Finite State Machine (FSM) Coding In VHDL

There is a special Coding style for State Machines in VHDL as well as in Verilog.

Let us consider below given state machine which is a “1011” overlapping sequence detector. Output becomes ‘1’ when sequence is detected in state S4 else it remains ‘0’ for other states.

 VHDL Code for FSM:

library IEEE;
use IEEE.STD_LOGIC_1164.ALL;
--Sequence detector for detecting the sequence "1011".
--Overlapping type.
entity seq_det is
port(   clk   : in std_logic;      --clock signal
        reset : in std_logic;      --reset signal
        S_in  : in std_logic;      --serial bit Input sequence   
        S_out : out std_logic);    -- Output        
end seq_det;
architecture Behavioral of seq_det is
--Defines the type for states in the state machine
type state_type is (S0,S1,S2,S3,S4);
--Declare the signal with the corresponding state type.
signal Current_State, Next_State : state_type;
begin
-- Synchronous Process
process(clk)
begin
    if( reset = '1' ) then                 --Synchronous Reset
        Current_State <= 'S0';
    elsif (clk'event and clk = '1') then   --Rising edge of Clock
        Current_State <= Next_State
    end if;
end process;
-- Combinational Process
Process(Current_State, S_in)
    begin
        case Current_State is
            when S0 =>                     
                S_out <= '0';
                if ( s_in = '0' ) then
                    Next_State <= S0;
                else   
                    Next_State <= S1;
                end if;
            when S1 =>
                S_out <= '1';  
                if ( S_in = '0' ) then
                    Next_State <= S3;
                else   
                    Next_State <= S2;
                end if;
            when S2 =>
                S_out <= '0';  
                if ( S_in = '0' ) then
                    Next_State <= S0;
                else   
                    Next_State <= S3;
                end if;
            when S3 =>
                S_out <= '1';  
                if (S_in = '0' ) then
                    Next_State <= S2;
                else   
                    Next_State <= S4;
                end if;
            when S4 =>
                S_out <= '1';  
                if ( S_in = '0' ) then
                    Next_State <= S2;
                else   
                    Next_State <= S1;
                end if;
            when others =>
                NULL;
        end case;
    end if;
end process;