FreeRTOS Parking Lot Simulation Using a Counting Semaphore

This project simulates a parking lot system on an Arduino UNO using FreeRTOS counting semaphores.

It demonstrates how multiple tasks can safely share a limited resource—parking spaces—without race conditions or conflicts.

Each car is represented by a FreeRTOS task that must acquire a semaphore before parking and release it when exiting.

Project Overview

In real-time systems, resources are often limited. FreeRTOS counting semaphores provide a clean solution to control how many tasks can access a resource simultaneously.

In this project:

• Parking spaces are limited to two
• Multiple “car” tasks attempt to park
• Tasks block automatically when no space is available
• A manual override button can free a space

Semaphore support is enabled with:

#include <Arduino_FreeRTOS.h>
#include <semphr.h>

Hardware Setup

✅ The exit button uses INPUT_PULLUP (pressed = LOW).

Semaphore Design

• Type: Counting Semaphore
• Max Count: 2 (number of parking spaces)
• Initial Count: 2

🧠 Conceptual Model:

The semaphore acts like a parking ticket dispenser.

A car must take a ticket (semaphore) to park.

If no ticket is available, the car waits.

Car Task Behavior

Car Task 1

This task attempts to park, occupies a slot, waits, and then exits.

void CarTask1(void *pvParameters) {
  while (1) {
    if (xSemaphoreTake(xParkingSemaphore, portMAX_DELAY) == pdTRUE) {
      digitalWrite(parkingLEDs[0], HIGH);
      digitalWrite(ENTRY_GATE_LED, HIGH);
      vTaskDelay(pdMS_TO_TICKS(200));
      digitalWrite(ENTRY_GATE_LED, LOW);

      Serial.print("Car 1: Parked. Spaces left: ");
      Serial.println(uxSemaphoreGetCount(xParkingSemaphore));

      vTaskDelay(pdMS_TO_TICKS(3000)); 

      digitalWrite(parkingLEDs[0], LOW);
      digitalWrite(EXIT_GATE_LED, HIGH);
      vTaskDelay(pdMS_TO_TICKS(200));
      digitalWrite(EXIT_GATE_LED, LOW);

      xSemaphoreGive(xParkingSemaphore);
      Serial.println("Car 1: Left parking");
    }
  }
}

Car Task 2

Similar to Car 1, but starts slightly later to show task staggering.

void CarTask2(void *pvParameters) {
  vTaskDelay(pdMS_TO_TICKS(1000)); // Stagger start

  while (1) {
    if (xSemaphoreTake(xParkingSemaphore, portMAX_DELAY) == pdTRUE) {
      digitalWrite(parkingLEDs[1], HIGH);
      vTaskDelay(pdMS_TO_TICKS(4000)); // Simulate parking time
      digitalWrite(parkingLEDs[1], LOW);

      xSemaphoreGive(xParkingSemaphore);
      Serial.println("Car 2: Left parking");
    }
  }
}

Manual Exit Override Task

This task monitors a button and forces a parking space to be released if pressed.

void ExitButtonTask(void *pvParameters) {
  while (1) {
    if (digitalRead(EXIT_BUTTON) == LOW) {
      if (uxSemaphoreGetCount(xParkingSemaphore) < TOTAL_PARKING_SPACES) {
        xSemaphoreGive(xParkingSemaphore);
        digitalWrite(OVERRIDE_LED, HIGH);
        Serial.println("Override: Manual exit triggered");
        vTaskDelay(pdMS_TO_TICKS(500));
        digitalWrite(OVERRIDE_LED, LOW);
      }
    }
    vTaskDelay(pdMS_TO_TICKS(100));
  }
}

Task Creation and Scheduler Control

All tasks and the counting semaphore are created in setup().

xParkingSemaphore = xSemaphoreCreateCounting(
  TOTAL_PARKING_SPACES,
  TOTAL_PARKING_SPACES
);

xTaskCreate(CarTask1, "Car1", 128, NULL, 1, NULL);
xTaskCreate(CarTask2, "Car2", 128, NULL, 1, NULL);
xTaskCreate(ExitButtonTask, "Exit", 128, NULL, 2, NULL);

The loop()

function remains empty because FreeRTOS manages execution:

void loop() {}

LED Behavior Summary

Key Learning Points

• Counting semaphores manage limited resources
• Tasks automatically block when resources are unavailable
• Semaphore count directly represents system capacity
• Manual overrides can safely interact with semaphore logic

Learning Outcomes

• Understand counting vs binary semaphores
• Learn how task blocking works in FreeRTOS
• Design scalable systems with shared resource limits

This project provides a clear, real-world visualization of counting semaphores, a critical concept in professional real-time embedded systems design.

That's all!

If you have any questions or suggestions, don’t hesitate to leave a comment below.