Polling and interrupts are two different mechanisms used in computer systems to help facilitate the communication and synchronization between devices. Polling involves continually checking the status of a device, while interrupts allow devices to signal when an event needs immediate attention. In this post, we'll further explore the differences between polling and interrupts, examining their advantages and disadvantages, as well as different applications where each may be used.

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Purpose of Polling and Interrupts

Polling and interrupts are used to efficiently manage and handle communication between the CPU and other devices such as I/Os and sensors in computing systems. Implementing such mechanisms helps to ensure timely responses to events and enhanced system performance. While polling and interrupts are used for a similar purpose, they each have different operation methods that come with their own advantages and disadvantages.

What is Polling?

Polling is a method where a device continuously checks the status of another device to determine if it needs attention. This involves sending requests or queries to the device, waiting for a response, and repeating this process in a loop until the desired condition or data is obtained.

For example, a microcontroller can poll a sensor to read its value or poll a serial port to see if there is any data available.

This can be analogous in a classroom setting where a teacher individually asks each student if they have a question. This method ensures that every student's question/input is attended to, but it can be time-consuming as the teacher needs to check with each student separately.

Advantages and Disadvantages of Polling

Polling is considered to be easy to implement and provides control over the data acquisition process. However, this often comes at the cost of increased resource usage and higher latency.

• Simplicity: It is a simpler mechanism to implement and provides straightforward control over the process.
• Compatibility: Polling can work on systems that do not support interrupt-driven mechanisms, making it widely applicable across various platforms.
• Predictability: Since the system checks for data at regular intervals, the behavior is predictable and can be fine-tuned for specific performance requirements.

• Resource Intensive: Constantly checking for data or events consumes system resources, including CPU cycles, leading to inefficiency and wasted power.
• Latency: There can be a delay between the occurrence of an event and the system detecting it, especially if the polling interval is long, leading to higher latency in event processing.
• Complexity in Real-Time Systems: In real-time applications where immediate responses are critical, polling may not be suitable.

What Are Interrupts?

Interrupts are signals sent by devices to the CPU to indicate that they need attention. When a device has data ready or an event occurs, it sends an interrupt signal to the CPU, causing the CPU to stop its current activities and attend to the interrupting device.

For example, a microcontroller can be interrupted by a sensor when it has a new value or by a serial port when it receives data.

Interrupts can be either hardware or software based. Hardware interrupts are triggered by external devices like keyboards, while software interrupts are generated by software components running on the processor.

Interrupts can compare to when a student raises their hand to ask a question. This signal immediately captures the teacher's attention, allowing for a quick response without needing to check every student individually, therefore making the process more efficient.

Advantages and Disadvantages of Interrupts

Interrupts offer real-time responsiveness and efficient multitasking capabilities but come with the trade-offs of complexity and potential disruptions. Proper design and handling are crucial to leverage the benefits of interrupts effectively.

• Efficiency: Systems that employ interrupts are generally more efficient as the CPU can focus on other tasks while waiting for external events, rather than constantly polling for them.
• Reduced Latency: Interrupt-driven communication can reduce latency. Devices can be serviced immediately when they have data to transfer, leading to faster response times.
• Real-time Responsiveness: Interrupts enable immediate responses to external events, making them essential for real-time applications where timely reactions are critical.
• Prioritization: Interrupts can be prioritized, allowing the system to respond to the most important events first, ensuring essential tasks are handled promptly.

• Complexity: Implementing and managing interrupts can be complex, involving intricate synchronization and handling mechanisms, especially in multi-device environments.
• Task Interference: In some cases, interrupts can restrict the simultaneous execution of tasks as the CPU's attention is frequently diverted to handle interrupt requests.
• Race Conditions: Improper handling of interrupts can lead to race conditions, where the order of execution becomes unpredictable, potentially causing unexpected behavior in the system.

When to Use Polling vs Interrupts

The choice between polling and interrupts depends on the specific requirements of the application.

Polling is ideal for events that occur at consistent and predictable intervals, allowing the system to check for updates periodically. It is also suitable for simple applications where a constant check for events doesn't significantly impact system performance and where a straightforward implementation is preferred.

For example, a smart home device such as a thermostat may use polling to periodically check the sensor data to monitor changes in the room temperature and adjust the temperature based on user settings.

Interrupts, on the other hand, are more ideal for when immediate responses to external events are critical, such as real-time systems. They are also best for event-driven applications where variable event timing is common.

Anti-lock braking systems (ABS) in cars is an example of how interrupts may be used in real-time systems. In this application, the sensors continuously monitor wheel speeds, and interrupts are used to alert the ABS controller when a wheel is about to lock. This instant notification ensures timely adjustments to prevent accidents.

Polling and Interrupts in Communication Protocols

In communication protocols such as I2C, SPI, USB, and CAN, both polling and interrupts are vital mechanisms in facilitating efficient data exchange in embedded systems.

Total Phase offers a line of host adapters and protocol analyzers for I2C, SPI, USB, CAN, and eSPI systems to help engineers streamline their debugging and developing efforts. Our host adapters allow engineers to emulate entire systems, including master and slave devices, to test and validate operations and responses between each other. Our protocol analyzers provide advanced visibility into the bus, allowing users to view bus communication and any errors in real time. These tools allow users to confirm systems and their operations, including polling or interrupt mechanisms, are working as expected.

I2C/SPI Host Adapters

The Aardvark I2C/SPI Host Adapter is well-suited for general-purpose testing and system emulation, the Cheetah SPI Host Adapter is specialized for high-speed memory programming, and the Promira Serial Platform is best for advanced speeds and applications.

I2C/SPI, USB, and eSPI Protocol Analyzers

For testing and debugging I2C, SPI, USB, and eSPI systems, Total Phase offers protocol analyzers that monitor bus traffic in real time. This allows engineers to verify communication between devices and ensure there are no protocol errors or bugs.

The Beagle I2C/SPI Protocol Analyzer can non-intrusively monitor I2C and SPI traffic, and our line of Beagle USB Protocol Analyzers can monitor a range of USB speeds, each with their own unique features and capabilities for debugging specific USB applications. Total Phase also offers an eSPI Analysis Application for the Promira Serial Platform to easily monitor eSPI traffic in real time.

CAN Interface

For CAN system development, Total Phase offers the Komodo CAN Duo Interface, which can act as both a non-intrusive bus monitor as well as an active CAN adapter simultaneously.

Conclusion

In summary, polling and interrupts are both pivotal mechanisms in ensuring efficient communication among devices within computer systems. While polling involves constant status checks, interrupts enable devices to promptly signal for urgent attention. This exploration of their differences, insight into their advantages and disadvantages, and when best to use each provides a better understanding of their essential roles in responsive and optimized computing systems.

For more information on how our tools can be helpful in your embedded applications, please email us at sales@totalphase.com.