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What Is Interrupt Handling in Embedded Systems?

Interrupt handling is a key feature in embedded systems that allows a microcontroller to respond immediately to important events. Instead of constantly checking for inputs, interrupts improve system efficiency and responsiveness—making them essential for real-time and embedded applications.

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What Is an Interrupt?

An interrupt is a signal that temporarily halts the main program’s execution so the system can respond to an external or internal event. After handling the event, the processor resumes its previous task. Interrupts make systems more responsive and power-efficient.

Examples of events that can trigger interrupts:

• A button press
• Sensor input (e.g., temperature threshold crossed)
• Timer overflow
• UART data received

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How Interrupt Handling Works

When an interrupt occurs:

1. The current execution is paused.
2. The system jumps to a specific function called the Interrupt Service Routine (ISR).
3. The ISR handles the event (e.g., reads sensor data).
4. Once complete, the system returns to the paused task.

This process is called interrupt handling.

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Types of Interrupts

✅ Hardware Interrupts

Triggered by external devices such as buttons, sensors, or communication modules (UART, SPI).

✅ Software Interrupts

Triggered by software instructions within the code—commonly used for system-level operations or triggering test conditions.

✅ Maskable Interrupts (IRQ)

Can be disabled (masked) by software when necessary.

✅ Non-maskable Interrupts (NMI)

High-priority interrupts that cannot be disabled. Used for critical situations like hardware failure or system reset.

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Key Terms in Interrupt Handling

• ISR (Interrupt Service Routine): The special function that executes when an interrupt occurs.
• Interrupt Vector Table: A table that stores the addresses of all ISRs.
• Context Saving: Saving the current program state before jumping to the ISR.
• Latency: The time between interrupt generation and ISR execution.
• Nesting: Handling new interrupts while another ISR is still running (can be risky if not managed well).

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Advantages of Interrupt Handling

• ✅ Real-Time Response: Interrupts allow the system to react immediately to critical inputs.
• ✅ Energy Efficiency: The processor can sleep and wake up only when needed.
• ✅ CPU Optimization: Reduces CPU load by avoiding constant polling.
• ✅ Multi-tasking Support: Enables better handling of multiple events.

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Polling vs Interrupts

Feature	Polling	Interrupts
CPU Usage	High – Constantly checks input	Low – Responds only when needed
Efficiency	Less efficient	Highly efficient
Complexity	Simpler to implement	Requires ISR and vector setup
Response Time	Delayed	Immediate

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How to Implement Interrupts (Example: ARM Cortex-M)

Using STM32 or Cortex-M microcontrollers, interrupt handling usually involves:

1. Enabling the peripheral interrupt (e.g., GPIO, Timer, UART).
2. Writing the ISR function.
3. Setting the priority using NVIC (Nested Vectored Interrupt Controller).
4. Clearing the interrupt flag within the ISR to avoid retriggering.

Example in C (simplified):

void EXTI0_IRQHandler(void) {
  if (EXTI->PR & EXTI_PR_PR0) {
    // Handle button press
    EXTI->PR |= EXTI_PR_PR0; // Clear interrupt flag
  }
}

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Challenges in Interrupt Handling

• ISR must be short and fast: Long ISRs can block other tasks or interrupts.
• Shared resource conflicts: If multiple ISRs or the main loop access the same data.
• Interrupt storm: Multiple interrupts in quick succession can overload the CPU.
• Debugging complexity: Harder to trace flow due to non-linear execution.

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Best Practices

• Keep ISRs as short and efficient as possible.
• Use volatile variables for shared data between ISR and main code.
• Use priorities wisely if multiple interrupts are used.
• Always clear interrupt flags to avoid re-triggering.

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Final Thoughts

Interrupt handling is a foundational concept in embedded systems. It allows your microcontroller to be fast, efficient, and real-time responsive. Whether you’re designing a sensor-based IoT system or a complex automotive control unit, mastering interrupts is key to effective embedded programming.

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