Nested interrupt handling in FreeRTOS

Introduction

Nested interrupt handling lets higher priority interrupts pause lower priority ones. This helps your system respond faster to urgent events.

When you have multiple interrupts with different importance levels.

When a critical sensor needs immediate attention even if another interrupt is running.

When you want to improve system responsiveness in real-time applications.

When handling time-sensitive tasks that must not wait.

When you want to avoid missing important events during long interrupt processing.

Syntax

FreeRTOS

void ISR_Handler(void) {
    BaseType_t xHigherPriorityTaskWoken = pdFALSE;

    // Handle interrupt

    // Clear interrupt flag

    // If a higher priority task was woken by ISR
    portYIELD_FROM_ISR(xHigherPriorityTaskWoken);
}

Use portYIELD_FROM_ISR() to request a context switch if needed.

Configure interrupt priorities properly to enable nesting.

Examples

Example of a high priority ISR that can preempt lower priority ISRs.

FreeRTOS

void HighPriorityISR(void) {
    BaseType_t xHigherPriorityTaskWoken = pdFALSE;
    // Handle high priority interrupt
    portYIELD_FROM_ISR(xHigherPriorityTaskWoken);
}

Example of a low priority ISR that can be interrupted by higher priority ISRs.

FreeRTOS

void LowPriorityISR(void) {
    BaseType_t xHigherPriorityTaskWoken = pdFALSE;
    // Handle low priority interrupt
    portYIELD_FROM_ISR(xHigherPriorityTaskWoken);
}

Sample Program

This program simulates two interrupts: low and high priority. The high priority interrupt can preempt the low priority one, showing nested interrupt handling.

FreeRTOS

#include "FreeRTOS.h"
#include "task.h"
#include "semphr.h"

volatile int highPriorityFlag = 0;
volatile int lowPriorityFlag = 0;

void HighPriorityISR(void) {
    BaseType_t xHigherPriorityTaskWoken = pdFALSE;
    highPriorityFlag = 1; // Set flag
    portYIELD_FROM_ISR(xHigherPriorityTaskWoken);
}

void LowPriorityISR(void) {
    BaseType_t xHigherPriorityTaskWoken = pdFALSE;
    lowPriorityFlag = 1; // Set flag
    // Simulate longer processing
    for (volatile int i = 0; i < 100000; i++) {}
    portYIELD_FROM_ISR(xHigherPriorityTaskWoken);
}

int main(void) {
    // Simulate interrupts firing
    LowPriorityISR();
    if (lowPriorityFlag) {
        printf("Low priority interrupt handled\n");
    }
    HighPriorityISR();
    if (highPriorityFlag) {
        printf("High priority interrupt handled\n");
    }
    return 0;
}

Important Notes

Make sure interrupt priorities are set correctly in your microcontroller to allow nesting.

Keep ISRs short to avoid long blocking of other interrupts.

Use portYIELD_FROM_ISR() to switch tasks safely after ISR.

Summary

Nested interrupts let urgent tasks interrupt less urgent ones.

Use proper priority settings and portYIELD_FROM_ISR() in FreeRTOS ISRs.

Keep interrupt code short and efficient for best results.

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