Interrupt priority levels in Embedded C - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to set the priority level of an interrupt to 3.

Embedded C

NVIC_SetPriority(IRQn, [1]);

Drag options to blanks, or click blank then click option'

A3

B0

C5

D7

Attempts:

3 left

💡 Hint

Common Mistakes

Using a priority level outside the allowed range.

Confusing priority number with interrupt number.

The function NVIC_SetPriority sets the priority of the interrupt. Here, priority level 3 is assigned.

2fill in blank

medium

Complete the code to enable the interrupt with IRQ number 10.

Embedded C

NVIC_EnableIRQ([1]);

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A5

B10

C3

D15

Attempts:

3 left

💡 Hint

Common Mistakes

Using the wrong IRQ number.

Confusing IRQ number with priority level.

The function NVIC_EnableIRQ enables the interrupt specified by its IRQ number. Here, IRQ 10 is enabled.

3fill in blank

hard

Fix the error in setting the priority level to the highest priority (0).

Embedded C

NVIC_SetPriority(IRQn, [1]);

Drag options to blanks, or click blank then click option'

A0

B3

C1

D7

Attempts:

3 left

💡 Hint

Common Mistakes

Using a non-zero number for highest priority.

Confusing priority levels with IRQ numbers.

Priority 0 is the highest priority level in many embedded systems. Setting it correctly ensures the interrupt has the highest priority.

4fill in blank

hard

Fill both blanks to create a dictionary mapping IRQ numbers to priority levels for IRQ 2 and IRQ 4.

Embedded C

const uint8_t irq_priorities[] = { [[1]] = 1, [[2]] = 3 };

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A2

B1

C4

D3

Attempts:

3 left

💡 Hint

Common Mistakes

Swapping IRQ numbers and priority values.

Using invalid IRQ numbers.

The array sets priority 1 for IRQ 2 and priority 3 for IRQ 4 using designated initializers.

5fill in blank

hard

Fill all three blanks to set priority 2 for IRQ 5, enable IRQ 5, and set priority 0 for IRQ 7.

Embedded C

NVIC_SetPriority([1], 2);
NVIC_EnableIRQ([2]);
NVIC_SetPriority([3], 0);

Drag options to blanks, or click blank then click option'

A5

B7

C3

D10

Attempts:

3 left

💡 Hint

Common Mistakes

Mixing IRQ numbers between calls.

Setting wrong priority levels.

The code sets priority 2 for IRQ 5, enables IRQ 5, and sets priority 0 (highest) for IRQ 7.

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