Implementing Deferred Interrupt Processing Patterns

Implementing Deferred Interrupt Processing Patterns

In this lesson, you will learn how to maintain system responsiveness by moving heavy-duty logic out of time-critical Interrupt Service Routines (ISRs) and into background tasks.

The Problem: ISR Bloat

When a hardware interrupt triggers, the CPU suspends all other task execution to run your ISR. If your ISR performs complex tasks—like processing a data packet, updating a display, or writing to a file system—it blocks the rest of your system. This "interrupt latency" can cause your RTOS to miss deadlines for other critical tasks, leading to instability or jitter.

The Solution: Deferred Processing

To keep your ISR lean, use the Deferred Interrupt Processing pattern. The ISR does the absolute minimum required: it clears the hardware interrupt flag and "signals" a high-priority task to handle the actual work. By offloading the processing to a task, you allow the RTOS scheduler to manage execution, ensuring that the interrupt-handling work doesn't starve other critical system components.

Implementation Example

Typically, you use a binary semaphore or a task notification to wake the worker task.

// High-priority worker task
void ProcessingTask(void *pvParameters) {
    while(1) {
        // Task blocks here until the ISR "gives" the semaphore
        if (xSemaphoreTake(xInterruptSemaphore, portMAX_DELAY)) {
            // Perform complex processing here
            HandleData(buffer);
        }
    }
}

// ISR (Minimal execution)
void Hardware_IRQHandler(void) {
    BaseType_t xHigherPriorityTaskWoken = pdFALSE;
    
    // Clear hardware interrupt flag
    ClearHardwareFlag();

    // Wake the processing task
    xSemaphoreGiveFromISR(xInterruptSemaphore, &xHigherPriorityTaskWoken);

    // Request context switch if the task we just woke has higher priority
    portYIELD_FROM_ISR(xHigherPriorityTaskWoken);
}

In this model, the ISR runs for only a few microseconds, while the ProcessingTask runs whenever the scheduler permits. This architectural choice prevents your hardware interrupts from becoming bottlenecks and makes your real-time system significantly more predictable and robust under heavy load.

Key Takeaway: Never perform complex logic inside an ISR; use semaphores or notifications to hand off work to a dedicated task to ensure system-wide responsiveness.