How do MCUs handle external interrupts?

Let%27s explore how microcontrollers (MCUs) handle external interrupts in detail.

## Understanding External Interrupts

External interrupts allow MCUs to respond to external events or signals. When a specific condition occurs (e.g., a button press, a sensor reading), the MCU interrupts its normal execution flow and jumps to a predefined interrupt service routine (ISR). Here are the key aspects of handling external interrupts:

1. Interrupt Sources:
- External interrupts can originate from various sources:
- GPIO Pins: External signals connected to GPIO pins (e.g., buttons, sensors).
- Peripheral Modules: Hardware peripherals (e.g., timers, UART, SPI) can generate interrupts.
- External Devices: Signals from external devices (e.g., sensors, communication interfaces).

2. Interrupt Vector Table (IVT):
- The MCU maintains an IVT that maps interrupt sources to their corresponding ISRs.
- Each interrupt source has a unique vector address pointing to its ISR.
- When an interrupt occurs, the MCU looks up the vector address and jumps to the appropriate ISR.

3. Interrupt Prioritization:
- MCUs prioritize interrupts based on their importance.
- Higher-priority interrupts preempt lower-priority ones.
- Developers can configure interrupt priorities to ensure critical tasks are handled promptly.

4. Interrupt Service Routine (ISR):
- The ISR is a function that executes when an interrupt occurs.
- It performs specific actions related to the interrupt source.
- ISRs should be concise, efficient, and non-blocking.

5. Debouncing:
- Mechanical switches or buttons can generate noisy signals (multiple transitions) during a single press.
- Debouncing techniques (e.g., software delay, hardware RC circuits) prevent false interrupts due to noise.

6. Nested Interrupts:
- MCUs support nested interrupts.
- If an interrupt occurs while handling another interrupt, the MCU saves the context and jumps to the new ISR.
- Proper nesting ensures timely handling of critical events.

7. Interrupt Enable/Disable:
- Developers enable or disable specific interrupts as needed.
- Disabling interrupts during critical sections (e.g., updating shared variables) prevents race conditions.

8. Edge Detection:
- External interrupts can be triggered by rising edges, falling edges, or both.
- The MCU configures the trigger condition based on the application requirements.

## Use Cases

1. Button Inputs:
- External interrupts handle button presses (e.g., for user interfaces).
- A button press generates an interrupt, allowing the MCU to respond immediately.

2. Sensor Inputs:
- Sensors (e.g., motion sensors, temperature sensors) trigger external interrupts.
- Real-time data acquisition and processing rely on interrupt-driven handling.

3. Communication Interfaces:
- UART, SPI, and I2C peripherals generate interrupts upon data reception.
- External devices communicate asynchronously with the MCU.

## Conclusion

External interrupts enhance MCU responsiveness, reduce polling overhead, and enable real-time event handling. Proper configuration and efficient ISRs are essential for effective interrupt management.

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References:
1. [Interrupts - Part 1: External interrupts • Wolles Elektronikkiste](https://wolles-elektronikkiste.de/en/interrupts-part-1-external-interrupts) ¹
2. [Multiple interrupt sources management for ST7 MCUs - STMicroelectronics](https://www.st.com/resource/en/application_note/an1044-multiple-interrupt-sources-management-for-st7-mcus-stmicroelectronics.pdf) ²
3. [Solved: How are multiple interrupts handled when they are executed?](https://community.st.com/t5/stm32-mcus-embedded-software/how-are-multiple-interrupts-handled-when-they-are-executed/td-p/299745) ³

(1) Interrupts - Part 1: External interrupts • Wolles Elektronikkiste. https://wolles-elektronikkiste.de/en/interrupts-part-1-external-interrupts.
(2) Multiple interrupt sources management for ST7 MCUs - STMicroelectronics. https://www.st.com/resource/en/application_note/an1044-multiple-interrupt-sources-management-for-st7-mcus-stmicroelectronics.pdf.
(3) Solved: How are multiple interrupts handled when they are .... https://community.st.com/t5/stm32-mcus-embedded-software/how-are-multiple-interrupts-handled-when-they-are-executed/td-p/299745.

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