Re: [avr-gcc-list] Bit-wise structure and unions

[David Brown]

Keith Gudger wrote:
> Below is a program which illustrates the following issue:
>
> When using AVR ports, I would like to define them with bit-wise
> structures.  This way I can easily access individual bits (or groups of
> bits).  e.g.:
>
> typedef struct port_reg {
>         unsigned bit0:1   ;
>         unsigned bit1:1   ;
>         unsigned unused:6 ;
> } PORT_REG;
>
> However, I cannot assign the 8 bit port value directly to a structure
> defined this way.  Here is the code and the gcc error:
>
>     PORT_REG *pLED_DIR = (PORT_REG *)0x37 ; ///< LED Port DIR REG PORT B
>     *pLED_DIR = (PORT_REG) 0x03 ;
>
> main.c:27: error: conversion to non-scalar type requested
>
> I can fix this problem by introducing a union:
>
> typedef union {
>     PORT_REG port_bits ;
>     uint8_t  byte ;
> } PORT_UNION ;
>
>    PORT_UNION *pLED_DIR = (PORT_UNION *)0x37 ; ///< LED Port DIR REG PORT B
>    PORT_UNION *pLED_OUT = (PORT_UNION *)0x38 ; ///< LED Port Output PORT B
>    pLED_DIR->byte = 0x3 ;
>
> But I would rather not have the union.  Is there any other way around
> this?  Thanks.
>
> Keith

Gcc supports anonymous unions as an extension, which can be used unless 
you have specified stricter standards compliance (i.e., it works with 
-std=gcc99, but not -std=c99):

typedef union {
        struct {
                unsigned bit0:1   ;
                unsigned bit1:1   ;
                unsigned unused:6 ;
        };
        unsigned char raw;
} PORT_REG;


PORT_REG *pLED_DIR = (PORT_REG *)0x37 ; ///< LED Port DIR REG PORT B

void foo(void) {
        pLED_DIR->raw = 0x03 ;
}

void bar(void) {
        pLED_DIR->bit1 = 1;
}


You want to make sure you don't have any name conflicts between the 
union parts.

Of course, your generated code will be very much more efficient if you 
use the port address directly (named in a macro), rather than via a pointer.



#define LED_DIR (*(PORT_REG *)0x37)

void foo2(void) {
        LED_DIR.raw = 0x03 ;
}

void bar2(void) {
        LED_DIR.bit1 = 1;
}



mvh.,

David