The Blue Pill is a mysterious, brandless development board for the STM32F103 microcontroller. You can buy one online for around $2. (Fun, right?)
This is a project to program it to act like a tiny USB flash drive.
- STM32F103C8 Datasheet - Includes an overview of the device, electrical information, and the memory map.
- RM0008 - The reference manual for the STM32F family of devices, detailing their memory and peripherals.
- PM0075 - How to access the device's flash memory.
- The Blue Pill on STM32-base - An unofficial overview of the board, including a schematic.
- USB in a NutShell - A hitchhiker's guide to USB implementation details. Quite outdated ("USB 2.0...is still yet to see day light on mainstream desktop computers"), but still useful.