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.

Practice

1.

What does a lower interrupt priority number mean in embedded C?

easy

A. It has lower priority and runs after others

B. It disables all other interrupts

C. It has higher priority and runs before others

D. It runs only when the CPU is idle

Solution

1. Step 1: Understand priority numbering

   In embedded C, interrupt priority levels are numbered so that lower numbers mean higher priority.

2. Step 2: Interpret priority effect

   Therefore, an interrupt with a lower number runs before interrupts with higher numbers.

3. Final Answer:

   It has higher priority and runs before others -> Option C

4. Quick Check:

   Lower number = higher priority [OK]

Hint: Lower number means higher priority, runs first [OK]

Common Mistakes:

• Thinking higher number means higher priority
• Confusing priority with interrupt enable/disable
• Assuming priority affects CPU idle time

2.

Which of the following is the correct way to set an interrupt priority level to 2 in embedded C?

void setup_interrupt() {
    // Set priority here
}

easy

A. NVIC_SetPriority(IRQn, HIGH);

B. NVIC_SetPriority(IRQn, 2);

C. NVIC_EnableIRQ(2);

D. NVIC_SetPriority(IRQn, 10);

Solution

1. Step 1: Identify correct function to set priority

   NVIC_SetPriority is the standard function to set interrupt priority in embedded C.

2. Step 2: Check correct priority value

   Passing 2 as the priority sets the interrupt priority level to 2 correctly.

3. Final Answer:

   NVIC_SetPriority(IRQn, 2); -> Option B

4. Quick Check:

   Use NVIC_SetPriority with numeric level [OK]

Hint: Use NVIC_SetPriority with number for priority [OK]

Common Mistakes:

• Using NVIC_EnableIRQ to set priority
• Passing symbolic constants instead of numbers
• Setting priority to a value outside allowed range

3.

Given the following code snippet, what will be the order of interrupt execution if both interrupts occur simultaneously?

NVIC_SetPriority(IRQn_UART, 3);
NVIC_SetPriority(IRQn_TIMER, 1);
// Both UART and TIMER interrupts triggered at the same time

medium

A. TIMER interrupt runs first, then UART

B. Both run simultaneously

C. UART interrupt runs first, then TIMER

D. No interrupt runs due to conflict

Solution

1. Step 1: Identify priority levels assigned

   UART interrupt priority is set to 3, TIMER interrupt priority is set to 1.

2. Step 2: Determine which priority is higher

   Lower number means higher priority, so TIMER (1) runs before UART (3).

3. Final Answer:

   TIMER interrupt runs first, then UART -> Option A

4. Quick Check:

   Lower priority number runs first [OK]

Hint: Lower priority number runs before higher [OK]

Common Mistakes:

• Assuming higher number runs first
• Thinking interrupts run simultaneously
• Ignoring priority settings

4.

Identify the error in this code snippet that sets interrupt priorities:

NVIC_SetPriority(IRQn_ADC, 5);
NVIC_SetPriority(IRQn_ADC, 0);
NVIC_EnableIRQ(IRQn_ADC);

What is the problem?

medium

A. Priority 0 is invalid and causes error

B. Setting priority twice causes conflict

C. No error, code works fine

D. Priority 0 is higher than priority 5, so last setting wins

Solution

1. Step 1: Analyze priority assignments

   The code sets priority 5 first, then overwrites it with priority 0 for the same interrupt.

2. Step 2: Understand priority effect

   Priority 0 is higher priority than 5, so the last setting (0) is effective.

3. Final Answer:

   Priority 0 is higher than priority 5, so last setting wins -> Option D

4. Quick Check:

   Last priority set is used [OK]

Hint: Last priority set is effective [OK]

Common Mistakes:

• Thinking priority 0 is invalid
• Assuming multiple priorities combine
• Ignoring order of calls

5.

You have three interrupts: IRQn_A, IRQn_B, and IRQn_C. You want IRQn_B to run first, then IRQn_C, and finally IRQn_A. Which priority settings achieve this?

hard

A. IRQn_B = 1, IRQn_C = 2, IRQn_A = 3

B. IRQn_A = 1, IRQn_B = 2, IRQn_C = 3

C. IRQn_C = 1, IRQn_A = 2, IRQn_B = 3

D. IRQn_B = 3, IRQn_C = 2, IRQn_A = 1

Solution

1. Step 1: Understand priority order needed

   IRQn_B should run first, then IRQn_C, then IRQn_A last.

2. Step 2: Assign priorities accordingly

   Lower number means higher priority, so IRQn_B = 1, IRQn_C = 2, IRQn_A = 3 matches the desired order.

3. Final Answer:

   IRQn_B = 1, IRQn_C = 2, IRQn_A = 3 -> Option A

4. Quick Check:

   Lower number runs first [OK]

Hint: Assign lower numbers to interrupts needing higher priority [OK]

Common Mistakes:

• Assigning higher numbers for higher priority
• Mixing up interrupt names and priorities
• Not following the required order