Tips on redefining a register bitfield in C

Question

I am struggling trying to come up with a clean way to redefine some register bitfields to be usable on a chip I am working with.

For example, this is what one of the CAN configuration registers is defined as:

extern volatile near unsigned char       BRGCON1;
extern volatile near struct {
  unsigned BRP0:1;
  unsigned BRP1:1;
  unsigned BRP2:1;
  unsigned BRP3:1;
  unsigned BRP4:1;
  unsigned BRP5:1;
  unsigned SJW0:1;
  unsigned SJW1:1;
} BRGCON1bits;

Neither of these definitions is all that helpful, as I need to assign the BRP and SJW like the following:

struct
{
    unsigned BRP:6;
    unsigned SJW:2;
} GoodBRGbits;

Here are two attempts that I have made:

Attempt #1:

union
{
    byte Value;
    struct
    {
        unsigned Prescaler:6;
        unsigned SynchronizedJumpWidth:2;
    };    
} BaudRateConfig1 = {NULL};
BaudRateConfig1.Prescaler = 5;
BRGCON1 = BaudRateConfig1.Value;

Attempt #2:

static volatile near struct
{
    unsigned Prescaler:6;
    unsigned SynchronizedJumpWidth:2;
} *BaudRateConfig1 = (volatile near void*)&BRGCON1;
BaudRateConfig1->Prescaler = 5;

Are there any "cleaner" ways to accomplish what I am trying to do? Also I am slightly annoyed about the volatile near casting in Attempt #2. Is it necessary to specify a variable is near?

Answer 1

Personally, I try to avoid using using bit fields for portability reasons. Instead, I tend to use bit masks so that I can explicitly control which bits are used.

For example (assuming the bit order is correct) ...

#define BRP0  0x80
#define BRP1  0x40
#define BRP2  0x20
#define BRP3  0x10
#define BRP4  0x08
#define BRP5  0x04
#define SJW0  0x02
#define SJW1  0x01

Masks can then be generated as appropriate and values assigned or read or tested. Better names for the macros can be picked by you.

Hope this helps.

Answer 2

I suggest that you dont mix up the bitfield declaration with the adressing of the hardware register.

Your union/struct declares how the bitfields are arranged, then you specify addressing and access restrictions when declaring a pointer to such a structure.

// foo.h
// Declare struct, declare pointer to hw reg

struct com_setup_t {
  unsigned BRP:6;
  unsigned SJW:2;
};

extern volatile near struct com_setup_t *BaudRateConfig1;

// foo.c
// Initialise pointer

volatile near struct com_setup_t *BaudRateConfig1 = 
(volatile near struct com_setup_t *)0xfff...;

// access hw reg
foo() {
  ...
  BaudRateConfig1->BRP = 3;
  ...
}

Regarding near/far I assume that the default is near unless far is specified, unless you can set the default pointer size to far using compiler switches.