Semaphore for ISR-to-task signaling in FreeRTOS

Introduction

A semaphore helps a task know when an interrupt has happened. It lets the interrupt tell the task to start working.

When an interrupt signals that new data is ready and a task should process it.

When a hardware event happens and a task needs to respond.

When you want to safely share information between an interrupt and a task.

When you want to pause a task until an interrupt says it can continue.

Syntax

FreeRTOS

SemaphoreHandle_t xSemaphore;

// In task:
if (xSemaphoreTake(xSemaphore, portMAX_DELAY) == pdTRUE) {
    // Process after ISR signals
}

// In ISR:
BaseType_t xHigherPriorityTaskWoken = pdFALSE;
xSemaphoreGiveFromISR(xSemaphore, &xHigherPriorityTaskWoken);
portYIELD_FROM_ISR(xHigherPriorityTaskWoken);

xSemaphoreTake() waits for the semaphore in the task.

xSemaphoreGiveFromISR() signals the semaphore from the interrupt safely.

Examples

Create a binary semaphore and wait forever in the task until ISR signals.

FreeRTOS

xSemaphore = xSemaphoreCreateBinary();

// Task waits for ISR signal
if (xSemaphoreTake(xSemaphore, portMAX_DELAY) == pdTRUE) {
    // Do work after ISR
}

ISR signals the semaphore and requests a context switch if needed.

FreeRTOS

void ISR_Handler(void) {
    BaseType_t xHigherPriorityTaskWoken = pdFALSE;
    xSemaphoreGiveFromISR(xSemaphore, &xHigherPriorityTaskWoken);
    portYIELD_FROM_ISR(xHigherPriorityTaskWoken);
}

Sample Program

This program creates a semaphore, a task waits for the semaphore, and an ISR signals it. The task prints a message when the ISR signals.

FreeRTOS

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

SemaphoreHandle_t xSemaphore;

void vTaskFunction(void *pvParameters) {
    while (1) {
        if (xSemaphoreTake(xSemaphore, portMAX_DELAY) == pdTRUE) {
            printf("Task: ISR signaled, processing now.\n");
        }
    }
}

void ISR_Handler(void) {
    BaseType_t xHigherPriorityTaskWoken = pdFALSE;
    xSemaphoreGiveFromISR(xSemaphore, &xHigherPriorityTaskWoken);
    portYIELD_FROM_ISR(xHigherPriorityTaskWoken);
}

int main(void) {
    xSemaphore = xSemaphoreCreateBinary();
    if (xSemaphore == NULL) {
        printf("Failed to create semaphore.\n");
        return 1;
    }

    xTaskCreate(vTaskFunction, "Task", 1000, NULL, 1, NULL);

    // Start scheduler (in real FreeRTOS)
    // vTaskStartScheduler();

    // Simulate ISR call for demo
    ISR_Handler();

    // Simulate task running once
    vTaskFunction(NULL);

    return 0;
}

Important Notes

Always use xSemaphoreGiveFromISR() inside interrupts to avoid problems.

Use portYIELD_FROM_ISR() to switch to a higher priority task immediately if needed.

Binary semaphores are good for signaling events between ISR and tasks.

Summary

Semaphores let tasks wait for signals from interrupts.

Use xSemaphoreGiveFromISR() in ISR and xSemaphoreTake() in tasks.

This helps tasks respond safely and quickly to hardware events.

Practice

1. What is the main purpose of using a semaphore in FreeRTOS for ISR-to-task signaling?

easy

A. To disable interrupts during task execution

B. To create a new task inside an ISR

C. To safely notify a task from an interrupt service routine (ISR)

D. To allocate memory dynamically in an ISR

Solution

1. Step 1: Understand semaphore role in FreeRTOS

   Semaphores are synchronization tools used to signal between tasks or between ISRs and tasks.

2. Step 2: Identify ISR-to-task signaling purpose

   In FreeRTOS, semaphores let an ISR notify a task safely without blocking or complex synchronization.

3. Final Answer:

   To safely notify a task from an interrupt service routine (ISR) -> Option C

4. Quick Check:

   Semaphore signals task from ISR = A [OK]

Hint: Semaphores signal tasks from ISRs safely [OK]

Common Mistakes:

• Thinking semaphores create tasks
• Confusing semaphores with disabling interrupts
• Using semaphores for memory allocation

2. Which FreeRTOS function is used inside an ISR to give a semaphore to a task?

easy

A. xSemaphoreGiveFromISR()

B. xSemaphoreTakeFromISR()

C. xSemaphoreCreateFromISR()

D. xSemaphoreGive()

Solution

1. Step 1: Recall ISR-safe semaphore functions

   FreeRTOS provides special functions for ISR context to avoid blocking and ensure safety.

2. Step 2: Identify the correct ISR semaphore give function

   The function to give a semaphore from an ISR is xSemaphoreGiveFromISR().

3. Final Answer:

   xSemaphoreGiveFromISR() -> Option A

4. Quick Check:

   Give semaphore from ISR = xSemaphoreGiveFromISR() [OK]

Hint: ISR uses xSemaphoreGiveFromISR() to signal tasks [OK]

Common Mistakes:

• Using xSemaphoreGive() inside ISR (not safe)
• Confusing take and give functions
• Assuming xSemaphoreCreateFromISR() exists

3. Given the code snippet below, what will be the output printed by the task?

SemaphoreHandle_t xSemaphore;

void ISR_Handler() {
    BaseType_t xHigherPriorityTaskWoken = pdFALSE;
    xSemaphoreGiveFromISR(xSemaphore, &xHigherPriorityTaskWoken);
}

void TaskFunction(void *pvParameters) {
    if (xSemaphoreTake(xSemaphore, portMAX_DELAY) == pdTRUE) {
        printf("Semaphore received\n");
    }
}

medium

A. Semaphore received twice

B. No output

C. Runtime error

D. Semaphore received

Solution

1. Step 1: Analyze ISR giving semaphore

   The ISR calls xSemaphoreGiveFromISR() which releases the semaphore.

2. Step 2: Analyze task waiting on semaphore

   The task waits indefinitely with xSemaphoreTake() and prints "Semaphore received" when it gets the semaphore.

3. Final Answer:

   Semaphore received -> Option D

4. Quick Check:

   Task prints after semaphore take = C [OK]

Hint: Task prints after successful semaphore take [OK]

Common Mistakes:

• Thinking semaphore is given twice
• Assuming no output if ISR runs
• Expecting runtime error without initialization

4. Identify the error in the following ISR code snippet for giving a semaphore:

void ISR_Handler() {
    xSemaphoreGive(xSemaphore);
}

medium

A. xSemaphoreGive() cannot be called from an ISR

B. Semaphore must be taken before giving

C. ISR must not use semaphores

D. xSemaphoreGive() requires a pointer argument

Solution

1. Step 1: Check function suitability for ISR

   xSemaphoreGive() is not safe to call inside an ISR because it may block or cause race conditions.

2. Step 2: Identify correct ISR function

   Use xSemaphoreGiveFromISR() inside ISRs to safely give semaphores.

3. Final Answer:

   xSemaphoreGive() cannot be called from an ISR -> Option A

4. Quick Check:

   Use xSemaphoreGiveFromISR() in ISR, not xSemaphoreGive() [OK]

Hint: Use xSemaphoreGiveFromISR() inside ISR, not xSemaphoreGive() [OK]

Common Mistakes:

• Calling xSemaphoreGive() inside ISR
• Ignoring ISR context restrictions
• Assuming semaphore must be taken before giving

5. You want a task to respond immediately when a hardware interrupt occurs. Which sequence correctly uses a semaphore for ISR-to-task signaling in FreeRTOS?

hard

A. Create counting semaphore; task calls xSemaphoreGive(); ISR calls xSemaphoreTakeFromISR()

B. Create binary semaphore; ISR calls xSemaphoreGiveFromISR(); task calls xSemaphoreTake() with portMAX_DELAY

C. Create mutex; ISR calls xSemaphoreGive(); task calls xSemaphoreTake() with zero timeout

D. Create binary semaphore; task calls xSemaphoreGiveFromISR(); ISR calls xSemaphoreTake()

Solution

1. Step 1: Identify correct semaphore type and usage

   Binary semaphore is suitable for signaling events from ISR to task.

2. Step 2: Confirm correct function calls and order

   ISR must call xSemaphoreGiveFromISR() to signal; task waits with xSemaphoreTake() blocking indefinitely (portMAX_DELAY) until signaled.

3. Final Answer:

   Create binary semaphore; ISR calls xSemaphoreGiveFromISR(); task calls xSemaphoreTake() with portMAX_DELAY -> Option B

4. Quick Check:

   Binary semaphore + ISR give + task take = A [OK]

Hint: ISR gives semaphore, task takes with infinite wait [OK]

Common Mistakes:

• Using mutex instead of semaphore
• Calling give in task and take in ISR
• Using zero timeout causing missed signals