在Executable file下编译UART例程出现如下错误：

[Error li1021]  The following symbols referenced in processor 'p0' could not be resolved:
        'ssync [_ssync]' referenced from '.\Debug\uart.doj'

Linker finished with 1 error
cc3089: fatal error: Link failed
Tool failed with exit/exception code: 1.
Build was unsuccessful.

请问是怎么回事，要怎么修改？谢谢