Interrupt Flag in Embedded C: What It Is and How It Works

In Embedded C, an interrupt flag is a special bit set by hardware to signal that an interrupt event has occurred. It tells the microcontroller to pause normal execution and run an interrupt service routine (ISR) to handle the event.

How It Works

Think of the interrupt flag as a small signal light inside your microcontroller. When a specific event happens, like a timer reaching zero or data arriving on a communication line, the hardware sets this flag to 'on'. This flag tells the microcontroller, "Hey, something needs your attention!"

When the interrupt flag is set, the microcontroller temporarily stops what it is doing and jumps to a special function called an Interrupt Service Routine (ISR). The ISR handles the event quickly, like answering a phone call, then clears the interrupt flag to turn off the signal light. After that, the microcontroller goes back to its normal work.

Example

This example shows how an interrupt flag can be checked and cleared in Embedded C for a simple timer interrupt.

c

#include <stdint.h>
#include <stdbool.h>

volatile bool timer_interrupt_flag = false;

// Simulated hardware register for interrupt flag
volatile uint8_t INTERRUPT_FLAG_REGISTER = 0;
#define TIMER_INTERRUPT_BIT 0x01

// Interrupt Service Routine (ISR) simulation
void timer_ISR() {
    // Clear the interrupt flag in hardware register
    INTERRUPT_FLAG_REGISTER &= ~TIMER_INTERRUPT_BIT;
    // Set software flag to indicate interrupt handled
    timer_interrupt_flag = true;
}

int main() {
    // Simulate timer interrupt event by setting the flag
    INTERRUPT_FLAG_REGISTER |= TIMER_INTERRUPT_BIT;

    // Check if interrupt flag is set
    if (INTERRUPT_FLAG_REGISTER & TIMER_INTERRUPT_BIT) {
        timer_ISR(); // Handle interrupt
    }

    if (timer_interrupt_flag) {
        // Interrupt was handled
        return 0; // Success
    } else {
        return 1; // No interrupt
    }
}

When to Use

Interrupt flags are used whenever your microcontroller needs to respond quickly to external or internal events without constantly checking for them. This is called interrupt-driven programming.

For example, use interrupt flags to:

• Detect when a button is pressed without wasting time checking it repeatedly.
• Handle data arriving on a serial port immediately.
• Respond to timer events for precise timing tasks.

This approach saves power and processing time because the microcontroller can do other tasks or sleep until an interrupt occurs.

Key Points

• An interrupt flag is a hardware bit that signals an interrupt event.
• The microcontroller checks this flag to run the interrupt service routine.
• The ISR clears the flag to acknowledge the event.
• Using interrupt flags allows efficient and responsive embedded programs.

Key Takeaways

An interrupt flag signals the microcontroller that an interrupt event has occurred.

The interrupt service routine (ISR) runs when the interrupt flag is set and clears it after handling.

Interrupt flags enable efficient event-driven programming in embedded systems.

Use interrupt flags to respond quickly to hardware events without constant polling.