C51: Accessing an External SFR in a C Program

Information in this article applies to:

• C51 All Versions

QUESTION

In C51, how can I declare SFRs in one source module and access them as external declarations in another source module?

ANSWER

First, you need to declare the SFR variables in an assembly program as shown below.

; ---------------------- A51 PROGRAM ------------------------
 LED_PORT        bit     091h    ; Port 1.1
 public  LED_PORT

 SWITCH_PORT     bit     0A2h    ; Port 2.2
 public  SWITCH_PORT

 LCD_DATA_PORT   data    0B0h    ; Port 3
 public  LCD_DATA_PORT

         END
; -----------------------------------------------------------

Then, you can write a C program to access these "external" SFRs. For example:

// ----------------------- C PROGRAM -------------------------
 extern bit LED_PORT;
 extern bit SWITCH_PORT;
 extern data unsigned char LCD_DATA_PORT;

.
 LED_PORT = 0;        /* turn off the led */

 a = SWITCH_PORT;     /* read the switch */

 LCD_DATA_PORT = 'A'; /* print A on the LCD */

// -----------------------------------------------------------

This is useful if you want to create a library of routines that access STUFF attached to the port pins of the 8051 like a push button on P1.1 and an LED on P2.3. However, you realize that the next project you do will have a button on a different port and an LED on still a different port. Furthermore, you want to make the library generic so you can use it over and over once it is debugged. Obviously, you need to have a way to "assign" the SFR addresses outside of the library.

There are several reasons why this is a cool idea. One is so that you can have a generic library of routines that you use that always work that you provide as a courtesy to clients. You give them the library but NOT the source code. To make this work, all you need to do is to change the addresses in the A51 program to match your hardware, link it in with the library, and PRESTO, you have external SFR definitions!

SEE ALSO

• C51: General Purpose SFR Interface
• C51: External SFR Access for 8051 Programs
• C51: Configurable SFR Bit Accesses
• BL51: Warning 1 (Unresolved External Symbol) Using SBITs

Last Reviewed: Thursday, February 25, 2021