ISR-to-task notification pattern in FreeRTOS - Mini Project: Build & Apply

ISR-to-task notification pattern

📖 Scenario: You are building a simple embedded system using FreeRTOS. The system has a button connected to an interrupt. When the button is pressed, the interrupt service routine (ISR) should notify a task to handle the button press event.

🎯 Goal: Create a FreeRTOS program where an ISR notifies a task using the ISR-to-task notification pattern. The task will print a message when it receives the notification.

📋 What You'll Learn

Create a task called ButtonTask that waits for a notification.

Create an ISR called ButtonISR that sends a notification to ButtonTask.

Use ulTaskNotifyTake in the task to wait for the notification.

Use xTaskNotifyGiveFromISR in the ISR to notify the task.

Print "Button pressed!" in the task when notified.

💡 Why This Matters

🌍 Real World

This pattern is used in embedded systems where hardware interrupts need to signal tasks to perform work without blocking the ISR.

💼 Career

Embedded software engineers use ISR-to-task notifications to write responsive and efficient real-time applications.

1

Create the ButtonTask task

Create a FreeRTOS task called ButtonTask that runs the function vButtonTask. Inside vButtonTask, write an infinite loop with no code inside yet.

FreeRTOS

void vButtonTask(void *pvParameters) {
    for (;;) {
        // Your code here
    }
}

int main(void) {
    // Your code here
    return 0;
}

Need a hint?

Use xTaskCreate to create the task and vTaskStartScheduler to start FreeRTOS.

2

Add a notification wait in ButtonTask

Inside the infinite loop of vButtonTask, add a call to ulTaskNotifyTake(pdTRUE, portMAX_DELAY) to wait indefinitely for a notification from the ISR.

FreeRTOS

void vButtonTask(void *pvParameters) {
    for (;;) {
        // Add ulTaskNotifyTake here to wait for notification
        // Your code here
    }
}

int main(void) {
    xTaskCreate(vButtonTask, "ButtonTask", configMINIMAL_STACK_SIZE, NULL, tskIDLE_PRIORITY + 1, NULL);
    vTaskStartScheduler();
    return 0;
}

Need a hint?

ulTaskNotifyTake blocks the task until a notification is received.

3

Create the ButtonISR to notify the task

Create an ISR function called ButtonISR that calls xTaskNotifyGiveFromISR to notify ButtonTask. Use a global variable xButtonTaskHandle to store the task handle. Also, declare xButtonTaskHandle as a global variable and assign it when creating the task in main.

FreeRTOS

TaskHandle_t xButtonTaskHandle = NULL;

void ButtonISR(void) {
    BaseType_t xHigherPriorityTaskWoken = pdFALSE;
    // Notify the task from ISR
    // Your code here
    portYIELD_FROM_ISR(xHigherPriorityTaskWoken);
}

int main(void) {
    // Assign task handle when creating the task
    // Your code here
    vTaskStartScheduler();
    return 0;
}

void vButtonTask(void *pvParameters) {
    for (;;) {
        ulTaskNotifyTake(pdTRUE, portMAX_DELAY);
    }
}

Need a hint?

Use xTaskNotifyGiveFromISR to notify the task safely from the ISR.

4

Print message when notified in ButtonTask

Modify vButtonTask so that after ulTaskNotifyTake returns, it prints "Button pressed!" using printf.

FreeRTOS

void vButtonTask(void *pvParameters) {
    for (;;) {
        ulTaskNotifyTake(pdTRUE, portMAX_DELAY);
        // Print "Button pressed!" here
        // Your code here
    }
}

Need a hint?

Use printf("Button pressed!\n") to show the message when the task is notified.