Dobrica Pavlinušić's random unstructured stuff
Blue Pill: Revision 4

stm32f103 board

arduino boot loader

triple usb serial

Function Pin and Port
USART3 TX PB10
USART3 RX PB11
USART2 TX PA2
USART2 RX PA3
USART1 TX PA9
USART1 RX PA10

USART TX pin RX pin  
3 PB10 PB11  
2 PA2 PA3  
1 PA9 PA10 BOOT=0

dpavlin@nuc:/nuc/stm32/pill_serial$ cat flash.sh
sudo ../stlink/build/Release/st-flash write src/pill_serial.bin 0x08000000