An unreachable state is excluded by the intended reset and transition rules, but its bit pattern can still exist in hardware. Recovery logic defines what to do if an illegal encoding is observed. Verification checks normal behavior, boundaries, reset and recovery rather than only a happy-path sequence.
Worked example
A two-bit controller uses IDLE = 00, RUN = 01 and DONE = 10. Encoding 11 is illegal. A default next-state rule can send 11 to IDLE while forcing safe outputs. Test it by deliberately starting or injecting that encoding in simulation and checking the specified recovery.
What to watch for
A default branch alone is not proof of fault tolerance: synthesis choices, output safety and fault models matter. Check what the implemented hardware preserves and whether recovery is observable.
Try it yourself
What basic properties should a three-state controller test include?
Answer: Reset reaches the initial state, each specified transition is correct, outputs match the state/input rule, and the unused encoding follows the defined recovery behavior.
How to study this topic
Begin with named states and a complete transition table. Then choose encodings and derive the logic. Trace inputs over several cycles, checking outputs as well as next states, including reset and unexpected encodings.
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