It is recommended to declare the FSM states as enumerated data types. Enumerated state types make HDL code generally more readable. It also facilitates increased flexibility in the synthesis implementation as you can select the encoding style used without modifying the HDL code. These aspects support greater design portability and support FSM error detection and recovery.
In safety-critical designs though, std_logic_vector can still be preferred.
Sigasi Visual HDL can ensure one of the following types is used for FSM state signals or variables:
- enumeration
- arrays of (u)logic, e.g.,
std_logic_vector - arrays of bit, e.g.,
bit_vector
The type that is configured by default is “enumeration”.
Example with “array of logic” as expected FSM state type:
process is
variable state : bit_vector(1 downto 0) := "00"; -- Unexpected FSM state type: expected array of (u)logic but was 'bit_vector'
variable state2 : std_logic_vector(1 downto 0) := "00";
begin
case state is
when "00" => state := "11";
when "11" => state := "00";
end case;
end process;
Note that this rule is set to
ignore by default. You can enable it in the VHDL Errors/Warnings preference page (Style Validation > Unexpected FSM state type).
You can also configure the FSM state type that is checked on this page.
Rule configuration
This rule can be disabled for your project, or its severity and parameters can be modified in the project linting settings. Alternatively, it can be manually configured with the following template:
237/severity/${path}={error|warning|info|ignore}
237/params/fsm_state_type/${path}={enumeration|bit_vector|std_logic_vector}