FSM_SAFE_STATE - 2023.2 English

Vivado Design Suite Properties Reference Guide (UG912)

Document ID
Release Date
2023.2 English

This attribute can be set in both the RTL and in the XDC.

The Vivado synthesis tool supports extraction of Finite State Machines (FSM) in a variety of configurations as determined by the FSM_ENCODING property, or the -fsm_extraction command line option for Vivado synthesis. Refer to the Vivado Design Suite User Guide: Synthesis (UG901) for more information.

A state machine can enter into an invalid, or “unreachable” state that causes the design to fail. FSM_SAFE_STATE tells synthesis to insert logic into the state machine that detects if there is an invalid state and then puts it into a known state on the next clock cycle. If an FSM enters an invalid state, the FSM_SAFE_STATE property defines a recovery state for use when an FSM is synthesized in the Vivado synthesis tool.

Tip: While providing for safe recovery of FSM states, this property can affect the quality of synthesis results, typically resulting in less performance with greater area.
Architecture Support
All architectures.
Applicable Objects
State machine registers.
  • reset_state: Return the state machine to the RESET state, as determined by the Vivado synthesis tool.
  • power_on_state: Return the state machine to the POWER_ON state, as determined by the Vivado synthesis tool.
  • default_state: Return the state machine to the default state, as defined by the state machine; even if that state is unreachable, using Hamming-2 encoding detection for one bit/flip.
  • auto_safe_state: implies Hamming-3 encoding.


Verilog Syntax
(* fsm_safe_state = "reset_state" *) reg [2:0] state; 
(* fsm_safe_state = "reset_state" *) reg [7:0] my_state;
VHDL Syntax
type count_state is (zero, one, two, three, four, five, six, seven); 
signal my_state : count_state;
attribute fsm_safe_state : string;
attribute fsm_safe_state of my_state : signal is "power_on_state";
XDC Syntax
set_property fsm_safe_state reset_state [get_cells state_reg*]

Affected Steps

  • Synthesis