ISR-safe API functions (FromISR suffix) in FreeRTOS - Mini Project: Build & Apply

Using ISR-safe API functions with FromISR suffix in FreeRTOS

📖 Scenario: You are programming a microcontroller using FreeRTOS. You want to safely send data from an interrupt service routine (ISR) to a task without causing problems in your program.

🎯 Goal: Build a simple program that creates a queue, sets up a variable to track if a context switch is needed, uses the ISR-safe function xQueueSendFromISR to send data from an ISR, and then prints the result.

📋 What You'll Learn

Create a queue called myQueue that can hold 5 integers.

Create a variable called higherPriorityTaskWoken of type BaseType_t and initialize it to pdFALSE.

Use xQueueSendFromISR to send the integer 100 to myQueue from an ISR context, passing &higherPriorityTaskWoken as the last argument.

Print the return value of xQueueSendFromISR.

💡 Why This Matters

🌍 Real World

In embedded systems, interrupts happen asynchronously. Using ISR-safe API functions like those with the FromISR suffix ensures safe communication between interrupts and tasks without causing crashes or data corruption.

💼 Career

Embedded software engineers often write interrupt handlers that must interact with FreeRTOS tasks safely. Knowing how to use FromISR functions is essential for writing reliable real-time applications.

1

Create a queue called myQueue

Create a queue called myQueue using xQueueCreate that can hold 5 integers.

FreeRTOS

# Create a queue called myQueue that holds 5 integers
# Your code here

Hint

Use xQueueCreate with 5 as the number of items and sizeof(int) as the size of each item.

2

Create a variable higherPriorityTaskWoken

Create a variable called higherPriorityTaskWoken of type BaseType_t and set it to pdFALSE.

FreeRTOS

#include "FreeRTOS.h"
#include "queue.h"

QueueHandle_t myQueue = xQueueCreate(5, sizeof(int));

// Create higherPriorityTaskWoken variable and set to pdFALSE
# Your code here

Hint

Use BaseType_t type and initialize with pdFALSE.

3

Send data from ISR using xQueueSendFromISR

Use xQueueSendFromISR to send the integer 100 to myQueue from an ISR context. Pass &higherPriorityTaskWoken as the last argument and store the return value in a variable called result.

FreeRTOS

#include "FreeRTOS.h"
#include "queue.h"

QueueHandle_t myQueue = xQueueCreate(5, sizeof(int));

BaseType_t higherPriorityTaskWoken = pdFALSE;

// Send 100 to myQueue from ISR and save return value in result
# Your code here

Hint

Declare an integer variable with value 100, then call xQueueSendFromISR with the queue, address of the integer, and address of higherPriorityTaskWoken.

4

Print the result of xQueueSendFromISR

Print the value of the variable result using printf.

FreeRTOS

#include "FreeRTOS.h"
#include "queue.h"
#include 

QueueHandle_t myQueue = xQueueCreate(5, sizeof(int));

BaseType_t higherPriorityTaskWoken = pdFALSE;

int valueToSend = 100;
BaseType_t result = xQueueSendFromISR(myQueue, &valueToSend, &higherPriorityTaskWoken);

// Print the result variable
# Your code here

Hint

Use printf to print the text and the value of result. The function xQueueSendFromISR returns pdTRUE (1) on success.

Practice

1.

What is the main reason to use FreeRTOS API functions with the FromISR suffix inside an interrupt service routine (ISR)?

easy

A. They disable all interrupts during execution.

B. They are safe to call from ISRs and handle interrupt context correctly.

C. They run tasks faster than normal API functions.

D. They automatically create new tasks inside the ISR.

Solution

1. Step 1: Understand ISR context constraints

   Inside an ISR, normal FreeRTOS API functions are unsafe because they may block or cause race conditions.

2. Step 2: Role of FromISR functions

   Functions with the FromISR suffix are designed to be safe in interrupt context by avoiding blocking and managing critical sections properly.

3. Final Answer:

   They are safe to call from ISRs and handle interrupt context correctly. -> Option B

4. Quick Check:

   FromISR functions = ISR safe calls [OK]

Hint: Use FromISR functions only inside interrupts for safety [OK]

Common Mistakes:

• Calling normal API functions inside ISR causing crashes
• Assuming FromISR functions disable interrupts
• Thinking FromISR creates tasks automatically

2.

Which of the following is the correct way to declare a variable to track if a higher priority task was woken inside an ISR?

?

easy

A. int higherPriorityTaskWoken = 0;

B. bool higherPriorityTaskWoken = false;

C. BaseType_t higherPriorityTaskWoken = pdFALSE;

D. volatile int higherPriorityTaskWoken = 1;

Solution

1. Step 1: Identify FreeRTOS type for ISR task wake flag

   FreeRTOS uses BaseType_t for portability and defines pdFALSE and pdTRUE for boolean values.

2. Step 2: Correct initialization

   The variable must start as pdFALSE to indicate no task was woken yet.

3. Final Answer:

   BaseType_t higherPriorityTaskWoken = pdFALSE; -> Option C

4. Quick Check:

   Use BaseType_t and pdFALSE for ISR flags [OK]

Hint: Use BaseType_t and pdFALSE to track task wake in ISR [OK]

Common Mistakes:

• Using plain int instead of BaseType_t
• Initializing flag to 1 instead of pdFALSE
• Using bool which may not be portable

3.

Consider this ISR code snippet:

BaseType_t higherPriorityTaskWoken = pdFALSE;
xQueueSendFromISR(queue, &data, &higherPriorityTaskWoken);
if (higherPriorityTaskWoken) {
    portYIELD_FROM_ISR();
}

What happens if higherPriorityTaskWoken is set to pdTRUE after xQueueSendFromISR?

medium

A. The ISR requests a context switch to the higher priority task immediately after ISR.

B. The ISR disables all interrupts until the higher priority task runs.

C. The queue send operation fails silently.

D. The higher priority task is blocked until the ISR finishes.

Solution

1. Step 1: Understand higherPriorityTaskWoken flag

   This flag indicates if a task with higher priority than the current one was made ready by the ISR.

2. Step 2: Role of portYIELD_FROM_ISR()

   If the flag is true, calling portYIELD_FROM_ISR() requests a context switch to that higher priority task immediately after ISR exit.

3. Final Answer:

   The ISR requests a context switch to the higher priority task immediately after ISR. -> Option A

4. Quick Check:

   higherPriorityTaskWoken true triggers context switch [OK]

Hint: If higherPriorityTaskWoken true, call portYIELD_FROM_ISR() [OK]

Common Mistakes:

• Thinking interrupts are disabled by portYIELD_FROM_ISR()
• Assuming queue send fails if flag is true
• Believing higher priority task waits for ISR to finish

4.

Identify the error in this ISR code snippet:

void ISR_Handler(void) {
    BaseType_t higherPriorityTaskWoken = pdFALSE;
    xSemaphoreGive(semaphore);
    if (higherPriorityTaskWoken) {
        portYIELD_FROM_ISR();
    }
}

What is the problem here?

medium

A. The variable higherPriorityTaskWoken should be global.

B. The semaphore is given twice causing overflow.

C. The portYIELD_FROM_ISR() call is outside the ISR.

D. The higherPriorityTaskWoken variable is not passed to xSemaphoreGiveFromISR.

Solution

1. Step 1: Check function used for semaphore give

   The code uses xSemaphoreGive, which is not safe inside ISR context.

2. Step 2: Correct ISR-safe function usage

   Inside ISR, xSemaphoreGiveFromISR must be used with higherPriorityTaskWoken passed by reference to track task wake.

3. Final Answer:

   The higherPriorityTaskWoken variable is not passed to xSemaphoreGiveFromISR. -> Option D

4. Quick Check:

   Use FromISR functions with higherPriorityTaskWoken [OK]

Hint: Always pass higherPriorityTaskWoken to FromISR functions [OK]

Common Mistakes:

• Using normal API functions inside ISR
• Not passing higherPriorityTaskWoken pointer
• Declaring higherPriorityTaskWoken as global unnecessarily

5.

You want to send data from an ISR to a task using a queue. Which sequence correctly uses FreeRTOS ISR-safe API functions to do this and ensure the higher priority task runs immediately if woken?

?

hard

A. BaseType_t higherPriorityTaskWoken = pdFALSE;
xQueueSendFromISR(queue, &data, &higherPriorityTaskWoken);
if (higherPriorityTaskWoken) portYIELD_FROM_ISR();

B. xQueueSend(queue, &data, 0);
BaseType_t higherPriorityTaskWoken = pdFALSE;
if (higherPriorityTaskWoken) portYIELD_FROM_ISR();

C. BaseType_t higherPriorityTaskWoken = pdTRUE;
xQueueSendFromISR(queue, &data, NULL);
portYIELD_FROM_ISR();

D. BaseType_t higherPriorityTaskWoken = pdFALSE;
xQueueSend(queue, &data, 0);
portYIELD_FROM_ISR();

Solution

1. Step 1: Use ISR-safe queue send function

   Inside ISR, xQueueSendFromISR must be used with a pointer to higherPriorityTaskWoken.

2. Step 2: Check higherPriorityTaskWoken and yield

   If higherPriorityTaskWoken is set to pdTRUE, call portYIELD_FROM_ISR() to request immediate context switch.

3. Final Answer:

   BaseType_t higherPriorityTaskWoken = pdFALSE; xQueueSendFromISR(queue, &data, &higherPriorityTaskWoken); if (higherPriorityTaskWoken) portYIELD_FROM_ISR(); -> Option A

4. Quick Check:

   Use FromISR + check flag + yield [OK]

Hint: Use xQueueSendFromISR with flag and yield if true [OK]

Common Mistakes:

• Using normal xQueueSend inside ISR
• Not passing higherPriorityTaskWoken pointer
• Calling portYIELD_FROM_ISR unconditionally