I worked for months with Quartus II and a Cyclone II device without getting big trouble. But since a few weeks I'm working on a new part of the design and I can't get this thing stable. I use modules I wrote long time ago an that worked well so far. But now, combinded with new modules (or slightly modified old ones) they do weird things - in simulation and on the hardware. The new design parts work well without the old ones (in simulation). The problem appears only in combination.
I tried a few things such as play with the Analysis and Synthesis Settings (restructure multiplexers = on/off etc.) or use another implementation for my state machines. Most of these things changed the behavior in any way. But the change that helped the one module to work in most cases had a negative effect on another module.
So how can I ever be sure that a part of my design, checked for proper operation, will work in future after adding new functionatity to the design.
Is the described behavior revealing of basic design mistakes?
Is it possible to separate a certain module, to define settings only for this one? Could that solve the problem at all?
Many thanks in advance!