This is a development board for (LQFP-48) STM32 microcontrollers like the STM32F103C8T6 and STM32F072C8T6. It is effectively 1/2 of the infamous 'blue-pill'.
Hold down the BOOT0/PA15 button while powering on (or press RESET and BOOT0 together, then release RESET) to access the built-in USB DFU bootloader. The BOOT0/PA15 button can also be used for user input while not in the bootloader.
PCB design was done using KiCad.
PCBs are available at OSHPARK (or upload the included gerber.zip to your favorite board house): https://oshpark.com/shared_projects/8USgL5pT
