Dunfield Development Services Inc.
Customer Support -- Application Note #5101
8051 MICRO-C
Defining additional SFRs for direct access
Applies to: [MICRO-C for 8051
]
Last updated: Sunday May 04, 2003
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PROCEDURE
As shipped, the MICRO-C 8051 compiler can handle all of the special function registers found in standard 8051 and 8052 type processors, by simply including
the file "8051reg.h". This file contains definitions for the register names, which are built into the assembler. Once this file has been #included, you
can read and write the registers just as if they were global 'C' variables.
EG: P1 = 0;
All SFR's are declared in 8051reg.h as "extern register char". The "extern" tells the compiler that the name is defined outside of the program module,
the "register" causes it to be accessed as a direct (8 bit internal address) entity, and the "char" defines it as an 8 bit quantity.
The standard names are known internally to the assembler, and therefore are not actually defined anywhere in the library. To fool the source linker
(SLINK) into thinking that it has resolved the external references, the SFR names are listed in the library index
file(s) as being defined within the
startup code (8051RLP?.ASM). Since these files are always included, the linker is satisfied that the variables are defined.
The address map for the 8051/52 Special Function Registers has many unused addresses. Several manufacturers have use these addresses to extend the basic
register set to include additional registers which control hardware unique to that manufacturers device. An example of this is the DS5000 processor from
Dallas Semiconductor. It defines two new SFR's, the "Memory Control" register (MCON) at address $C6, and the "Timer Access" register (TA) at address $C7.
To access these new registers in MICRO-C, you must accomplish two things:
- Inform the assembler of the direct address for the register This is done by adding a new file to the library, which contains EQU
statements giving the SFR names addresses.
- Inform the compiler of the name and type of the register This is done by adding definitions to 8051reg.h as described above.
Here is an example command sequence for defining the new DS5000 registers. You may use your favorite editor instead of the "copy con" & "type" commands:
C:\> cd \mc\lib51 ; Position to 8051 library
C:\MC\MC51> copy con extreg.asm ; Create a new file
MCON EQU $C6 ; Memory Control register address
TA EQU $C7 ; Timed Access register address
^Z ; End of file (exit)
C:\MC\MC51> \mc\slib i=TINY a=extreg ; Add to TINY library
C:\MC\MC51> \mc\slib i=SMALL a=extreg ; Add to SMALL library
C:\MC\MC51> \ms\slib i=COMPACT a=extreg ; Add to COMPACT library
C:\MC\MC51> \mc\slib i=MEDIUM a=extreg ; Add to MEDIUM library
C:\MC\MC51> \mc\slib i=LARGE a=extreg ; Add to LARGE library
C:\MC\MC51> cd \mc ; Switch to home directory
C:\MC> type con >>8051reg.h ; Append to 8051reg.h
extern register char MCON, TA; ; 'C' definitions
^Z ; end of file
C:\MC>
NOTE: If you make changes/additions to the registers defined in "extreg.asm"
you will have to REMOVE and re-ADD it to all memory models (using SLIB) so that the .LIB files will be updated.
If you prefer, you can use inline assembly for the EQU's instead of making the library file. This has the advantage that you don't have to mess with
the library, and keeps the changes all in one place:
C:\MC> type con >>8051reg.h ; Append to 8051reg.h
asm { ; Begin inline assembly
MCON EQU $C6 ; Memory Control register address
TA EQU $C7 ; Timed Access register address
} ; End inline assembly
extern register char MCON, TA; ; 'C' definitions
^Z ; end of file
C:\MC>
If you prefer NOT to modify any of the original Micro-C files, you can just as easily place the above in a separate file (lets call it newreg.h),
and #include that file when you wish to access the new registers.
Also, if you prefer to make the SFR's appear as unsigned values (useful when I/O ports hold 8 bit unsigned quantities), change the 'C' declaration to:
extern register unsigned char ...
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