---
description: Embedded C/C++ rules for MCU, STM32, HAL, interrupts, DMA, memory constraints, and hardware-focused testing
globs: ["**/*.c", "**/*.h", "**/*.cpp", "**/*.hpp", "**/*.ioc", "CMakeLists.txt", "Makefile", "platformio.ini"]
alwaysApply: false
---

# Embedded MCU, STM32, and HAL Rules

## Project Structure

- Keep board support, drivers, middleware, application logic, and tests separate.
- Isolate generated CubeMX or vendor code from hand-written application code.
- Put hardware abstraction behind narrow interfaces so logic can be tested without hardware.
- Document clock tree, pin mappings, peripheral ownership, and interrupt priorities.

## STM32 HAL and Peripherals

- Initialize peripherals in one place and avoid hidden reconfiguration.
- Check return values from HAL calls and handle timeout/error cases.
- Keep blocking HAL calls out of time-critical paths.
- Use DMA for high-throughput UART, SPI, I2C, ADC, or timer capture paths when appropriate.
- Document buffer ownership and lifetime for DMA operations.
- Use `volatile` only for memory shared with ISRs or hardware registers.

## Interrupts and Concurrency

- Keep ISRs short and deterministic.
- Defer heavy work from interrupts to the main loop, RTOS task, or event queue.
- Protect shared data with critical sections, atomics, queues, or RTOS primitives.
- Avoid dynamic allocation in interrupts.
- Make interrupt priority decisions explicit.

## Memory and Timing

- Avoid heap allocation in firmware unless the project explicitly allows it.
- Check stack usage for ISRs and RTOS tasks.
- Keep lookup tables `const` so they can live in flash.
- Use fixed-width integer types for hardware-facing code.
- Add timeouts for hardware waits.
- Treat watchdog configuration as part of application design, not a late add-on.

## Testing and Debugging

- Unit test pure logic on host builds.
- Use hardware-in-the-loop tests for peripheral behavior.
- Add assertions for impossible hardware states in debug builds.
- Use SWD/JTAG, logic analyzers, and serial logs with rate limits.
- Keep fault handlers useful: capture reset reason, fault registers, and build version when possible.

## Common Mistakes

- Do not modify generated files unless the workflow preserves changes.
- Do not busy-wait forever on hardware flags.
- Do not share buffers between DMA and CPU without synchronization.
- Do not assume peripheral reset state after low-power modes.