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

Need a 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

Need a 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

Need a 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

Need a hint?

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