[avr-chat] Passing pins through link

[Keith Gudger]

I need to pass port addresses and pin locations from one file to a
library, so that the addresses and pins are resolved at link time.  (The
library can not know which port or pin the user will define).

Here's what I've tried:

typedef struct
{
        TWI_PINS pin_reg,        ///< pin input register
                 dir_reg,        ///< data direction register
                 port_reg;       ///< port output register
} TWI_PORT;  // A port for a bit-banged TWI

It's use in main.c is:

TWI_PORT *pTWI = (TWI_PORT *)0x33 ;  // Port C

The file in the library references the port this way:

       pTWI->port_reg

which works just as I wanted:

 122 0088 E091 0000             lds r30,pTWI
 123 008c F091 0000             lds r31,(pTWI)+1
 124 0090 8281                  ldd r24,Z+2

Now I need a way to pass in to the library the proper pin to tweak.  I
tried this:

typedef struct
{
        unsigned clock_pin:1; ///< Clock pin on port pin 0
        unsigned data_pin:1;  ///< Data  pin on port pin 1
        unsigned nused_pin:6; ///< Temporarily not used pins
} TWI_PINS ;

and then the library references the pin like this:

    pTWI->port_reg.data_pin = 1;

But the compiler hard codes the pin value:

 125 0092 E091 0000             lds r30,pTWI
 126 0094 F091 0000             lds r31,(pTWI)+1

Does anyone have a suggestion for how to pass the pin position to my
library file?  I guess I could define a global variable:

const uint8_t clock_pin = _BV(0);
const uint8_t data_pin  = _BV(1);

But this is not as clean as the bitwise structure definition.  Ideas?  
Thanks.

Keith