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ARM FIQ handling problem

Hi all,
I'm experiencing some problems in using FIQ and IRQ over and ARM7TDMI.
I have a "slow" (10ms) timer used for eCos scheduling and one, faster (100us), used by my application to do its work.
Due realtime constrain of my app I have to treat the faster as FIQ, so it is not delayed by others IRQ.

Here is where my problems rise: everything works fine (IRQ and FIQ routine are called in the right way) only for a while, after that something goes wrong and an undefined exception is rised. The instruction is usually placed somewhere in user code (most of times in the thread idle routine).

I have also noticed that when the exception is rised the __exception_stack is corrupted.
Please also note that when setting the faster timer as IRQ too there are no problems at all...

To me it sounds like there are some problems in the interaction between IRQ and FIQ, maybe in some situation that happen only not so often...

I have noticed that FIQ are handled much like IRQ, only with the special care of setting FIQ disable flags too.
So them both use the same __exception_stack as temporary stack: what if an IRQ is interrupted by a FIQ, which uses the same __exception_stack to save its status (or is it impossible to have such a kind of problem due FIQ disabling while the IRQ code is using that stack)?

Any clue about where can be the problem?

Thanks in advance!

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