5. Trouble Shooting
5.1. Flash file system
- MicroPython REPL prompt is not displayed. - Re-program the MicroPython image file. 
- Confirm serial port connection. The connection must be crossed that the board TXD pin is connected with USB-serial conversion’s RXD signal pin. 
- If the prompt is not displayed suddenly, try to do factory file system boot mode as the final method. Please note that all files are removed forever.