struct variable access

ScottC — Wed, 10 May 2006 18:29:35 GMT

I am using the following and can'y access the variable flags1. I can only change bits

Also, the struct defaults to 2 byte. I can't seem to change it to 1 byte. I'm new to C.

Thanks,

struct
      {
       byte flow0              :1;     /* b0 */
       byte flow1              :1;     /* b1 */
       byte temp0              :1;     /* b2 */
       byte temp1              :1;     /* b3 */
       byte pres0              :1;     /* b4 */
       byte pres1              :1;     /* b5 */
       byte reverse            :1;     /* b6 */
       byte capfont            :1;     /* b7 */
       byte ampm24             :1;     /* b8 */
       byte h24                :1;     /* b9 */
       byte underline          :1;     /* b10 */
       byte volstd             :1;     /* b11 */
       byte pwrsav             :1;     /* b12 */
       byte baklit             :1;     /* b13 */
       byte mdyform            :1;     /* b14 */
       byte magnify            :1;     /* b15 */
      }
flags1;

#define flow0 flags1.flow0
#define flow1 flags1.flow1
#define temp0 flags1.temp0
#define temp1 flags1.temp1

#define reverse flags1.reverse
#define pres0 flags1.pres0
#define pres1 flags1.pres1
#define capfont flags1.capfont

#define ampm24 flags1.ampm24
#define h24 flags1.h24
#define underline flags1.underline
#define volstd flags1.volstd

#define pwrsav flags1.pwrsav
#define baklit flags1.baklit
#define mdyform flags1.mdyform
#define magnify flags1.magnify

Re: struct variable access

UK_CF_FAE — Thu, 11 May 2006 03:33:16 GMT

Hi Scott,

Yes, I see your problem. Your structure is declared correctly, but this is a bit-field type declaration. What you have declared is a bit-field with 16 entries, so the compiler uses an integer (ie 16-bits, 2 bytes) to store the bit field. The fact that you need to use the storage class 'byte' to represent the datatype "bit" ie byte flow : 1 is a relic of C, I'm afraid.

If you abandon the entries 8-15, you'll find CW uses a byte for this bit-field.

Look at the "device".h files - eg mc9s08qb8.h and you'll see plenty of these bit-fields where we have defined the registers of the device.

Mark.

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