-- Classic 2-Process State Machine and Test Bench
-- MEALY TYPE STATE MACHINE EXAMPLE
-- dowload from: www.fpga.com.cn & www.pld.com.cn

library ieee;
use ieee.std_logic_1164.all;

ENTITY fsm IS
    PORT(clock,x : IN BIT; z : OUT BIT);
END fsm;
-------------------------------------------------
ARCHITECTURE behaviour OF fsm IS

TYPE state_type IS (s0,s1,s2,s3);
SIGNAL present_state,next_state : state_type;

BEGIN
--state register process
state_reg:PROCESS
          BEGIN
            WAIT UNTIL clock'EVENT AND clock = '1';
            present_state <= next_state;
          END PROCESS;
--combinational logic feedback process
 fb_logic:PROCESS(present_state,x)
          BEGIN
            CASE present_state IS
                WHEN s0 =>
                IF x = '0' THEN z <= '0'; next_state <= s0;
                ELSE z <= '1'; next_state <= s2;
                END IF;

                WHEN s1 =>
                IF x = '0' THEN z <= '0'; next_state <= s0;
                ELSE z <= '0'; next_state <= s2;
                END IF;

                WHEN s2 =>
                IF x = '0' THEN z <= '1'; next_state <= s2;
                ELSE z <= '0'; next_state <= s3;
                END IF;

                WHEN s3 =>
                IF x = '0' THEN z <= '0'; next_state <= s3;
                ELSE z <= '1'; next_state <= s1;
                END IF;
            END CASE;
          END PROCESS;
END behaviour;
-----------------------------------------------------------

--STIMULUS GENERATOR FOR FSM

ENTITY fsm_stim IS
    PORT (clock,x: OUT BIT; z: IN BIT);
END fsm_stim;

ARCHITECTURE behavioural OF fsm_stim IS
BEGIN

--clock pulses : __--__--__--__--__--__
--x input      : _____------------_____
--each '-' represents 5 ns.

    clock <= '0' AFTER 0 ns,
             '1' AFTER 10 ns, --clock 1
             '0' AFTER 20 ns,
             '1' AFTER 30 ns, --clock 2
             '0' AFTER 40 ns,
             '1' AFTER 50 ns, --clock 3
             '0' AFTER 60 ns,
             '1' AFTER 70 ns, --clock 4
             '0' AFTER 80 ns,
             '1' AFTER 90 ns, --clock 5
             '0' AFTER 100 ns;

    x <= '0' AFTER 0 ns,
         '1' AFTER 25 ns,
         '0' AFTER 85 ns;

END behavioural;

-----------------------------------------------

ENTITY fsm_bench IS
END fsm_bench;

ARCHITECTURE structural OF fsm_bench IS
    COMPONENT fsm_stim PORT (clock,x: OUT BIT; z: IN BIT); END COMPONENT;
    COMPONENT fsm PORT (clock,x: IN BIT; z: OUT BIT); END COMPONENT;
    SIGNAL clock,x,z: BIT;
BEGIN
    generator:fsm_stim PORT MAP(clock,x,z);
    circuit:fsm PORT MAP(clock,x,z);
END structural;