C51编译警告“UNRESOLVED EXTERNAL SYMBOL”

*** WARNING L1: UNRESOLVED EXTERNAL SYMBOL
    SYMBOL: LED_OFF
    MODULE: .\OutputFiles\Timer.obj (TIMER)
*** WARNING L1: UNRESOLVED EXTERNAL SYMBOL
    SYMBOL: LED_ON
    MODULE: .\OutputFiles\Timer.obj (TIMER)
*** WARNING L1: UNRESOLVED EXTERNAL SYMBOL
    SYMBOL: LED
    MODULE: .\OutputFiles\Timer.obj (TIMER)
*** WARNING L2: REFERENCE MADE TO UNRESOLVED EXTERNAL
    SYMBOL: LED
    MODULE: .\OutputFiles\Timer.obj (TIMER)
    ADDRESS: 0048H
*** WARNING L2: REFERENCE MADE TO UNRESOLVED EXTERNAL
    SYMBOL: LED
    MODULE: .\OutputFiles\Timer.obj (TIMER)
    ADDRESS: 004DH
*** WARNING L2: REFERENCE MADE TO UNRESOLVED EXTERNAL
    SYMBOL: LED
    MODULE: .\OutputFiles\Timer.obj (TIMER)
    ADDRESS: 0052H
*** WARNING L2: REFERENCE MADE TO UNRESOLVED EXTERNAL
    SYMBOL: LED
    MODULE: .\OutputFiles\Timer.obj (TIMER)
    ADDRESS: 0056H
*** WARNING L2: REFERENCE MADE TO UNRESOLVED EXTERNAL
    SYMBOL: LED
    MODULE: .\OutputFiles\Timer.obj (TIMER)
    ADDRESS: 0058H
*** WARNING L2: REFERENCE MADE TO UNRESOLVED EXTERNAL
    SYMBOL: LED_ON
    MODULE: .\OutputFiles\Timer.obj (TIMER)
    ADDRESS: 0099H
*** WARNING L2: REFERENCE MADE TO UNRESOLVED EXTERNAL
    SYMBOL: LED_OFF
    MODULE: .\OutputFiles\Timer.obj (TIMER)
    ADDRESS: 00A0H
在用C51编译器出现上面的警告，害我郁闷半天，没道理啊，我仔细检查了两遍没问题，结果错误出在没有将一个源文件添加进入工程 ，看来我还是个初学者，居然犯这样的低级错误。