And I added a pin-toggle to the interrupt-routine as well one as to the main-routine, just before the if-statements. The interrupt is triggered approx. 25 times per second, the main-routine quite a lot more often (my poor old scope was almost at the end of its capabilities ...).

So the interrupt happens, thus forget the timer for now.

There are some strange things here are you sure it is not a watchdog or some such (a typical simulator "oversight") try a toggle pin at main() before the while(1), Ill bet you a beer at FrankA (easy when I am 3000 miles away) that it toggles.

Erik