Usually a microcontroller/DSP manufacturer would provide a register definition header along with a development tool. However, some header files do not come in a good way, which offers difficulties in C/C++ programming. So before coding in C, do some foundation work in the register header file if it was not good shaped.
A good shape of a register header file shall have structure definition in both integer and bits format, as well as assigned to the respective register address.
Example: Renesas SH7047, port A data register L (PADRL) is a 16-bit readable/writable register that stores port A data. It shall be defined in header file (sh7047reg.h) as the following.
struct st_pa // struct PA
{
union // PADRL
{
unsigned short WORD; // Word Access
struct // Byte Access
{
unsigned char H; // High byte
unsigned char L; // Low byte
} BYTE;
struct // Bit Access
{
unsigned char B15:1; // Bit 15
unsigned char B14:1; // Bit 14
unsigned char B13:1; // Bit 13
unsigned char B12:1; // Bit 12
unsigned char B11:1; // Bit 11
unsigned char B10:1; // Bit 10
unsigned char B9 :1; // Bit 9
unsigned char B8 :1; // Bit 8
unsigned char B7 :1; // Bit 7
unsigned char B6 :1; // Bit 6
unsigned char B5 :1; // Bit 5
unsigned char B4 :1; // Bit 4
unsigned char B3 :1; // Bit 3
unsigned char B2 :1; // Bit 2
unsigned char B1 :1; // Bit 1
unsigned char B0 :1; // Bit 0
} BIT;
} DRL;
};
#define PA (*(volatile struct st_pa *)0xFFFF8382) // PA Address
then in C code, you can access the register either by word, by byte, or by bits as shown below.
#include “sh7047reg.h”
PA.DRL.WORD = 0x1234; //access word
PA.DRL.BYTE.H = 0x12; //access high byte
PA.DRL.BYTE.L = 0x34; //access low byte
PA.DRL.BIT.B0 = 1; //access LSB
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