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pleditor
Joined: 27 Feb 2013
Posts: 3
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| Char Identifier in a Function |
 Posted: Wed Feb 27, 2013 1:26 am |
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Cant get this code to work :-/ "attempt to create a pointer to a constant" error
| Code: |
void led(char[4] color)
{
if (color[0] == 'g') ledA(1,1);
if (color[0] == 'b') ledA(1,2);
if (color[0] == 'r') ledA(1,3); //reading only the first digit (r)
}
main()
{
led("r00");
}
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This code works perfect 
| Code: |
void led()
{
char color[4] = "r00";
if (color[0] == 'g') ledA(1,1);
if (color[0] == 'b') ledA(1,2);
if (color[0] == 'r') ledA(1,3); //reading only the first digit (r)
}
main()
{
led();
}
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Last edited by pleditor on Wed Feb 27, 2013 10:22 am; edited 1 time in total |
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Mike Walne
Joined: 19 Feb 2004
Posts: 1785
Location: Boston Spa UK
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| Posted: Wed Feb 27, 2013 2:44 am |
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Post SHORT complete compilable code which illustrates your problem.
Then we can try it.
Mike |
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Ttelmah
Joined: 11 Mar 2010
Posts: 19553
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| Posted: Wed Feb 27, 2013 3:42 am |
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This is down to the difference in constant strings.
A search in the forum will find a lot about this, and full explanations
Key is that the PIC, does not have a classic memory architecture, with a single 'flat' memory space, containing both ROM and RAM. The ROM is actually in a different memory map to the RAM. Now this has the advantage that the chip can be accessing both memories 'at once', pre-fetching the next instruction from ROM, while working with the RAM at the same time, without having to have cache memory. When you call:
led("r00");
"r00", is _not_ a char string in RAM. You are declaring a ROM string. This is in the second memory space, and on older PIC's,this memory space is completely inaccessible. Hence the compiler has to generate a _program_ which can be called with a value saying 'which character is wanted, not a string that can actually be accessed as an array.....
On later chips this can be programmed round. The compiler command:
#device PASS_STRINGS=IN_RAM
will tell the compiler to automatically copy ROM strings into RAM so they can be normally accessed as arrays.
The forum search will explain what happens otherwise.
Best Wishes |
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pleditor
Joined: 27 Feb 2013
Posts: 3
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| Posted: Wed Feb 27, 2013 10:24 am |
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| Ttelmah wrote: | This is down to the difference in constant strings.
A search in the forum will find a lot about this, and full explanations
Key is that the PIC, does not have a classic memory architecture, with a single 'flat' memory space, containing both ROM and RAM. The ROM is actually in a different memory map to the RAM. Now this has the advantage that the chip can be accessing both memories 'at once', pre-fetching the next instruction from ROM, while working with the RAM at the same time, without having to have cache memory. When you call:
led("r00");
"r00", is _not_ a char string in RAM. You are declaring a ROM string. This is in the second memory space, and on older PIC's,this memory space is completely inaccessible. Hence the compiler has to generate a _program_ which can be called with a value saying 'which character is wanted, not a string that can actually be accessed as an array.....
On later chips this can be programmed round. The compiler command:
#device PASS_STRINGS=IN_RAM
will tell the compiler to automatically copy ROM strings into RAM so they can be normally accessed as arrays.
The forum search will explain what happens otherwise.
Best Wishes |
wow, thanks for the explanation. I am working with the pic16F887, as soon i get the chance i will try to get this solved. |
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pleditor
Joined: 27 Feb 2013
Posts: 3
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| Posted: Thu Feb 28, 2013 4:08 pm |
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| i did add the line "#device PASS_STRINGS=IN_RAM" and everything works as expected. Thanks again :-) |
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