Changing global var on a function that doesn't use it !!!

Study · Joined: 09 Apr 2009 Posts: 29

Hi friends
I have a program that has one global var (unsigned int16) and I'm using 18F452.
I use this variable in main and timer1_int. I have some function too. When I call one function it does its work fine but changes my global variable too!!! (I never use this variable on that function).
I mean before calling that function my global variable is ok in main but after come back from the function it changes (I don't use global var on that function).
I have looked at asm program in lst file too. I didn't see any work on global var by that function but it changes!!
What's the problem?
Regards
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Sorry if i have much mistakes, my english is not good.
Thanks a lot for your helps.
Have a good time

vinniewryan · Joined: 29 Jul 2009 Posts: 154 Location: at work

Please post the code and your compiler version.

Try writing a test code and comment out certain lines of code in the 'function in question' to narrow down exactly what function or line of code is changing your global variable.

I can't say much more without seeing the code.
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Vinnie Ryan

Ttelmah · Joined: 11 Mar 2010 Posts: 19328

Most likely, something going beyond the area it is meant to.

Do you use any arrays, or pointer in the function?.
The 'classic' problem is (for example), having an array of eight characters, and writing an 8 character string to this. Forgetting that a 'string', always has an extra character for the terminator. This then results in the next variable in memory getting corrupted....

So, open the .sym file. This shows 'where' variables are stored in memory. Find where the global variable is stored. Then look at what is stored immediately in front of this. Is this a variable in the problem function?. Then look at how this is declared, and used. The variables immediately in front, are the 'most likely' to cause a problem.
Beyond this, are you using pointers anywhere in the function?. If so, triple check that declarations are correct for these, and indexes are correct. Then the same for arrays. Another 'classic', is forgetting that in C, arrays are 'zero referenced', so if you create an array of twenty elements, you can use a[0] to a[19]. If you accidentally use a[20], this results in the next variable getting corrupted....

Best Wishes

Study · Joined: 09 Apr 2009 Posts: 29

Thanks a lot Ttelmah
I think the problem is what you say, I have global array too that i use it on function, it stors at: 019-01C and my global var is on 01D-01E.
what should i do now?
_________________
Sorry if i have much mistakes, my english is not good.
Thanks a lot for your helps.
Have a good time

Gabriel · Joined: 03 Aug 2009 Posts: 1067 Location: Panama

Check your counter (index) for traversing the Array.

Its probably going out of bounds.

Most likely in the function that over writes your variable contains a loop.

Check its limits.

I think I just said the same thing twice.
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CCS PCM 5.078 & CCS PCH 5.093

Study · Joined: 09 Apr 2009 Posts: 29

I got answer, Thanks a lot for your helps friends Smile

_________________
Sorry if i have much mistakes, my english is not good.
Thanks a lot for your helps.
Have a good time