Nested interrupt handling in FreeRTOS - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to enable nested interrupts by setting the correct priority grouping.

FreeRTOS

NVIC_SetPriorityGrouping([1]);

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

ANVIC_PRIORITYGROUP_3

BNVIC_PRIORITYGROUP_2

CNVIC_PRIORITYGROUP_4

DNVIC_PRIORITYGROUP_1

Attempts:

3 left

💡 Hint

Common Mistakes

Choosing a priority grouping that disables subpriority levels.

Using NVIC_PRIORITYGROUP_4 which disables nested interrupts.

NVIC_PRIORITYGROUP_3 allows 4 bits for pre-emption priority and 0 bits for subpriority, enabling nested interrupts effectively.

2fill in blank

medium

Complete the code to set the priority of an interrupt to allow nesting.

FreeRTOS

NVIC_SetPriority(USART1_IRQn, [1]);

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

A5

B15

C0

D255

Attempts:

3 left

💡 Hint

Common Mistakes

Setting priority to 0 which disables nesting by making it highest priority.

Using 255 which is invalid for priority levels.

Priority 5 is a moderate priority level allowing higher priority interrupts to pre-empt this one.

3fill in blank

hard

Fix the error in the interrupt handler declaration to support nesting.

FreeRTOS

void [1](void) {
    // Interrupt code
}

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

AUSART1_Interrupt

BUSART1_IRQHandler

CUSART1_Handler

Attempts:

3 left

💡 Hint

Common Mistakes

Using incorrect handler names that do not match the vector table.

Misspelling or wrong casing in the handler function name.

The correct interrupt handler name for USART1 is USART1_IRQHandler, matching the vector table.

4fill in blank

hard

Fill both blanks to correctly enable and set priority for a nested interrupt.

FreeRTOS

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

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

ATIM2_IRQn

BUSART2_IRQn

CTIM3_IRQn

DUSART1_IRQn

Attempts:

3 left

💡 Hint

Common Mistakes

Using different interrupt names in enable and set priority calls.

Choosing interrupts not configured for nesting.

Enabling and setting priority for TIM2_IRQn allows nested interrupts on TIM2.

5fill in blank

hard

Fill all three blanks to create a nested interrupt safe critical section using FreeRTOS API.

FreeRTOS

taskENTER_CRITICAL();
[1]();
// Critical code
[2]();
NVIC_SetPriority([3], 1);

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

AportDISABLE_INTERRUPTS

BportENABLE_INTERRUPTS

CTIM4_IRQn

DportSET_INTERRUPT_MASK

Attempts:

3 left

💡 Hint

Common Mistakes

Forgetting to disable or enable interrupts properly.

Setting priority on wrong interrupt number.

Disable interrupts before critical code, enable after, and set priority for TIM4_IRQn to 1 for nesting.

Practice

1.

What is the main purpose of nested interrupt handling in FreeRTOS?

easy

A. Allow higher priority interrupts to interrupt lower priority ones

B. Disable all interrupts during task switching

C. Prevent any interrupt from occurring during critical sections

D. Run all interrupts sequentially without interruption

Solution

1. Step 1: Understand interrupt priorities

   Nested interrupts allow interrupts with higher priority to interrupt those with lower priority.

2. Step 2: Relate to FreeRTOS behavior

   FreeRTOS supports nested interrupts to ensure urgent tasks get immediate attention.

3. Final Answer:

   Allow higher priority interrupts to interrupt lower priority ones -> Option A

4. Quick Check:

   Nested interrupts = higher priority interrupts interrupting lower priority [OK]

Hint: Nested interrupts let urgent interrupts pause less urgent ones [OK]

Common Mistakes:

• Thinking all interrupts are disabled during task switching
• Believing interrupts run only sequentially without nesting
• Confusing nested interrupts with disabling interrupts

2.

Which of the following is the correct way to yield from an ISR in FreeRTOS?

void ISR_Handler(void) {
    BaseType_t xHigherPriorityTaskWoken = pdFALSE;
    // Some ISR code
    // ...
    // Yield if needed
    ?
}

easy

A. taskYIELD();

B. portYIELD_FROM_ISR(xHigherPriorityTaskWoken);

C. vTaskDelay(1);

D. portEND_SWITCHING_ISR(xHigherPriorityTaskWoken);

Solution

1. Step 1: Identify correct ISR yield function

   In FreeRTOS, portYIELD_FROM_ISR is used to request a context switch from an ISR safely.

2. Step 2: Check other options

   taskYIELD is for tasks, vTaskDelay is for delaying tasks, portEND_SWITCHING_ISR is not a FreeRTOS API.

3. Final Answer:

   portYIELD_FROM_ISR(xHigherPriorityTaskWoken); -> Option B

4. Quick Check:

   Yield from ISR = portYIELD_FROM_ISR [OK]

Hint: Use portYIELD_FROM_ISR to yield safely inside ISRs [OK]

Common Mistakes:

• Using taskYIELD inside ISR causing errors
• Calling vTaskDelay inside ISR which is invalid
• Confusing portEND_SWITCHING_ISR with portYIELD_FROM_ISR

3.

Given the following FreeRTOS ISR code snippet, what will be the value of xHigherPriorityTaskWoken after execution?

void ISR_Timer(void) {
    BaseType_t xHigherPriorityTaskWoken = pdFALSE;
    xSemaphoreGiveFromISR(xSemaphore, &xHigherPriorityTaskWoken);
    portYIELD_FROM_ISR(xHigherPriorityTaskWoken);
}

Assume the semaphore unblocks a higher priority task waiting.

medium

A. Undefined behavior

B. pdFALSE

C. pdTRUE

D. Semaphore handle value

Solution

1. Step 1: Understand xSemaphoreGiveFromISR behavior

   This function sets xHigherPriorityTaskWoken to pdTRUE if a higher priority task is unblocked.

2. Step 2: Given higher priority task is unblocked

   Since the semaphore unblocks a higher priority task, xHigherPriorityTaskWoken becomes pdTRUE.

3. Final Answer:

   pdTRUE -> Option C

4. Quick Check:

   Higher priority task unblocked = xHigherPriorityTaskWoken pdTRUE [OK]

Hint: xHigherPriorityTaskWoken is pdTRUE if higher priority task unblocked [OK]

Common Mistakes:

• Assuming xHigherPriorityTaskWoken stays pdFALSE always
• Confusing semaphore handle with boolean flag
• Ignoring the effect of unblocking higher priority tasks

4.

Identify the error in this nested interrupt handling code snippet:

void ISR_UART(void) {
    BaseType_t xHigherPriorityTaskWoken = pdFALSE;
    xQueueSendFromISR(xQueue, &data, NULL);
    portYIELD_FROM_ISR(xHigherPriorityTaskWoken);
}

medium

A. BaseType_t should be int instead

B. portYIELD_FROM_ISR should not be called inside ISR

C. xQueueSendFromISR cannot be used in ISRs

D. xHigherPriorityTaskWoken should be passed to xQueueSendFromISR

Solution

1. Step 1: Check xQueueSendFromISR parameters

   The function requires a pointer to xHigherPriorityTaskWoken to update it if a higher priority task is woken.

2. Step 2: Identify missing argument

   Passing NULL means xHigherPriorityTaskWoken is never updated, so portYIELD_FROM_ISR may not yield correctly.

3. Final Answer:

   xHigherPriorityTaskWoken should be passed to xQueueSendFromISR -> Option D

4. Quick Check:

   Pass xHigherPriorityTaskWoken pointer to FromISR APIs [OK]

Hint: Always pass xHigherPriorityTaskWoken pointer to FromISR APIs [OK]

Common Mistakes:

• Passing NULL instead of &xHigherPriorityTaskWoken
• Calling portYIELD_FROM_ISR without updating flag
• Using wrong variable types for task woken flag

5.

You want to implement nested interrupt handling where a high priority interrupt can preempt a low priority one and safely switch tasks if needed. Which combination of steps is correct?

1. Configure interrupt priorities properly
2. Use portYIELD_FROM_ISR() inside ISRs
3. Disable all interrupts during ISR execution
4. Use FromISR API versions for FreeRTOS calls inside ISRs

Choose the best combination.

hard

A. 1, 2, and 4 only

B. 1 and 3 only

C. 2, 3, and 4 only

D. All four steps

Solution

1. Step 1: Configure interrupt priorities

   Proper priority setup allows nested interrupts to work correctly.

2. Step 2: Use portYIELD_FROM_ISR and FromISR APIs

   These ensure safe task switching and API calls inside ISRs.

3. Step 3: Avoid disabling all interrupts during ISR

   Disabling all interrupts prevents nesting, so step 3 is incorrect.

4. Final Answer:

   1, 2, and 4 only -> Option A

5. Quick Check:

   Nested interrupts need priorities + FromISR APIs + portYIELD_FROM_ISR [OK]

Hint: Use priorities and FromISR APIs; don't disable all interrupts [OK]

Common Mistakes:

• Disabling all interrupts prevents nesting
• Forgetting to use FromISR API versions
• Ignoring interrupt priority configuration