Recovering from Illegal States

The Compiler generates logic for a state machine using the state machine design you specify in a Text Design File (.tdf). If you specify a state machine design that explicitly declares state bits and also does not use one-hot encoding, the Compiler can generate logic where state bit values are not assigned to valid states. These unassigned or incorrectly assigned state bit values are called illegal states. A state machine that enters an illegal state—for example, as a result of setup time (tSU) or hold time (tH) violations—can produce erroneous outputs.You can make a state machine recover from an illegal state in one of the following ways:

NOTE Although a state machine can recover from an illegal state it enters due to a tSU or tH violation, Altera® recommends that state machine inputs meet all tSU and tH requirements.

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