Unit Testing of individual routines is crucially important.

There are different philosophies on where "input and resource validation" should reside.

I personally believe it should reside in each routine and should resolve when necessary to a uniform "error trap" mechanism. This is due to the likelyhood of these routines ending up in libraries where they are used long after specific knowledge of there inner workings is forgotten. This also beg's the requirement for the best possible source annotation.

Also, Shankara's comment regarding interrupts is also very important as is "straddles" unit testing of the individual interrupt routines and their "effect" at the time of Integration Testing.

At the time of Functional Testing there are certain individuals who "break" code better than others. In my experience these individuals are very valuable employees
and produce results far better than if the originating coding group tests their own systems.

regards,
p