Phase 3: Building a FreeRTOS application on STM32L475 (B-L475E-IOT01A) from Scratch

No IDE | GCC | Make | OpenOCD

🧠 The Mission

Escaping the abstraction of proprietary IDEs to build a production-grade FreeRTOS environment from raw source code. This phase focuses on the “plumbing” of embedded systems—mastering the build process, dependency management, and hardware bring-up without the “magic” of code generators.

🛠 Core Objectives

1. Toolchain Independence

GCC & Make: Replacing “Click-to-Build” buttons with manual compilation rules, vpath source discovery, and wildcard file handling.
Linker Mechanics: Understanding memory layout (.text, .data, .bss) and manually configuring the STM32L475 Flash/RAM partitioning via .ld scripts.
Garbage Collection: Utilizing -ffunction-sections and --gc-sections to strip unused HAL drivers from the final binary.

2. Manual BSP Integration (The “Hunt”)

Driver Extraction: Manually gathering CMSIS core headers, STM32L4 HAL drivers, and Startup Assembly code from vendor SDKs.
Dependency Resolution: Solving complex include chains (Legacy headers, Extended _ex drivers) and eliminating conflicting template files.
Interrupt Mapping: Manually mapping FreeRTOS kernel handlers (SVC, PendSV, SysTick) to the STM32 vector table.

3. OS Configuration

Minimal Config: Crafting a FreeRTOSConfig.h from scratch, tailored specifically for the Cortex-M4F.
Hook Management: configuring the OS to run without standard hooks (Idle, Tick, Stack Overflow) for initial bring-up simplicity.

🚀 Key Milestones

The Clean Architecture: A modular project structure separating Drivers, Middlewares, and Application code.
The Bulletproof Makefile: A robust build script capable of compiling the entire HAL + OS stack.
Alive on Hardware: A validated Green LED blink (PB14) driven by the FreeRTOS scheduler, flashed and verified using OpenOCD.