Air Quality Monitor - STM32 RTOS Project

A production-quality embedded system for indoor air quality monitoring, built on the STM32F446RE microcontroller with FreeRTOS and Modbus RTU communication.

Project Overview

This firmware implements a complete air quality sensor system that acquires environmental data (temperature, humidity, VOC index), logs it to non-volatile memory, and exposes it via Modbus RTU for remote monitoring and control.

Technical Architecture

Hardware Platform:

• MCU: STM32F446RE (ARM Cortex-M4, 180 MHz)
• Sensors: Sensirion SHT3x (temperature/humidity), SGP40 (VOC index)
• Storage: SPI FRAM for non-volatile data logging
• Communication: UART-based Modbus RTU slave

Software Stack:

• RTOS: FreeRTOS with custom task architecture
• Drivers: Register-level peripheral drivers using CMSIS (no HAL)
• Communication Protocol: Custom Modbus RTU implementation
• Development: Bare-metal embedded C with modular architecture

Key Features & Technical Skills Demonstrated

Low-Level Firmware Development

• Register-level peripheral drivers for GPIO, UART, SPI, I2C using CMSIS
• Interrupt-driven communication with DMA support
• Hardware abstraction layers for sensor and storage interfaces
• Clock tree configuration and power management

Real-Time Operating System (FreeRTOS)

• Multi-task architecture with 5 concurrent application tasks:
  • Sensor Data Acquisition Task
  • System Health Monitor Task
  • Error Handler Task
  • Modbus Slave Task
  • Modbus Data Manager Task
• Synchronization mechanisms: Queues, semaphores, mutexes, task notifications, event groups
• Non-blocking peripheral drivers using RTOS primitives
• Watchdog integration for fault recovery

Communication & Data Management

• Custom Modbus RTU slave implementation with register mapping
• SPI FRAM driver for reliable data logging
• I2C sensor drivers with error handling and retry logic
• Structured data flow between tasks using queues and shared memory

Professional Software Engineering Practices

• Modular architecture with clear separation of concerns
• Scalable design patterns for embedded systems
• Comprehensive error handling and fault detection
• Production-ready code structure suitable for commercial products

Technology Stack

Layer	Technology
Hardware	STM32F446RE, Sensirion sensors, SPI FRAM
RTOS	FreeRTOS (tasks, queues, semaphores, mutexes)
Peripherals	UART, SPI, I2C, ADC, DMA, Timers, GPIO
Protocol	Modbus RTU slave
Language	Embedded C
Toolchain	ARM GCC, STM32CubeIDE

System Architecture

┌─────────────────────────────────────────────┐
│          FreeRTOS Application Layer          │
├─────────────────────────────────────────────┤
│  Sensor Task  │  Health Monitor  │  Modbus   │
│  Error Handler │  Data Manager               │
├─────────────────────────────────────────────┤
│        Custom Peripheral Drivers             │
│    (UART, SPI, I2C, GPIO, ADC, Timers)      │
├─────────────────────────────────────────────┤
│              CMSIS Core                      │
├─────────────────────────────────────────────┤
│            STM32F446RE Hardware              │
└─────────────────────────────────────────────┘

Project Highlights

• Production-oriented design: Emphasis on maintainability, scalability, and robustness
• Complete system implementation: From power-on to operational data acquisition and communication
• Professional code quality: Modular structure, comprehensive error handling, clear documentation
• Real-world applicability: Demonstrates skills directly transferable to commercial embedded products