Page 124 - Programming Microcontrollers in C

More Structures 109

following the colon. The first four fields are each 1 bit wide, and the
final field ALL is 4-bits wide. These bits can be turned on by:
FLAGS.OUT1 = FLAGS.OUT2 = 1;

or off
FLAGS.OUT1 = FLAGS.0UT2 = 0;

and they can be tested

if(FLAGS.PB1 == 0 && FLAGS.PB2 == 1)
...

Some of the compilers have special bit constructs. These con­
structs are usually structs that have either 8- or 16-bits within the
field. These structs are useful as Boolean variables.
When setting up a microcontroller program, the programmer will
frequently want to have bit fields at specific locations in memory.
These bit fields can be used as I/O ports, control registers and even
arrays of bits to be used internally as flags. An approach to this prob­
lem is found in the bit array.

typedef struct
{
bit_0 :1;
bit_1 :1;
bit_2 :1;
bit_3 :1;
bit_4 :1;
bit_5 :1;
bit_6 :1;
bit_7 :1;
} BITS:

A macro definition is used to create a variable:
#define PORTA (*( BITS *) 0x1000)

With these definitions, instruction statements like
PORTA.bit_7 = 0;

if(PORTA.bit_3 == 1 && PORTA.bit_2 == 0)
etc. can be used in dealing with the bits within this memory location.