union _word
	{
		uint16_t	word;
		uint8_t		byte[2];
	} wrd;

	wrd.byte[0] = data;
	wrd.byte[1] = reg;
-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-
http://www.avrfreaks.net/index.php?name=PNphpBB2&file=viewtopic&t=67601

   typedef union { 
       volatile unsigned char flinps; 
       struct { 
          unsigned char b0 : 1; 
          unsigned char b1 : 1; 
          unsigned char menubtileft : 1; 
          unsigned char menubtimiddle : 1; 
          unsigned char menubtiright : 1; 
          unsigned char b5 : 1; 
          unsigned char b6 : 1; 
          unsigned char b7 : 1; 
       }; 
    } flinps_union_t; 
     
    typedef union { 
       volatile unsigned char buttons; 
       struct { 
          unsigned char b0 : 1; 
          unsigned char b1 : 1; 
          unsigned char menubtbleft : 1; 
          unsigned char menubtbmiddle : 1; 
          unsigned char menubtbright : 1; 
          unsigned char b5 : 1; 
          unsigned char b6 : 1; 
          unsigned char b7 : 1; 
       }; 
    } buttons_union_t; 

 typdef struct { 
    flinps_union_t flinps; 
    buttons_union_t buttons; 
    unsigned char buttprsd;        
    unsigned char fdsmsm;        
    unsigned char tmpinps; 
 } mydata_struct_t; 

 mydata_struct_t mydata; 

 int main(void) { 
     mydata.flinps.menubtileft = 1; 
     mydata.buttons.menubtbright = 0; 
     mydata.buttprsd = 11; 
     mydata.fdsmsm = 22; 
     mydata.tmpinps = 33; 
 }
-------------------------------------------------------------------------------

Как брать отдельные байты от слова?

http://www.avrfreaks.net/index.php?name=PNphpBB2&file=printview&t=20709&start=0

A UNION is the most elegant solution and designed to perform just this task. 

Code:

 union u_type            //Setup a Union 
 { 
   unsigned int IntVar; 
   unsigned char Bytes[2]; 
 } 
  temp;                    //Assign a var name to the Union 


 void main(void) 
 { 
  temp.IntVar=65535;          //Assign a value to the Int var of the Union 
  HighByte=temp.Bytes[1];  //Get the High Byte (255) 
  LowByte=temp.Bytes[0];   //Get the Low Byte (255) 
 }

----------

into a series of char (8-bit) pointers. Example: 
Code:

 unsigned char low_byte; 
 unsigned char high_byte; 
 unsigned int the_value; 
 ... 
 the_value = 1234; 
 low_byte = * ((unsigned char *)&the_value); 
 high_byte = * ((unsigned char *)((&the_value)+1)); 


 or a char-sized pointer could be used the same way. [I'd probably use the pointer method for repetitive uses within an app and/or many-byte variables.] 

Code:

 unsigned char low_byte; 
 unsigned char high_byte; 
 unsigned int the_value; 
 unsigned char *the_ptr; 
 ... 
 the_value = 1234; 
 the_ptr = * ((unsigned char *)&the_value); 
 low_byte = *the_ptr; 
 the_ptr++; 
 high_byte = *the_ptr;

----------

the bset way: 

 it only compile in ONe line asm code ! 

Code:

 #define LowB(x) (*((unsigned char*)  &(##x)+1)) 
 #define HighB(x) (*((unsigned char*) &(##x)+0)) 



 for ex for low byte use LowB(x)

только тут наверное младший байт без смещение (лежит первым), а старший 
байт -- +1. И тогда это совпадёт с предыдущим примером.