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5.7 Choosing the Right Tool: A Practical Comparison

Deciding which synchronization mechanism to use is a key skill in embedded programming. Use the following table and questions as a guide.

Mechanism Purpose Transfers Data? When to Use Primary Risk
Critical Section Mutual Exclusion No Protecting a few lines of code that modify a shared variable. Must be extremely fast. Halts system responsiveness by disabling all interrupts. Can easily break real-time deadlines.
Semaphore Signaling No Notifying a task that a specific event has occurred. Deferring ISR work to a task. Does not help with transferring the actual data associated with the event.
Queue Data Transfer Yes Sending data of any type from an ISR to a task for processing. Minor overhead for copying data into the queue. May not be suitable for very large data structures.

Decision-Making Guide

When designing an interaction between an ISR and a task, ask yourself these questions:

  1. Do I need to pass data from the ISR to the task?

    • Yes: Use a Queue. This is the safest and most robust solution for transferring data.

    • No: Go to question 2.

  2. Is my goal simply to wake up a task to do some work when an interrupt occurs?

    • Yes: Use a Semaphore. It is a lightweight and highly efficient signaling mechanism.

    • No: Go to question 3.

  3. Do I only need to protect a single, simple variable (like a counter or flag) during a very quick read-modify-write operation?

    • Yes: A Critical Section is an option, but only if the operation is genuinely just a few lines of code. Be aware of the impact on system latency.

    • No: Re-evaluate your design. You likely need a semaphore or a queue.

In modern RTOS development, Queues and Semaphores are almost always preferred over Critical Sections for managing ISR-task interactions. They provide cleaner, safer, and more scalable solutions that have less impact on the overall real-time performance of your system.

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