Embedded systems often rely on the use of interrupts to handle time-critical events and external inputs. Interrupts allow the processor to suspend its current execution and handle an event that requires immediate attention. In this article, we will explore how to handle interrupts in C++ for embedded systems.

Understanding Interrupts

Interrupts can be triggered by various sources such as timers, external hardware signals, or communication interfaces. When an interrupt occurs, the processor suspends its current task, saves its execution context, and jumps to the interrupt service routine (ISR) associated with that interrupt.

Defining an ISR

In C++, an ISR is typically defined as a static member function within a class or as a standalone function. It is important to note that ISRs should be kept short and simple, as they execute in an interrupt context and can disrupt regular program flow.

Here’s an example of defining and implementing an ISR in C++ using the ARM Cortex-M architecture:

class InterruptHandler {
public:
    static void ISR() {
        // ISR implementation
    }
};

// Example of an actual interrupt vector table entry
extern "C" void EXTI0_IRQHandler() {
    InterruptHandler::ISR();
}

The InterruptHandler class defines the static ISR function, which can be called by the actual interrupt vector table entry. The extern "C" declaration is used to ensure correct linkage for C++ code.

Registering the ISR

Once the ISR is defined, it needs to be registered with the interrupt controller. The exact procedure to register an ISR depends on the specific microcontroller and development environment being used.

Typically, the steps involve:

1. Identifying the interrupt source and corresponding interrupt number.
2. Setting up the interrupt controller to enable the corresponding interrupt.
3. Mapping the interrupt number to the ISR function.

Here’s a simplified example of how to register an ISR using the ARM Cortex-M architecture and the STM32 HAL (Hardware Abstraction Layer) library:

// Enable interrupt for EXTI0 line
HAL_NVIC_SetPriority(EXTI0_IRQn, 0, 0);
HAL_NVIC_EnableIRQ(EXTI0_IRQn);

// Map the interrupt number to the ISR function
InterruptVectorTable[EXTI0_IRQn] = InterruptHandler::ISR;

In this example, the HAL_NVIC_SetPriority and HAL_NVIC_EnableIRQ functions are used to enable and set the priority for the EXTI0 interrupt. The InterruptVectorTable is an array of function pointers representing the interrupt vector table, where the ISR function is mapped to the corresponding interrupt number.

Interrupt Service Routine Implementation

Inside the ISR, it is crucial to handle the interrupt in a timely manner and perform only the necessary operations. Complex or time-consuming operations should be avoided as they can delay the execution of other time-sensitive tasks.

Here are a few guidelines for implementing ISRs in embedded systems:

• Keep the ISR short and focused.
• Avoid blocking or lengthy operations.
• Use non-blocking techniques, such as flags or queues, to communicate with the non-interrupt context of the application.
• Ensure proper atomicity when accessing shared data between the ISR and other parts of the code.

Conclusion

Interrupt handling is a crucial aspect of embedded systems programming. By understanding the basics of interrupt handling in C++ and following best practices for ISR implementation, developers can effectively handle time-critical events and improve the responsiveness of their embedded applications.

#EmbeddedSystems #InterruptHandling