unexpected change of variables (solved)

gs · Joined: 22 Aug 2005 Posts: 30 Location: Ioannina - Greece

Compiler version PCH 4.084
PIC18F4550

In my application I see that certain variables change, when a specific code part is executed. The change is always on the same byte, same bit.
If I define a dummy variable, the problem is temporarily solved, but this is not the solution, because this might also happen anywhere in the program in variables that I dont check externally.

Have you ever met this behavior before?

I know I have to post a part of code for you to reproduce the problem but I have to strip it down first as it is fairly big file.

Thanks a lot in advance
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meereck · Joined: 09 Nov 2006 Posts: 173

Perhaps, you are addressing a wrong index which is beyond the variable size. Such as

cseusy · Joined: 15 Apr 2006 Posts: 12

See responses to my post called "randomly changing variables". Maybe they can shed some light. I have been fighting the same or similar problem for two weeks. A friend who is an expert computer scientist says this is called memory stomping. That is an accurate description. I can always change code to make the problem go away, then when I add more code, the "stomp" manifests somewhere else. Best of luck! If you find the solution, please post it.....pleeeeeeeeaaassse!!!

gs · Joined: 22 Aug 2005 Posts: 30 Location: Ioannina - Greece

It is very big code to strip it down. It uses 18F4550 and a glcd 128*64

I use graphics.c and HDM64GS12.h math and string functions.

FvM · Joined: 27 Aug 2008 Posts: 2337 Location: Germany

If I understand right, you already found out, that the glcd_rect() call writes somehow out of bounds. You can set a breakpoint on write to the specific variable address and identify, which exact instruction does the write. You should be able to understand, if the compiler does something wrong or it's a coding error, e.g. in the subroutine.

gs · Joined: 22 Aug 2005 Posts: 30 Location: Ioannina - Greece

Finally I solved the problem.

cseusy, did you also use graphics lcd?

I use fast glcd

My problem was created because I was displaying text at the bottom right corner of the lcd and the y value was one pixel over the proper value.

100% my coding fault.

I wrote a routine to display value with an option to select the polarity of the text displayed and the background of the text. That background is always 1 pixel around the text displayed. The routine that draws that background goes out of the glcd displaydata table and writes in the next memory positions.

I have to confess that I was quite brute while debugging today...

Thanks a lot for your time
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