4.13 Debugging: Finding Causes of Unknowns X’s, or unknowns, can occur in simulation when hardware behavior is undetermined. These X’s can potentially cause problems in real hardware when unaddressed. Riviera-PRO provides a number of tools capable of finding the causes of unknowns. This video will demonstrate these tools such as Xtrace, Cause Finder, Waveform Viewer, Drivers/Readers, and Dataflow.