Why does my test for long==0 in timer interrupt not work ?

[nurquhar]

I am having problems with 'if(!timerX)' in the following code.

For some reason testing the 16bit timerX value for zero in my interrupt sometimes gives the wrong result.

I have managed to fix it by reverting the timerX value back to an 8bit int x and then testing that. Indeed I can also solve the bug by changing the defanition of timerX to an 8bit int. But this limits the range for my app.

I am using PCM3.249, PIC16F877A 20mhz and have device *=16 enabled, so bank switching is in play.

Can anyone explain why the compiler is getting it wrong ?

I have included below the FIXED and BROKEN source and asm listings :-



Source BROKEN version

[Ttelmah]

The code should work.
You perhaps need to be aware that the 'make8' version, will test as '0', a lot more frequently than the 16bit version.
You are taking the upper byte, and the low byte, and adding them with an 8bit result. You will get 'zero' for this, when the high byte=1, and the low byte 255, then again when the high byte=2, and the low byte 254, etc. etc.. It'll actually give the zero result, every 254 counts, while the 16bit version will only give zero every 65536 counts.
The correct 8bit equivalent, is:

x = make8(timerX,0) | make8(timerX, 1) ;

If this behaves as the 16bit version does, then the problem is not in timerX, but in what you think it should do...

Best Wishes

[nurquhar]

Dear Ttelmah

I will certainly try the OR make8 idea. But at the moment I am always testing with timerX being load with 0x64 (100) so 1 + 255 = 0 should not occur. Indeed I can also solve the problem by changing timerX to be an 8bit int.

I was wondering what happens if the interrupt occurs whilst an access to a variable in another bank was taking place. I did try clearing all the bank bits in STATUS at the begnining of the interrupt but this did not seem to help.

[ckielstra]

Both code versions look to me like correct C implementations.

The compiled code however for both versions looks incorrect. The broken version doesn't do any bank switches which could be OK, I don't have the symbol file, but the results you see are wrong.
The 'fixed' version does bank switching but is reading both bytes of the TimerX variable from different banks, this is wrong.

Something is screwing up your compiler. Forward your complete source code to CCS and/or have a look outside the ISR for errors.

[nurquhar]

Having tried the make8() | make8() I now see what should have been obvious. I was setting up the interrupt with timerX initialy set to zero. Which is wrong because it first decrements the value and thus it rolls over to 0xFFFF. Which screws up the timings I was trying to achive.

I am curious about this bank switching during the interupt, or should I say the lack of it. I can only guess that CCS must be stacking the STATUS reg before/after my interrupt funct and that it only does a bank switch if and when it has varibles not in bank 0 that need to be accessed.

[ckielstra]

[Ttelmah]