Detailed analysis of the interrupt of the microcontroller

When the MCU CPU is processing an event A, another event B may occur and request immediate attention from the CPU (interrupt happens). The CPU temporarily pauses its current task and moves to handle event B (interrupt response and service). After completing the handling of event B, the CPU returns to where it left off with event A and continues processing it (interrupt return). This entire process is known as an interrupt.

For example, imagine you're washing clothes when your phone rings. This interruption causes you to pause your laundry and answer the call. Once the call is over, you return to your laundry. This scenario illustrates how an interrupt works in real life.

MCU interrupts are typically divided into two categories: internal and external. External interrupts are generated by devices connected to the microcontroller, such as sensors or buttons. These signals are transmitted through specific input/output pins of the MCU. Most MCUs have one or more IO ports that can detect external interrupt signals when set to input mode. There are five common conditions for triggering an external interrupt: a high-to-low transition, low-to-high transition, high level, low level, or a change in state.

To use an external interrupt, the peripheral device must provide a signal that matches the type supported by the MCU. However, some trigger types like level-based signals may not work continuously unless the signal changes state. For instance, if a device triggers an interrupt via a falling edge, it needs to reset the signal after the interrupt is processed so that the next interrupt can be triggered properly.

Some peripherals include an interrupt clear mechanism, which allows the MCU to reset the interrupt signal and enable subsequent triggers. This ensures that the interrupt can be used repeatedly without manual intervention.

External interrupts also support advanced features such as pulse width measurement or counting, which are built on top of basic trigger modes. These techniques are less commonly used but can be useful in specialized applications.

Internal interrupts, on the other hand, are generated by modules within the MCU itself, such as timers, UARTs, or ADCs. These interrupts do not rely on external pins but instead use internal flags stored in registers. When an internal interrupt occurs, the corresponding flag is set, and it must be cleared manually or automatically depending on the MCU's design.

Interrupt control is managed through various registers that allow developers to enable or disable specific interrupts. A global interrupt enable bit is usually present, and individual bits control each interrupt source. To ensure stability, it’s important to disable unused interrupts at startup to prevent unintended behavior.

Each interrupt requires a dedicated service routine, which is a section of code that handles the interrupt. If no routine is provided, the MCU may crash or behave unpredictably. Developers often write only the necessary routines to save memory space and improve efficiency.

The MCU uses an interrupt vector table to locate the correct service routine when an interrupt occurs. Some MCUs have fixed addresses for each interrupt, while others use a single address with an interrupt number to identify the source. This affects the speed and complexity of interrupt handling.

Interrupts can also have priority levels, allowing higher-priority interrupts to preempt lower ones. This feature is not available on all MCUs but is essential in systems requiring fast response times. Nesting allows multiple interrupts to be handled sequentially, ensuring critical tasks are addressed first.

In summary, interrupts are a powerful tool for managing asynchronous events in microcontroller systems. Understanding their types, handling methods, and configuration options is crucial for developing efficient and reliable embedded applications.