Microchip Technology SW006022-2N Hoja De Datos
Device-Related Features
2012 Microchip Technology Inc.
DS52071B-page 87
4.6
USING SFRS
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 device mem-
ory mapped, which means that they appear at, and can be accessed using, specific
addresses in the device’s data memory space. Individual bits within some registers
control independent features. Some registers are read-only; some are write-only. See
your device data sheet for more information.
operation or that of peripheral modules on the device. These registers are device mem-
ory mapped, which means that they appear at, and can be accessed using, specific
addresses in the device’s data memory space. Individual bits within some registers
control independent features. Some registers are read-only; some are write-only. See
your device data sheet for more information.
Memory-mapped SFRs are accessed by special C variables that are placed at the
address of the register. These variables can be accessed like any ordinary C variable
so that no special syntax is required to access SFRs.
address of the register. These variables can be accessed like any ordinary C variable
so that no special syntax is required to access SFRs.
The SFR variable identifiers are predefined in header files and are accessible once you
have included the <xc.h> header file (see Section 4.3 “Device Header Files”) into
your source code. Structures with bit-fields are also defined so you may access bits
within a register in your source code.
have included the <xc.h> header file (see Section 4.3 “Device Header Files”) into
your source code. Structures with bit-fields are also defined so you may access bits
within a register in your source code.
A linker script file for the appropriate device must be linked into your project to ensure
the SFR variable identifiers are linked to the correct address. MPLAB IDE will link in a
default linker script, but a linker script file must be explicitly specified if you are driving
the command-line toolchain. Linker scripts have a .gld extension (e.g.
p30F6014.gld)
the SFR variable identifiers are linked to the correct address. MPLAB IDE will link in a
default linker script, but a linker script file must be explicitly specified if you are driving
the command-line toolchain. Linker scripts have a .gld extension (e.g.
p30F6014.gld)
and basic files are provided with the compiler.
The convention in the processor header files is that each SFR is named, using the
same name that appears in the data sheet for the part – for example, CORCON for the
Core Control register. If the register has individual bits that might be of interest, then
there will also be a structure defined for that SFR, and the name of the structure will be
the same as the SFR name, with “bits” appended. For example, CORCONbits for the
Core Control register. The individual bits (or bit fields) are named in the structure using
the names in the data sheet – for example PSV for the PSV bit of the CORCON
register.
same name that appears in the data sheet for the part – for example, CORCON for the
Core Control register. If the register has individual bits that might be of interest, then
there will also be a structure defined for that SFR, and the name of the structure will be
the same as the SFR name, with “bits” appended. For example, CORCONbits for the
Core Control register. The individual bits (or bit fields) are named in the structure using
the names in the data sheet – for example PSV for the PSV bit of the CORCON
register.
Here is the complete definition of CORCON (subject to change):
/* CORCON: CPU Mode control Register */
extern volatile unsigned int CORCON
__
attribute
_ _
((
_ _
sfr
_ _
));
typedef struct tagCORCONBITS {
unsigned IF :1; /* Integer/Fractional mode */
unsigned RND :1; /* Rounding mode */
unsigned PSV :1; /* Program Space Visibility enable */
unsigned IPL3 :1;
unsigned ACCSAT :1; /* Acc saturation mode */
unsigned SATDW :1; /* Data space write saturation enable */
unsigned SATB :1; /* Acc B saturation enable */
unsigned SATA :1; /* Acc A saturation enable */
unsigned DL :3; /* DO loop nesting level status */
unsigned :4;
} CORCONBITS;
extern volatile CORCONBITS CORCONbits _ _attribute_ _((_ _sfr_ _));
See MPLAB Assembler, Linker and Utilities for PIC24 MCUs and dsPIC DSCs User’s
Guide (DS51317) for more information on using linker scripts.
Guide (DS51317) for more information on using linker scripts.
For example, the following is a sample real-time clock. It uses an SFR, e.g. TMR1, as
well as bits within an SFR, e.g. T1CONbits.TCS. Descriptions for these SFRs are
found in the p30F6014.h file (this file will automatically be included by <xc.h> so you
do not need to include this into your source code). This file would be linked with the
device specific linker script which is p30F6014.gld.
well as bits within an SFR, e.g. T1CONbits.TCS. Descriptions for these SFRs are
found in the p30F6014.h file (this file will automatically be included by <xc.h> so you
do not need to include this into your source code). This file would be linked with the
device specific linker script which is p30F6014.gld.
Note:
The symbols CORCON and CORCONbits refer to the same register and will
resolve to the same address at link time.
resolve to the same address at link time.