不论是将XBee设计在底板上，还是设计在转接板上，应该预留用XCTU调试XBee模块所需的接口。也就是可以用XCTU通过USB转TTL模块来给XBee供电，升级固件和做异常时固件恢复的工作。

1. Set RTS to low
2. Set BRK to high
3. Wait until the module is reset
4. Set BRK to low
5. Set RTS to high
6. Change baudrate to 115200
During a max time of 20 seconds:
7. Write 0x0D and try to read the bootloader prompt ("BL >")
If the bootloader prompt can be read during the max timeout, finish. Otherwise, fail