Task notifications are FreeRTOS's fastest, lightest signalling mechanism — 45% faster than a semaphore with zero extra RAM. Every task has a built-in 32-bit notification value. This part covers when notifications beat semaphores and event groups, and exactly how to use all four notification actions.

✓Understand the 32-bit notification value and its four action modes
✓Use ulTaskNotifyTake() as a zero-overhead binary or counting semaphore
✓Use xTaskNotifyWait() as a 32-flag event group without extra memory
✓Send notifications safely from ISRs with vTaskNotifyGiveFromISR()

Notification Actions

➕

Increment

Increment the notification value. Acts as a counting semaphore Give with no overhead.

eIncrement

🚩

Set Bits

OR bits into the notification value — 32 independent event flags with no extra memory.

eSetBits

🔢

Set Value

Write a 32-bit value directly to the notification register. Receive it with xTaskNotifyWait.

eSetValueWithOverwrite

Code

ISR to Task — notification vs semaphore

notify_isr.c

TaskHandle_t xProcessHandle;

// ISR — notification is faster than xSemaphoreGiveFromISR
void EXTI0_IRQHandler(void) {
    BaseType_t xWoken = pdFALSE;
    vTaskNotifyGiveFromISR(xProcessHandle, &xWoken);
    portYIELD_FROM_ISR(xWoken);
    HAL_GPIO_EXTI_IRQHandler(GPIO_PIN_0);
}

// Task: ulTaskNotifyTake acts like a counting semaphore Take
void vProcessTask(void *pv) {
    while(1) {
        uint32_t n = ulTaskNotifyTake(pdTRUE, portMAX_DELAY);
        ProcessEvents(n); // n = how many ISRs fired
    }
}

// Multi-flag events with xTaskNotifyWait
#define EVT_SENSOR  (1<<0)
#define EVT_UART    (1<<1)
#define EVT_BTN     (1<<2)

void vEventTask(void *pv) {
    uint32_t flags;
    while(1) {
        xTaskNotifyWait(0, 0xFFFFFFFF, &flags, portMAX_DELAY);
        if(flags & EVT_SENSOR) ReadSensor();
        if(flags & EVT_UART)   HandleUART();
        if(flags & EVT_BTN)    HandleButton();
    }
}

⚠️

Notifications are one-to-one only

A task notification targets a single specific task. For broadcast events to multiple tasks, use an Event Group. For data transfer, use a Queue.

Continue the Series

Work through all 8 parts of the FreeRTOS Deep Dive to master real-time embedded systems programming.

Part 3: Software Timers & Idle Hook Patterns → Related Project →