Unknown value (X)
Beginner
A spot in the chip whose value during a test can’t be predicted, so the tester has to ignore it.
Novice
A value that simulation can’t predict, marked X. It comes from things like memories that were never written or paths too slow to settle. The tester must ignore X bits, or it would fail good chips.
Expert
Sources include uninitialized memories, non-scan flops, analog and black-box boundaries, and false or multicycle paths at speed. X’s cost observability, and in a MISR or XOR compactor one X can hide many good bits, so they are blocked at the source or masked.