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BL51: Warning 1 (Unresolved External Symbol) Using SBITs

Information in this article applies to:

  • C51 Version 5.50a and later


I want to use a bit of Port P1 for a chip select line. I have declared the chip select as follows:

sbit CS=P1^4;

In a second module I need to access this SFR, so I declare it as an extern:

extern bit CS;

When I link, I receive the following linker error:

Warning L1: Unresolved External Symbol
Symbol: CS
Module: second module name

Where have I made a mistake?


Unfortunately, you can't declare sbits as extern. You must declare the sbit in each C file (or create a header file for it). Just include the SBIT definition you already have in each file.


  • Review the REG??.H include files in the \C51\INC directory for examples of how you can declare SFRs in an include file.
  • Refer to Warning L1 in the BL51 User's Guide.


Last Reviewed: Thursday, February 25, 2021

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