Hello Zhou Yigang,
If you had installed the CY3684 DVK, you will find the FX2LPSerial.c and FX2LPSerial.h files in the following directory:
The CY3684 DVK can be installed from the below link:
These files FX2LPSerial.c and FX2LPSerial.h can be used to debug the firmware. Any information can be printed on a virtual terminal on the host. Please refer to the app note AN58009 (http://www.cypress.com/file/44386/download ) for more details on implementation of this serial port debugging.
Hope this helps.