Introduction to RTOS Part 9 - Hardware Interrupts | Digi-Key Electronics

Summary

This tutorial dives into integrating hardware interrupts with FreeRTOS on the ESP32 microcontroller, focusing on configuring hardware timers and handling interrupts efficiently. Key topics include interrupt service routines (ISRs), synchronization between tasks and ISRs, critical sections, semaphores, and managing shared data. Practical examples include a blinky LED with timer interrupts and an advanced challenge to sample ADC values and compute averages in real time. The tutorial emphasizes safe data access techniques and provides valuable insights into interrupt handling, making it ideal for embedded system developers looking to improve task synchronization and real-time data processing.

Takeaways

• 😀 Hardware interrupts are essential in embedded systems for responding to asynchronous events such as timers or user inputs.
• 😀 Interrupt service routines (ISRs) typically have a higher priority than tasks in FreeRTOS, ensuring they interrupt tasks when triggered.
• 😀 FreeRTOS allows for hardware interrupts to pause tasks, forcing the CPU to handle the ISR before resuming task execution.
• 😀 Microcontrollers like the ESP32 have built-in peripherals that generate interrupts, such as timers and communication buses, to notify the CPU of certain events.
• 😀 To manage synchronization between ISRs and tasks, spin locks and critical sections are often used to protect shared resources in a FreeRTOS environment.
• 😀 In FreeRTOS, the `volatile` qualifier is important for variables shared between ISRs and tasks to prevent the compiler from optimizing them out.
• 😀 Special ISR synchronization techniques, such as using semaphores and direct-to-task notifications, help communicate data between ISRs and tasks without blocking the ISR.
• 😀 ISRs should remain as short as possible to avoid blocking other interrupts and prevent delayed or missed event handling.
• 😀 FreeRTOS provides APIs such as `from_isr` functions that help manage synchronization from within an ISR, ensuring non-blocking behavior.
• 😀 A common use case for interrupt and task synchronization is collecting sensor data in an ISR and processing it in a task, such as calculating averages of ADC readings.

Q & A

What is the primary function of hardware interrupts in embedded systems?

Hardware interrupts allow a microcontroller to respond to asynchronous events, such as a timer expiring or a button being pressed, by temporarily halting the current task to handle the event in an Interrupt Service Routine (ISR).

How does FreeRTOS prioritize hardware interrupts compared to tasks?

In FreeRTOS, hardware interrupts have a higher priority than any task, regardless of the task's priority. This ensures that when an interrupt occurs, the processor immediately executes the associated ISR, pausing the execution of the current task.

What is the significance of the 'volatile' keyword when working with variables in ISRs?

The 'volatile' keyword tells the compiler that the value of a variable may be changed outside the scope of the current execution, particularly by an ISR. This prevents the compiler from optimizing the variable, ensuring it is properly updated in the ISR.

What is a spinlock, and how is it used in FreeRTOS?

A spinlock is a type of lock that prevents other tasks in the system from entering a critical section by continuously checking if the lock is available. It’s used in FreeRTOS to protect shared resources and avoid race conditions, especially when working with multiple cores.

What is the role of critical sections in FreeRTOS, and how do they help with interrupt synchronization?

Critical sections temporarily disable interrupts to protect shared resources from being accessed by multiple tasks simultaneously. This ensures that operations on critical data are atomic and that race conditions are avoided.

What is the purpose of using a binary semaphore in the context of ISRs and FreeRTOS tasks?

A binary semaphore is used to signal between an ISR and a FreeRTOS task. The ISR 'gives' the semaphore when it completes its work, which unblocks a waiting task and allows it to process the data or perform further actions.

What is the difference between a regular mutex and a spinlock in the context of FreeRTOS?

A mutex can block tasks, allowing the system to efficiently wait for resources, whereas a spinlock causes the task to continuously check for resource availability without blocking, which can be useful for critical sections where immediate action is needed.

How does task synchronization improve the reliability of embedded systems that use ISRs?

Task synchronization ensures that tasks and ISRs work together smoothly without interference. By using mechanisms like semaphores or task notifications, data processed by an ISR can be safely handled by tasks, improving the reliability and predictability of embedded systems.

What are direct-to-task notifications in FreeRTOS, and how do they improve ISR-task communication?

Direct-to-task notifications allow tasks to be notified more quickly and efficiently compared to semaphores. They provide a fast, lightweight mechanism for notifying a specific task that some data or event is ready, reducing the overhead of using traditional synchronization methods like semaphores.

What challenge is presented at the end of the video, and what are the key requirements for the solution?

The challenge is to create a system where an ISR samples data from an ADC 10 times per second, stores it in a buffer, and wakes up a task to compute the average of these samples. Task B then handles serial communication, allowing the user to retrieve the average when prompted. The system requires proper synchronization between the ISR and tasks to manage data safely.