David Fussell wrote:
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I was wondering about assertion times of the output pin during the fetch cycle, but since both flash and ram are identical, would this be a problem?


Exact, this was the problem, during this switching the current code fetch was corrupted.

And the solution:
The code, which switch from one to the other must be executed in one of these exclusively.
E.g. the Flash was bigger than the RAM and outside the RAM size always the Flash was accessed (or vice versa).

There are also a similiar Dallas application note, since on some Dallas devices the internal code ROM can be switched of.


Peter