LIBRARY ieee;USE ieee.std_logic_1164.all;USE ieee.std_logic_arith.all;ENTITY stop_watch_reference IS  port(    CLK        : in std_logic;   --4096 Hz    RESET      : in std_logic;   -- LOW active    KEY1       : in std_logic;   -- LOW active    KEY2       : in std_logic;   -- LOW active    DISPL_1    : out    std_logic_vector (6 DOWNTO 0);    DISPL_10   : out    std_logic_vector (6 DOWNTO 0);    DISPL_100  : out    std_logic_vector (6 DOWNTO 0);    DISPL_1000 : out    std_logic_vector (6 DOWNTO 0)  );END stop_watch_reference;ARCHITECTURE reference_stop_watch OF stop_watch_reference IS  signal STOP_TIME    :  integer;   signal STOP_TIME_L1 :  integer;   signal STOP_TIME_L2 :  integer;   signal STOP_TIME_L3 :  integer;   signal STOP_TIME_L4 :  integer;   signal CLK_COUNT    :  integer;   type T_STATE is (START, MEASURE_CONT, MEASURE_STOP, MEASURE_INTERM);  signal ACTUAL_STATE, NEXT_STATE : T_STATE;  function int_2_7seg (    insig : integer)    return std_logic_vector is    variable pattern : std_logic_vector(6 downto 0);  begin  -- int_2_7seg    case insig is       when 0 => pattern := "1111110";      when 1 => pattern := "1100000";      when 2 => pattern := "1011011";      when 3 => pattern := "1110011";      when 4 => pattern := "1100101";      when 5 => pattern := "0110111";      when 6 => pattern := "0111111";      when 7 => pattern := "1100010";      when 8 => pattern := "1111111";      when 9 => pattern := "1110111";      when others => pattern := "0000000";    end case;    return pattern;  end int_2_7seg;BEGIN  -----------------------------------------------------------------------------------  process(CLK)  begin    if CLK'event and CLK = '1' then      STOP_TIME_L1 <= STOP_TIME;      STOP_TIME_L2 <= STOP_TIME_L1;      STOP_TIME_L3 <= STOP_TIME_L2;      STOP_TIME_L4 <= STOP_TIME_L3;    end if;  end process;  -----------------------------------------------------------------------------------    process(STOP_TIME_L4)  begin    DISPL_1000 <= int_2_7seg( ( STOP_TIME_L4/1000 ) rem 10 ) after 6800 us;    DISPL_100  <= int_2_7seg( ( STOP_TIME_L4/100  ) rem 10 ) after 6800 us;    DISPL_10   <= int_2_7seg( ( STOP_TIME_L4/10   ) rem 10 ) after 6800 us;    DISPL_1    <= int_2_7seg( ( STOP_TIME_L4      ) rem 10 ) after 6800 us;  end process;    -----------------------------------------------------------------------------------    process (CLK_COUNT,ACTUAL_STATE)  begin    case ACTUAL_STATE is      when MEASURE_INTERM =>            STOP_TIME <= STOP_TIME;      when others =>          STOP_TIME <= CLK_COUNT*100/4096;          end case;  end process;  -----------------------------------------------------------------------------------    process (CLK,ACTUAL_STATE,RESET)  begin    if reset='1' then      CLK_COUNT <= 0;    else      case ACTUAL_STATE is        when START =>           CLK_COUNT <= 0;        when MEASURE_CONT =>           if CLK'event and CLK = '1' then            CLK_COUNT <= CLK_COUNT +1;          end if;        when MEASURE_INTERM =>                if CLK'event and CLK = '1' then            CLK_COUNT <= CLK_COUNT +1;          end if;       when others =>             end case;    end if;  end process;  -----------------------------------------------------------------------------------    process (KEY1, KEY2,RESET)  begin    if reset='1' then      ACTUAL_STATE <= START;    else      case ACTUAL_STATE is        when START =>           if KEY1'event and KEY1 = '1' then            ACTUAL_STATE <= MEASURE_CONT;          end if;        when MEASURE_CONT =>           if KEY1'event and KEY1 = '1' then            ACTUAL_STATE <= MEASURE_STOP;          elsif KEY2'event and KEY2 = '1' then            ACTUAL_STATE <= MEASURE_INTERM;          end if;        when MEASURE_STOP =>           if KEY2'event and KEY2 = '1' then            ACTUAL_STATE <= START;          elsif KEY1'event and KEY1 = '1' then            ACTUAL_STATE <= MEASURE_CONT;          end if;        when MEASURE_INTERM =>           if KEY1'event and KEY1 = '1' then            ACTUAL_STATE <= MEASURE_STOP;          elsif KEY2'event and KEY2 = '1' then            ACTUAL_STATE <= MEASURE_CONT;          end if;        when others  =>        end case;    end if;  end process;END reference_stop_watch;