SFRs are declared in the same fashion as other C variables. The only difference is that the data type specified is sfr
rather than char
. For example:
sfr P0 = 0x80; /*Port-0, address 80h */
sfr P1 = 0x90; /*Port-0, address 90h */
sfr P2 = 0xA0; /*Port-0, address A0h */
sfr P3 = 0xB0; /*Port-0, address B0h */
, and P3
are the SFR name declarations. Names for sfr
variables are defined just like other C variable declarations. Any symbolic name may be used in an sfr
The address specification after the equal sign (=) must be a numeric constant. (Expressions with operators are not allowed.) This constant expression must lie in the SFR address range (0x80 to 0xFF).
With typical 8051 applications, it is often necessary to access individual bits with an SFR. The C51 compiler makes this possible with the sbit
data type. The sbit
data type allows you to access bit-addressable SFRs. For example:
This declaration defines EA
to be the SFR bit at address 0xAF
. On the 8051, this is the enable all
bit in the interrupt enable register.
Not all SFRs are bit-addressable. Only those SFRs whose address is evenly divisible by 8 are bit-addressable. The lower nibble of the SFR's address must be 0 or 8. For example, SFRs at 0xA8 and 0xD0 are bit-addressable, whereas SFRs at 0xC7 and 0xEB are not. To calculate an SFR bit address, add the bit position to the SFR byte address. So to access bit 6 in the SFR at 0xC8, the SFR bit address would be 0xCE (0xC8 + 6).
Any symbolic name can be used in an sbit
declaration. The expression to the right of the equal sign (=) specifies an absolute bit address for the symbolic name. There are three variants for specifying the address: