Implementing Efficient Interrupt Service Routine Handlers

In this lesson, you will learn how to write lean, high-speed Interrupt Service Routines (ISRs) to keep your embedded system responsive and prevent hardware event bottlenecks.

The Goal of a Lean ISR

When a hardware peripheral triggers an interrupt, the processor pauses its current work to execute your ISR. Because the system is effectively "on hold" during this time, any delay in your code can cause missed events, jittery timing, or even system instability. To stay efficient, follow the "short and sweet" rule: perform the absolute minimum work necessary to clear the hardware flag and hand off data.

Keep it Minimal

Your ISR should never include heavy logic, long calculations, or blocking functions (like printf or delay loops). Instead, treat the ISR as a fast bridge. If you receive data, move it from the peripheral register to a buffer or set a flag for the main() loop to process later.

Consider this inefficient vs. efficient approach:

// INEFFICIENT: Heavy processing inside ISR
void EXTI0_IRQHandler(void) {
    if (GPIO_Read_Bit(PORTA, 0)) {
        uint32_t data = UART_Read();
        // Do heavy filtering or math here...
        // This keeps the CPU in the ISR too long!
        UART_Send(process(data)); 
        Clear_Interrupt_Flag();
    }
}

// EFFICIENT: Quick hand-off
volatile uint32_t raw_data;
volatile bool data_ready = false;

void EXTI0_IRQHandler(void) {
    // 1. Capture the data immediately
    raw_data = UART_Read();
    
    // 2. Set a flag for main() to handle the rest
    data_ready = true;
    
    // 3. Clear hardware flag and exit
    Clear_Interrupt_Flag();
}

Why This Matters

By offloading the "heavy lifting" to your main() loop, you ensure that the processor returns to normal execution as quickly as possible. This allows the system to remain responsive to other higher-priority interrupts. Always keep your ISRs focused solely on the hardware handshake and data acquisition; let your background tasks handle the complexity.

Key Takeaway: To maintain system responsiveness, keep ISRs extremely brief by offloading data processing to the main loop and exiting as quickly as the hardware allows.