A Simple And Cool Flight Controller (ASAC FC) is a flight controller based on the rp2040 microcontroller, completely open-source.
The initial design of the flight controller is done with off-the-shelf components such as an Raspberry Pi Pico and an MPU-6050 as the IMU. This is mainly to make a proof of concept. The second iteration includes a custom PCB with an rp2040 and BMI270 as the IMU.
I then built a drone with 4x single motor ESCs (xsd7a), flysky receiver (FS-a8s) and 3D-printed the frame. I have flewn the drone and it flies surprisingly well for the simple controller, amazing! :)
I know there's downsides of using the rp2040 as a flight controller, but it also has multiple benefits such as being incredibly cheap, quite powerful (up to 133 MHz), it contains two cores, programmable I/O pins (PIO), has an extensive C/C++ SDK, and most importantly, is available!