Interrupt priority levels in Embedded C

Introduction

Interrupt priority levels help decide which interrupt to handle first when many happen at the same time. This keeps your program running smoothly and quickly.

When multiple hardware devices can send signals to the processor at the same time.

When you want urgent tasks to stop less important tasks immediately.

When managing time-critical operations like sensors or communication.

When you want to avoid missing important events by handling them first.

When designing embedded systems that must respond quickly and reliably.

Syntax

Embedded C

/* Example syntax to set interrupt priority in embedded C */
void set_interrupt_priority(int interrupt_number, int priority_level) {
    // platform-specific code to assign priority
}

Priority levels are usually numbers; lower or higher numbers mean higher priority depending on the system.

Setting priorities is hardware and compiler specific; check your microcontroller's manual.

Examples

This sets the timer interrupt to priority level 1, which is the highest priority in this example.

Embedded C

/* Set Timer interrupt to highest priority */
set_interrupt_priority(TIMER_INTERRUPT, 1);

This sets the UART interrupt to priority level 5, lower than the timer interrupt.

Embedded C

/* Set UART interrupt to lower priority */
set_interrupt_priority(UART_INTERRUPT, 5);

Sample Program

This program simulates setting interrupt priorities for 3 interrupts and finds which one has the highest priority (lowest number). It then prints the interrupt number with the highest priority.

Embedded C

#include <stdio.h>

// Simulated interrupt priority levels
int interrupt_priorities[10];

void set_interrupt_priority(int interrupt_number, int priority_level) {
    if (interrupt_number >= 0 && interrupt_number < 10) {
        interrupt_priorities[interrupt_number] = priority_level;
    }
}

int get_highest_priority_interrupt() {
    int highest_priority = 1000; // large number
    int interrupt_num = -1;
    for (int i = 0; i < 10; i++) {
        if (interrupt_priorities[i] != 0 && interrupt_priorities[i] < highest_priority) {
            highest_priority = interrupt_priorities[i];
            interrupt_num = i;
        }
    }
    return interrupt_num;
}

int main() {
    // Set priorities
    set_interrupt_priority(2, 3); // Interrupt 2 priority 3
    set_interrupt_priority(5, 1); // Interrupt 5 priority 1 (highest)
    set_interrupt_priority(7, 4); // Interrupt 7 priority 4

    int highest = get_highest_priority_interrupt();
    printf("Highest priority interrupt is: %d\n", highest);
    return 0;
}

Important Notes

Interrupt priority numbers can be different depending on your hardware; always check your device's datasheet.

Higher priority interrupts can interrupt lower priority ones, but not vice versa.

Be careful to avoid priority inversion, where a low priority interrupt blocks a higher priority one.

Summary

Interrupt priority levels help manage multiple interrupts by deciding which to handle first.

Lower numbers usually mean higher priority, but check your system.

Setting priorities correctly helps your embedded system respond quickly and reliably.