FSM output signal assignment

[ehsan_iut]

I'm involved in designing a rather simple state machine. I was looking at the "language template" of the FSM in Xilinx ISE. In that templates, the output is assigned to internal signal at each state. Then these internal signals are assigned to the output ports in a clocked process (The same process in which the state transition occurs). Is it necessary to latch the outputs of the FSMs? Does anybody know the reason or have any experiences? What do u usually do?

Thanks alot!

 

[saikat]

I have also noticed the same during one of my project. I personally feel that there is no need to use two separate processes for implementing a FSM, one for output latching and another for state transition.

I have implemented FSMs for UART and HDLC cores in my project and used a single process for updating the outputs and next state. All signals are updating at clock edge. The design is running perfectly fine in simulations and on board also.

I thought these are just different ways of implementing FSMs.

Any different opinion is welcome!

 

[j_andr]

/.../Any different opinion is welcome!

not a different opinion, just a note;
if you need the valid output just one clock cycle after
the FSM gets to a particular state you have to use extra
FF to latch the output;
if output state is required at the same cycle as FSM
you must not use the extra FF;
if the FSM controls many out lines - i.e address/data
to an external chip - latching outputs removes skew of lines;
---

 

[craftor]

Here is the VHDL code:

--Mealy Machine
library ieee;
use ieee.std_logic_1164.all;

entity mealy is
port (
clock, reset : in std_logic;
data_in : in std_logic_vector(1 downto 0));
data_out : out std_logic;
end mealy;

architecture arc of mealy is
type state_values is (st0, st1, st2, st3, st4);
signal pres_state, next_state : state_values;
begin
statereg : process(clock, reset) begin
if (reset = '0') then
pres_state <= st0;
elsif (clock'event and clock='1') then
pres_state <= next_state;
end if;
end process statereg;
fsm : process(pres_state, data_in) begin
case pres_state is
when st0 =>
case data_in is
when "00" => next_state <= st0;
when "01" => next_state <= st4;
when "10" => next_state <= st1;
when "11" => next_state <= st2;
when others => null;
end case;
when st1 =>
case data_in is
when "00" => next_state <= st0;
when "10" => next_state <= st2;
when others => next_state <= st1;
end case;
when st2 =>
case data_in is
when "00" => next_state <= st1;
when "01" => next_state <= st1;
when "10" => next_state <= st3;
when "11" => next_state <= st3;
when others => null;
end case;
when st3 =>
case data_in is
when "01" => next_state <= st4;
when "11" => next_state <= st4;
when others => next_state <= st3;
end case;
when st4 =>
case data_in is
when "11" => next_state <= st4;
when others => next_state <= st0;
end case;
when others => next_state <= st0;
end case;
end process fsm;
outputs : process( pres_state, data_in ) begin
case pres_state is
when st0 =>
case data_in is
when "00" => data_out <= '0';
when others => data_out <= '1';
end case;
when st1 =>
data_out <= '0';
when st2 =>
case data_in is
when "00" => data_out <= '0';
when "01" => data_out <= '0';
when others => data_out <= '1';
end case;
when st3 =>
data_out <= '1';
when st4 =>
case data_in is
when "10" => data_out <= '1';
when "11" => data_out <= '1';
when others => data_out <='0';
end case;
end process outputs;
end arc;