Question

I am using MPLABx and the HI Tech PICC compiler. My target chip is a PIC16F876. By looking at the pic16f876.h include file, it appears that it should be possible to set the system registers of the chip by referring to them by name.

For example, within the CCP1CON register, bits 0 to 3 set how the CCP and PWM modules work. By looking at the pic16f876.h file, it looks like it should be possible to refer to these 4 bits alone, without change the value of the rest of the CCP1CON register.

However, I have tried to refer to these 4 bits in a variety of ways with no success.

I have tried;

CCP1CON.CCP1M=0xC0; this results in "error: struct/union required
CCP1CON:CCP1M=0xC0; this results in "error: undefined identifier "CCP1M"

but both have failed. I have read through the Hi Tech PICC compiler manual, but cannot see how to do this.

From the pic16f876.h file, it looks to me as though I should be able to refer to these subsets within the system registers by name, as they are defined in the .h file.
Does anyone know how to accomplish this?

Excerpt from pic16f876.h

// Register: CCP1CON
volatile unsigned char           CCP1CON             @ 0x017;
// bit and bitfield definitions
volatile bit CCP1Y               @ ((unsigned)&CCP1CON*8)+4;
volatile bit CCP1X               @ ((unsigned)&CCP1CON*8)+5;
volatile bit CCP1M0              @ ((unsigned)&CCP1CON*8)+0;
volatile bit CCP1M1              @ ((unsigned)&CCP1CON*8)+1;
volatile bit CCP1M2              @ ((unsigned)&CCP1CON*8)+2;
volatile bit CCP1M3              @ ((unsigned)&CCP1CON*8)+3;
#ifndef _LIB_BUILD
volatile union {
struct {
    unsigned    CCP1M               : 4;
    unsigned    CCP1Y               : 1;
    unsigned    CCP1X               : 1;
};
struct {
    unsigned    CCP1M0              : 1;
    unsigned    CCP1M1              : 1;
    unsigned    CCP1M2              : 1;
    unsigned    CCP1M3              : 1;
};
} CCP1CONbits @ 0x017;
#endif

Answer 1

You need to access the bitfield members through an instance of a struct. In this case, that is CCP1CONbits. Because it is a bitfield, you only need to have the number of significant bits as defined in the bitfield, not the full eight bits in your code.

So:

CCP1CONbits.CCP1M = 0x0c;

Should be the equivalent of what you are trying to do. If you want to set all eight bits at once you can use CCP1CON = 0xc0. That would set the CCP1M bits to 0x0c and all the other bits to zero.

The header you gave also has individual bit symbols, so you could do this too:

CCP1M0 = 1;
CCP1M1 = 1;
CCP1M2 = 0;
CCP1M3 = 0;

Although the bitfield approach is cleaner.