Interrupt priority levels in Embedded C - Mini Project: Build & Apply

Interrupt Priority Levels

📖 Scenario: You are working on a simple embedded system that handles multiple interrupts. Each interrupt has a priority level. Higher priority interrupts should be handled before lower priority ones.Imagine you have three interrupts: Timer, UART, and ADC. You want to assign priority levels to them so the system knows which interrupt to handle first.

🎯 Goal: Build a small program that sets up interrupt priority levels using variables and then prints the priority order.

📋 What You'll Learn

Create variables for three interrupts: timer_priority, uart_priority, and adc_priority with exact integer values.

Create a variable highest_priority to hold the highest priority value.

Use conditional statements to find which interrupt has the highest priority.

Print the name of the interrupt with the highest priority.

💡 Why This Matters

🌍 Real World

Embedded systems often have multiple interrupts. Setting priority levels helps the system decide which interrupt to handle first, ensuring important tasks get immediate attention.

💼 Career

Understanding interrupt priorities is essential for embedded software developers working on microcontrollers, real-time systems, and hardware interfacing.

1

Set interrupt priority levels

Create three integer variables called timer_priority, uart_priority, and adc_priority with values 2, 1, and 3 respectively.

Embedded C

# Create interrupt priority variables
// Your code here

Hint

Use int to declare variables and assign the exact values.

2

Create a variable for highest priority

Create an integer variable called highest_priority and set it to 0.

Embedded C

int timer_priority = 2;
int uart_priority = 1;
int adc_priority = 3;

// Create highest_priority variable
// Your code here

Hint

Initialize highest_priority to zero before comparing.

3

Find the highest priority interrupt

Use if statements to compare timer_priority, uart_priority, and adc_priority. Set highest_priority to the highest value among them.

Embedded C

int timer_priority = 2;
int uart_priority = 1;
int adc_priority = 3;

int highest_priority = 0;

// Compare priorities and set highest_priority
// Your code here

Hint

Check each priority and update highest_priority if the current one is greater.

4

Print the highest priority interrupt

Use if statements to check which interrupt has the highest_priority value. Then print the exact text "Highest priority interrupt: TIMER", "Highest priority interrupt: UART", or "Highest priority interrupt: ADC" accordingly using printf.

Embedded C

int timer_priority = 2;
int uart_priority = 1;
int adc_priority = 3;

int highest_priority = 0;

if (timer_priority > highest_priority) {
    highest_priority = timer_priority;
}
if (uart_priority > highest_priority) {
    highest_priority = uart_priority;
}
if (adc_priority > highest_priority) {
    highest_priority = adc_priority;
}

// Print the highest priority interrupt
// Your code here

Hint

Use printf to print the exact text with a newline. Compare highest_priority to each interrupt priority.

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