FreeRTOS Weather MQTT – Project C1

An application running on Raspberry Pi 5 using the FreeRTOS POSIX/Linux Simulator. The program periodically fetches weather data from the OpenWeatherMap API and publishes it to an MQTT broker.

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How It Works

The application consists of two FreeRTOS tasks and one software timer.

Timer – Odczyt_Danych

• Fires every 2 hours (default: PERIOD_ASK = 7 200 000 ms)
• Wakes up the vWeatherTask task via xTaskNotifyGive

Task – vWeatherTask (data acquisition)

1. Waits for a notification from the timer
2. Iterates over a grid of measurement points defined in the latitudes[] and longitudes[] arrays
3. For each point, performs an HTTP request to the OpenWeatherMap API using curl and retrieves a JSON response
4. Parses the response using the cJSON library, extracting:
   • coord.lon / coord.lat – coordinates
   • main.temp – temperature in Celsius
   • wind.speed – wind speed in m/s
   • weather[0].description – weather description
5. Builds a JSON array from all measurement points and sends it to jsonQueue
6. Signals vSendData that data is ready via dataReadySem

Task – vSendData (MQTT publishing)

1. Waits on the dataReadySem semaphore
2. Receives the JSON array from the queue
3. Serializes the JSON to a string and calls mosquitto_pub via system()
4. Publishes the data to topic projekt/pogoda/C1 on the configured broker

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File Structure

.
├── main_blinky.c                  # Main application logic (tasks, timer)
├── main.c                         # Entry point – calls main_blinky()
├── cJSON/
│   ├── cJSON.c                    # JSON parsing library
│   └── cJSON.h
├── FreeRTOSConfig.h               # FreeRTOS configuration
├── CMakeLists.txt                 # CMake build system
├── Makefile                       # Alternative build file
└── README.md

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Requirements

• Raspberry Pi 5 running Linux (Raspberry Pi OS or Ubuntu)
• FreeRTOS POSIX/Linux Simulator
• curl – for fetching data from OpenWeatherMap
• mosquitto-clients – for publishing via MQTT (mosquitto_pub)
• An API key from openweathermap.org
• A running MQTT broker accessible on the network (e.g. Mosquitto)

Installing dependencies on Raspberry Pi OS / Debian

sudo apt update
sudo apt install curl mosquitto-clients build-essential cmake

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Setup and Running

1. Clone the repository

git clone <REPOSITORY_URL>
cd <repository-directory>

2. Configure credentials and broker

In main_blinky.c, set the following defines:

#define MQTT_HOST    "your_broker_ip"
#define MQTT_PORT    1883
#define MQTT_TOPIC   "projekt/pogoda/C1"
#define OWM_API_KEY  "your_api_key"

Optionally, adjust the measurement points in the latitudes[] and longitudes[] arrays.

3. Build

mkdir build && cd build
cmake ..
make

Or using the Makefile:

make

4. Run

./FreeRTOS_Weather

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MQTT Payload Format

Data published to the topic is a JSON array:

[
  {
    "lon": 14.1,
    "lat": 49.0,
    "temp": 18.45,
    "speed": 3.2,
    "description": "light clouds"
  }
]

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FreeRTOS Synchronization

Mechanism	Usage
xTimerCreate	Periodic wake-up every 2 hours
xTaskNotifyGive	Timer notifies vWeatherTask
xSemaphoreCreateMutex (czekaj_na_dane)	Protects the data acquisition critical section
xSemaphoreCreateBinary (dataReadySem)	Signals that data is ready to send
xQueueCreate (jsonQueue)	Passes the JSON pointer between tasks

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FreeRTOS POSIX/Linux Simulator

This project uses the FreeRTOS Simulator for Linux, which allows FreeRTOS code to run natively on a Linux system without dedicated embedded hardware. Each FreeRTOS task is mapped to a POSIX thread (pthread), so queues, semaphores, and timers all work as they would on a microcontroller. This makes Raspberry Pi 5 a convenient development and deployment platform.

More information: https://github.com/FreeRTOS/FreeRTOS-Simulator-for-Linux