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Can't access bit field bits
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CMatic



Joined: 11 Jan 2012
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Can't access bit field bits
PostPosted: Tue Jan 19, 2016 11:06 am     Reply with quote

CCS PCM C Compiler, Version 5.026

I have defined a structure as follows:
Code:

typedef struct { 
   int1 Units:1;        //bit0 - Units LSB
   int1 Sound:1;        //bit 1- sound
   int1 Alarm:1;        //bit 2- Alarm
   int1 Bcklgt:1;       //bit 3- Bcklgt
   int1 Autopwroff:1;   //bit 4- Autopwroff
   int1 BatLow:1;       //bit 5- BatLow
   int1 Reserved:1;     //bit 6- Reserved
   int1 Factory:1;      //bit 7- Factory MSB
} user_t;   

user_t config = 0x80;


And when I try to use the variable config.Factory in another module it gives me error with Factory underlined as the error. I am using extern config in the other module. When I hover my cursor on config it shows all elements but Factory shows "Expecting union/structure". Please help and thanks in advance.
RF_Developer



Joined: 07 Feb 2011
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Re: Can't access bit field bits
PostPosted: Tue Jan 19, 2016 11:25 am     Reply with quote

You have not defined a structure with a byte bit field. you have defined a structure with eight int1s (i.e. booleans) each of one bit wide, which they are anyway!

Even with a byte-wide bit field you would not be able to access all the bits, i.e. the byte this way. To do that requires a union:

Code:

typedef union user_t {
   int8 All_Bits;
   struct {
   int8 Units:1;        //bit0 - Units LSB
   int8 Sound:1;        //bit 1- sound
   int8 Alarm:1;        //bit 2- Alarm
   int8 Bcklgt:1;       //bit 3- Bcklgt
   int8 Autopwroff:1;   //bit 4- Autopwroff
   int8 BatLow:1;       //bit 5- BatLow
   int8 Reserved:1;     //bit 6- Reserved
   int8 Factory:1;      //bit 7- Factory MSB
   } Single_Bits;
} My_union;

...

My_union.All_Bits = 0x80;

My_union.Single_Bits.BatLow = FALSE;

...

if (My_union.Single_Bits.Factory)
{
  // Do stuff
}


You need to check up on how to define bit fields and unions. Also, you do need to be careful of the bit order. CCS C provides a more certain, but non-portable way of accessing bits. Personally, I wouldn't use bit fields at all, just booleans.

PS: I hope I got that right, my bit field knowledge is not all it should be, and my unions can be a bit sketchy.
CMatic



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PostPosted: Tue Jan 19, 2016 11:41 am     Reply with quote

RF_Developer Thanks very much for replying. Yes, I can see why you also don't want to use bit fields. Since I am trying to conserve EEPROM space, can you suggest what would be a better way to save users configurations?
gaugeguy



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PostPosted: Tue Jan 19, 2016 12:22 pm     Reply with quote

#bit will do what you want

int8 config
#bit Units = config.0

#bit Factory = config.7
CMatic



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PostPosted: Tue Jan 19, 2016 1:22 pm     Reply with quote

gaugeguy wrote:
#bit will do what you want

int8 config
#bit Units = config.0

#bit Factory = config.7


Thanks for the comment, I am trying this out now and will post my results.
CMatic



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PostPosted: Tue Jan 19, 2016 6:14 pm     Reply with quote

Guageguy I tried your #bit suggestion but it did not work.

I tried the config.Factory = 1; And the compiler did compile it correctly, however, when I tried to use this statement in another module, this statement does not work? Am I missing something here?

Thanks
drolleman



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PostPosted: Tue Jan 19, 2016 9:51 pm     Reply with quote

you didn't do it properly. #bit works I use it in almost every project.
CMatic



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PostPosted: Tue Jan 19, 2016 10:16 pm     Reply with quote

drolleman wrote:
you didn't do it properly. #bit works I use it in almost every project.


I tried the following and the compiler shows error on config.0 when I hover over it, it says "expecting a declaration" ?

here is the code snip.
int8 config = 0x80;
#bit Units = config.0;
#bit Sound = config.1;
#bit Alarm = config.2;
#bit Bcklgt = config.3;
#bit Autopwroff = config.4;
#bit BatLow = config.5;
#bit Reserved = config.6;
#bit Factory = config.7;
drolleman



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PostPosted: Tue Jan 19, 2016 11:43 pm     Reply with quote

put complete code up.

what you have here factory will be 1 all other bits will be 0

if config = 0x34 = 00110100b

then

#bit Units = 0
#bit Sound = 0;
#bit Alarm = 1;
#bit Bcklgt =1;
#bit Autopwroff = 0;
#bit BatLow = 1;
#bit Reserved = 0;
#bit Factory = 0;
CMatic



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PostPosted: Wed Jan 20, 2016 12:23 am     Reply with quote

Don't mean any disrespect but the code is very large and in 7 modules.
The structure is as follows:
Code:

struct { 
    int8 Units:1;        //bit0 - Units LSB
    int8 Sound:1;        //bit 1- sound
    int8 Alarm:1;        //bit 2- Alarm
    int8 Bcklgt:1;       //bit 3- Bcklgt
    int8 Autopwroff:1;   //bit 4- Autopwroff
    int8 BatLow:1;       //bit 5- BatLow
    int8 Reserved:1;     //bit 6- Reserved
    int8 Factory:1;      //bit 7- Factory MSB
 } config;   

#locate config = 0x2000 //define where this variable will be located
#bit Units = config.0;
#bit Sound = config.1;
#bit Alarm = config.2;
#bit Bcklgt = config.3;
#bit Autopwroff = config.4;
#bit BatLow = config.5;
#bit Reserved = config.6;
#bit Factory = config.7;

Using the above structure, I am able to set Factory bit = 1, as follows:

Code:
config.Factory =1;

However, in my other module (where I declare:extern config), when I try to use config.Factory = 1; I get error saying "Expecting structure/union", and this is where I don't understand the compiler? Thanks for looking into it.

This is working in my main function.
gaugeguy



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PostPosted: Wed Jan 20, 2016 7:24 am     Reply with quote

You should not use both the structure and #bit.
Delete the structure and access the bits as

Factory = TRUE; or Factory = 1;

config = 0x80;
PCM programmer



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PostPosted: Wed Jan 20, 2016 7:51 am     Reply with quote

Quote:
Don't mean any disrespect but the code is very large and in 7 modules.

You still have not told the people working on your problem what PIC you
are using, and if you are using Multiple Compilation Units or not.
drolleman



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PostPosted: Wed Jan 20, 2016 2:22 pm     Reply with quote

your structure uses 8 bytes of data. if you use

int8 config;

this uses 1 byte of data or 8 bits isn't that what you want?
Ttelmah



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PostPosted: Wed Jan 20, 2016 2:32 pm     Reply with quote

drolleman wrote:
your structure uses 8 bytes of data. if you use

int8 config;

this uses 1 byte of data or 8 bits isn't that what you want?


No. He is using bitfields. The :1 means use one bit of the int8, as if this is an integer. You can use sizes like :4 to give you nibble sized integers.

Used correctly, this is perfectly acceptable, but his declarations are problematic. The bitfield, it needs to be declared in all modules using it.
CMatic



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PostPosted: Wed Jan 20, 2016 3:20 pm     Reply with quote

PCM programmer The pic is 16f1825 and there is no multiple compilations
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