Interrupt priority levels in Embedded C - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is an interrupt priority level in embedded systems?

An interrupt priority level is a number that tells the system which interrupt is more important. Higher priority interrupts can stop lower priority ones to be handled first.

beginner

Why do we use interrupt priority levels?

We use interrupt priority levels to manage multiple interrupts. This helps the system respond quickly to important events without ignoring less urgent ones.

intermediate

How does a higher priority interrupt affect a lower priority interrupt?

A higher priority interrupt can pause or 'preempt' a lower priority interrupt. The system handles the higher priority interrupt first, then returns to the lower one.

intermediate

In embedded C, how do you typically set interrupt priority levels?

You set interrupt priority levels using special registers or functions provided by the microcontroller's library, often by assigning a priority number to each interrupt source.

intermediate

What happens if two interrupts have the same priority level?

If two interrupts have the same priority, the system usually handles them in the order they occur or based on a fixed hardware scheme, but they do not preempt each other.

What does a higher interrupt priority level mean?

A It is ignored by the system

B It runs only after all lower priority interrupts

C It is handled before lower priority interrupts

D It disables all interrupts

Higher priority interrupts are handled first, preempting lower priority ones.

How are interrupt priorities usually assigned in embedded C?

A By writing interrupt code in a specific order

B By setting priority numbers in special registers

C By disabling interrupts globally

D By using delay loops

Interrupt priorities are set by assigning numbers in hardware registers or using library functions.

What happens if a lower priority interrupt is running and a higher priority interrupt occurs?

A The lower priority interrupt is paused and the higher priority interrupt runs

B The lower priority interrupt continues without interruption

C Both interrupts run at the same time

D The system crashes

Higher priority interrupts preempt lower priority ones to be handled first.

If two interrupts have the same priority, how does the system decide which to handle first?

A Usually by the order they occur or hardware scheme

B The system ignores both

C The system merges them into one interrupt

D The system randomly picks one

Same priority interrupts are handled in occurrence order or by fixed hardware rules.

Why is it important to assign correct interrupt priority levels?

A To disable all interrupts

B To slow down the system

C To avoid using interrupts

D To ensure critical tasks are handled quickly

Correct priorities help the system respond fast to important events.

Explain how interrupt priority levels help manage multiple interrupts in an embedded system.

Think about which interrupt runs first and how the system decides.

Describe how you would set interrupt priority levels in embedded C code.

Consider hardware registers or API calls.

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