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snoop911
Joined: 24 Aug 2007
Posts: 4
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| 'union' question + CCS Compiler |
 Posted: Fri Mar 13, 2009 11:15 am |
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Hi,
Not sure if this is a general c-syntax/functionality or if its a compiler specific question...
In embedded devices where memory is scarce, I'd like to define a structure that can hold an 8 bit value that will be used to drive an output port. At the same time however, in the code, I would like to access individual bits without using the 8 bit value/mask.
Something like:
| Code: | typedef struct
{
union
{
uint8 OutputPortA;
int1 led0;
int1 led1;
int1 led2;
int1 led3;
int1 led4;
int1 led5;
int1 led6;
int1 led7;
} uSelection;
} PORT_A_OUTPUT; |
Will the above reserve a single 8 bit chunk, such that setting/clearing any of the individual led bits will automatically update OutputPortA? Similarly, modifying OutputPortA will affect the individual single bit values?
I know most compilers don't operate on single bits so perhaps this is not possible? |
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Heath
Joined: 21 Dec 2007
Posts: 41
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| Posted: Fri Mar 13, 2009 12:09 pm |
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As a quick answer, look up the #bit directive. Maybe that something like what you're looking for.
I think what you suggest is possible but your code doesn't look right to me. I haven't tested it though. |
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Ttelmah
Guest
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| Posted: Fri Mar 13, 2009 3:51 pm |
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The problem is that the union as shown, will allocate an eight byte chunk as 'OutputPortA', but then 8 single bits _each addressing the same bit_.
In a union, each entity, talks to the same memory. You are showing nine separate entities.
The union syntax, would be:
| Code: |
union {
uint8 WholePort;
int1 LED[8];
} PORT_A_OUTPUT;
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(You don't need to fiddle around with a structure definition as well - just use the union, and typedef it if needed - though see below for why a structure definition might be used).
The you can access PORT_A_OUTPUT.WholePort to write all eight bits, and PORTA_OUTPUT.LED[0] to LED[7] to access the individual bits.
Even better, you can map this directly to the port register if required. Having defined the variable, then use:
| Code: |
#byte PORT_A_OUTPUT=0xnnn
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Where 'nnn' is the memory address of the port (depends on whether you are using a PIC16, or PIC18).
Now, using an array of bits, requires a V4 compiler. The alternative, is to declare an 8bit structure, and then declare a union of this to the unsigned int8.
Best Wishes |
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snoop911
Joined: 24 Aug 2007
Posts: 4
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| Posted: Fri Mar 13, 2009 4:04 pm |
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Thanks a bunch!
Not many compilers work with 'bit' types, so adding 'union' into the mix, was my concern. I'll give a try and post how it goes.
BTW, how would the mapping go from the bit to the byte? For example, would LED[7] correspond to the most-significant-bit of the WholePort or would the compiler map LED[0] to that? I'm guessing the same mapping convention would apply if an LED struct it used in place of the array.
Cheers |
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