7.5 Using SFRs From C Code

The Special Function Registers (SFRs) are registers which control aspects of the MCU operation or that of peripheral modules on the device. These registers are memory mapped, which means that they appear at specific addresses in the device memory map. With some registers, the bits within the register control independent features.

Memory-mapped SFRs are accessed by special C variables that are placed at the addresses of the registers and use special attributes. These variables can be accessed like any ordinary C variable so that no special syntax is required to access SFRs.

The SFR variables are predefined in header files and will be accessible once the <xc.h> header file (see 7.2 Device Header Files) has been included into your source code. Structures are also defined by these header files to allow access to bits within the SFR.

The names given to the C variables, which map over the registers and bit variables, or bit fields, within the registers are based on the names specified in the device data sheet. The names of the structures that hold the bit fields will typically be those of the corresponding register followed by bits. For example, the following shows code that includes the generic header file, clears PORTB as a whole and sets bit 2 of PORTB using the structure/bit field definitions

#include <xc.h>
int main(void)
{
  PORTBCLR = 0xFFFFu;
  PORTBbits.RB2 = 1;
  PORTBSET= _PORTB_RB2_MASK;
}

For use with assembly, the PORTB register is declared as: .extern PORTB.

To confirm the names that are relevant for the device you are using, check the device specific header file that <xc.h> will include for the definitions of each variable. These files will be located in the pic32mx/include/proc directory of the compiler and will have a name that represents the device. There is a one-to-one correlation between device and header file names that will be included by <xc.h>, for example, when compiling for a PIC32MX360F512L device, the <xc.h> header file will include <proc/p32mx360f512l.h>. Remember that you do not need to include this chip-specific file into your source code; it is automatically included by <xc.h>.

Some of the PIC32 SFRs have associated registers that allow the bits within the SFR to be set, cleared or toggled atomically. For example, the PORTB SFR has the write-only registers PORTBSET, PORTBCLR and PORTBINV associated with it. Writing a '1' to a bit location in these registers sets, clears or toggles, respectively, the corresponding bit in the PORTB SFR. So to set bit 1 in PORTB, you can use the following code:

PORTBSET = 0x2;

or alternatively, using macros provided in the device header files:

PORTBSET = _PORTB_RB1_MASK;