ISR-to-task notification pattern in FreeRTOS - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of task notification from ISR

What will be the output of this FreeRTOS code snippet when the ISR sends a notification to the task?

FreeRTOS

#include "FreeRTOS.h"
#include "task.h"
#include <stdio.h>

TaskHandle_t xTaskHandle = NULL;

void vTaskFunction(void *pvParameters) {
    uint32_t ulNotificationValue;
    for (;;) {
        xTaskNotifyWait(0x00, 0xffffffff, &ulNotificationValue, portMAX_DELAY);
        printf("Notification received with value: %lu\n", ulNotificationValue);
    }
}

void vISR_Handler(void) {
    BaseType_t xHigherPriorityTaskWoken = pdFALSE;
    xTaskNotifyFromISR(xTaskHandle, 42, eSetValueWithOverwrite, &xHigherPriorityTaskWoken);
    portYIELD_FROM_ISR(xHigherPriorityTaskWoken);
}

int main() {
    xTaskCreate(vTaskFunction, "Task", 1000, NULL, 1, &xTaskHandle);
    // Simulate ISR call
    vISR_Handler();
    // Normally scheduler would be started here
    return 0;
}

A Notification received with value: 42

B Notification received with value: 0

C No output, task never receives notification

D Compilation error due to missing portYIELD_FROM_ISR definition

💡 Hint

The scheduler is not started, so the task does not run to process the notification.

The code creates the task and simulates an ISR call sending notification value 42, but the scheduler is never started (commented out). Therefore, the task never executes and produces no output.

🧠 Conceptual

intermediate

Understanding notification value overwrite behavior

If an ISR sends multiple notifications to a task using xTaskNotifyFromISR with eSetValueWithOverwrite before the task processes them, what will the task receive?

A The task receives the first notification value only

B The task receives the last notification value sent by the ISR

C The task receives all notification values in order

D The task receives a sum of all notification values

💡 Hint

eSetValueWithOverwrite replaces the previous notification value.

Using eSetValueWithOverwrite causes the notification value to be overwritten if multiple notifications arrive before the task processes them. So only the last value is received.

🔧 Debug

advanced

Identify the error in ISR notification code

What error will occur when running this ISR notification code snippet?

FreeRTOS

void vISR_Handler(void) {
    BaseType_t xHigherPriorityTaskWoken = pdFALSE;
    xTaskNotifyFromISR(NULL, 10, eSetValueWithOverwrite, &xHigherPriorityTaskWoken);
    portYIELD_FROM_ISR(xHigherPriorityTaskWoken);
}

A Deadlock because task handle is NULL

B No error, notification sent successfully

C Compilation error due to missing xHigherPriorityTaskWoken variable

D Runtime error due to NULL task handle passed to xTaskNotifyFromISR

💡 Hint

xTaskNotifyFromISR requires a valid task handle to send notification.

Passing NULL as the task handle to xTaskNotifyFromISR causes a runtime error or undefined behavior because the function expects a valid task handle to notify.

📝 Syntax

advanced

Syntax correctness of ISR notification call

Which option shows the correct syntax for sending a notification from an ISR to a task with notification value 5 and using eIncrement action?

A xTaskNotifyFromISR(xTaskHandle, 5, eIncrement, &xHigherPriorityTaskWoken);

B xTaskNotifyFromISR(xTaskHandle, 5, eSetBits, xHigherPriorityTaskWoken);

C xTaskNotifyFromISR(xTaskHandle, 5, eIncrement);

D xTaskNotifyFromISR(xTaskHandle, 5, eSetValueWithOverwrite, &xHigherPriorityTaskWoken);

💡 Hint

Check the parameters and their order for xTaskNotifyFromISR.

The correct syntax requires the task handle, notification value, action, and pointer to BaseType_t variable for priority yield. Option A matches this exactly.

🚀 Application

expert

Determine the number of notifications received by the task

Given the following scenario: An ISR sends three notifications to a task using xTaskNotifyFromISR with eSetBits action, setting bits 0x01, 0x02, and 0x04 respectively, before the task calls xTaskNotifyWait. How many bits will the task receive in total after unblocking?

FreeRTOS

void vISR_Handler(void) {
    BaseType_t xHigherPriorityTaskWoken = pdFALSE;
    xTaskNotifyFromISR(xTaskHandle, 0x01, eSetBits, &xHigherPriorityTaskWoken);
    xTaskNotifyFromISR(xTaskHandle, 0x02, eSetBits, &xHigherPriorityTaskWoken);
    xTaskNotifyFromISR(xTaskHandle, 0x04, eSetBits, &xHigherPriorityTaskWoken);
    portYIELD_FROM_ISR(xHigherPriorityTaskWoken);
}

void vTaskFunction(void *pvParameters) {
    uint32_t ulNotificationValue;
    xTaskNotifyWait(0x00, 0xffffffff, &ulNotificationValue, portMAX_DELAY);
    // ulNotificationValue contains the bits set by ISR
}

A The task receives 0x07 (bits 0x01 | 0x02 | 0x04 combined)

B The task receives only the last bit 0x04

C The task receives 0x03 (bits 0x01 | 0x02 combined)

D The task receives 0x00 because bits are overwritten

💡 Hint

eSetBits action combines bits using OR operation.

Using eSetBits causes the notification value bits to be combined (ORed). Sending 0x01, 0x02, and 0x04 results in 0x07 total bits set when the task unblocks.