I finally added minimum task scheduling for the RTOS. I had originally designed a simple kernel to create a buffer of scheduled tasks in QEMU, but after learning that the simulation didn’t actually support the specific MCU that my friend and I are using for the real drone, I had to pivot. I switched over to using STM32CubeMX for the actual bare-metal configuration and Renode to handle running the emulation. Right now, the kernel just executes a task based on a basic timing loop and priority, waits for it to completely finish, and then moves on to the next task.

To be honest, it isn’t super useful yet because it only covers tasks in what I call the “scheduled zone” of the drone. This includes background stuff like blackbox flight data management, but it leaves out the most critical parts like the PID control loops and streaming real-time UART data from the IMU. Those features need to fire their own asynchronous interrupt signals at the highest possible hardware priority so they can instantly pause the optional background tasks and execute the critical code that actually keeps the drone stable and flying.

Next up, I need to write a short piece of assembly to handle saving the current CPU state and registers of whatever task is currently running when an interrupt hits. It shouldn’t be too bad since it will only take about 30 lines of code. I know I haven’t been posting devlogs for the past couple of weeks even though I’ve been grinding on this, but that’s mostly because we spent a ton of time initially planning out the architecture of the drone. We wanted to map out the software boundaries early so my friend and I could go our separate ways working on different systems without having massive merge conflicts later on.