Safe FSM Implementation

XST can add logic to your FSM implementation that will let your state machine recover from an invalid state. If during its execution, a state machine gets into an invalid state, the logic added by XST will bring it back to a known state, called a recovery state. This is known as Safe Implementation mode. To activate Safe FSM implementation, select the Safe Implementation option from the Synthesis Properties dialog box in Project Navigator or apply the SAFE_IMPLEMENTATION constraint to the hierarchical block or signal that represents the state register. See "Safe Implementation" in Chapter 5 for details about the SAFE_IMPLEMENTATION constraint.

By default, XST automatically selects a reset state as the recovery state. If the FSM does not have an initialization signal, XST selects a power-up state as the recovery state. You can manually define the recovery state by applying the RECOVERY_STATE constraint. See "Recovery State" in Chapter 5 for details about the RECOVERY_STATE constraint.