. When to Use Polling
6. When to Use Interrupts
7. Real-Life Example: Button-Press Detection
8. Performance and Power Considerations
9. Conclusion

Introduction

Microcontrollers are the heartbeat of embedded systems, responding to real-world events and managing hardware through a series of control structures. Two fundamental methods for handling peripheral communication and external events are Polling and Interrupts. While both have their place in the developer’s toolbox, knowing when and how to use each can drastically affect performance, responsiveness, and power consumption.

What is Polling?

Polling is a method where the CPU continuously checks the status of a device or flag in a loop. This means the processor remains active, repeatedly querying peripherals for data or status updates, even when nothing has changed.

Code Example: Polling for a Button Press

Pros:

• Simple and easy to implement
• Good for short, predictable tasks
• No need for extra hardware or interrupt configuration

Cons:

• CPU stays busy, wasting cycles
• Poor power efficiency
• Not scalable when handling multiple inputs

Understanding Interrupts

Interrupts allow the microcontroller to respond to external or internal events only when they occur. Instead of continuously checking a flag, the MCU “waits” and gets interrupted only when a specified event happens. This leads to more efficient CPU usage and quicker response time for time-critical events.

Code Example: Interrupt-Based Button Detection (AVR)

Pros:

• Efficient use of CPU resources
• Better for real-time applications
• Lowers power consumption in sleep modes

Cons:

• Slightly more complex to implement
• Improper use can lead to missed or nested interrupts
• Must handle ISR timing carefully to avoid blocking

Key Differences Between Polling and Interrupts

When to Use Polling

• When the system is simple or has minimal peripherals
• When consistent sampling is required (e.g., ADC at fixed intervals)
• In systems where power isn’t a concern
• During debugging or early prototyping stages

When to Use Interrupts

• In real-time systems where response time is critical
• For asynchronous events like UART reception or external GPIO changes
• In battery-powered devices that rely on power-saving modes
• When handling multiple peripherals simultaneously

Real-Life Example: Button-Press Detection

Imagine you're developing a battery-powered door lock. Using polling to constantly check the keypad or door sensor would drain power quickly. By using interrupts, the MCU can remain in a low-power state and only wake up when a button is pressed, significantly extending battery life.

Performance and Power Considerations

Polling continuously uses clock cycles, which can be a major drawback in energy-sensitive applications. Interrupts allow the system to sleep until needed, reducing power consumption and allowing the CPU to allocate time more effectively across tasks. However, care must be taken with Interrupt Service Routines (ISRs) to avoid excessive processing or blocking other interrupts.

Conclusion

Choosing between polling and interrupts is not just a technical decision—it's an architectural one. Polling offers simplicity and control but at the cost of CPU time and power. Interrupts, while more complex, provide a responsive, efficient, and scalable solution for modern embedded systems.

A well-designed embedded application often uses a combination of both. For instance, polling might be used for low-priority tasks in the main loop, while interrupts handle urgent or time-critical events.

Understanding the strengths and trade-offs of each method is key to designing robust, responsive, and energy-efficient systems.