Build target 'Target 1'
assembling STARTUP.A51...
compiling 10PWM.c...
compiling UART.c...
linking...
*** WARNING L16: UNCALLED SEGMENT, IGNORED FOR OVERLAY PROCESS
SEGMENT: ?PR?_UARTPUTCHAR?UART
*** WARNING L1: UNRESOLVED EXTERNAL SYMBOL
SYMBOL: _PRINTS
MODULE: 10PWM.obj (10PWM)
*** WARNING L2: REFERENCE MADE TO UNRESOLVED EXTERNAL
SYMBOL: _PRINTS
MODULE: 10PWM.obj (10PWM)
ADDRESS: 075BH
Program Size: data=148.0 xdata=0 code=1965
creating hex file from "10PWM"...
"10PWM" - 0 Error(s), 3 Warning(s).
请问如何解决？附KEIL4工程文件。