--      
-- TCNTR.vhd - Behavioral Model
-- 1 minute timer.
-- uses behavior of asynchronous counter.
--
--      
-- timer.vhd 
-- Version: v1.1
--

Library IEEE;
use IEEE.STD_LOGIC_1164.all;
use IEEE.STD_LOGIC_UNSIGNED.all;

entity TIMER is
	port (C:   in STD_LOGIC;                       -- clock (100Hz)
	      EN:  in STD_LOGIC;                       -- enable
	      RST: in STD_LOGIC;                       -- reset
	      RUN: in STD_LOGIC;                       -- start/stop ('1' means run)
	      Q3:  out STD_LOGIC_VECTOR (3 downto 0);  -- 10    Sec
	      Q2:  out STD_LOGIC_VECTOR (3 downto 0);  -- 1     Sec
	      Q1:  out STD_LOGIC_VECTOR (3 downto 0);  -- 1/10  Sec
	      Q0:  out STD_LOGIC_VECTOR (3 downto 0)); -- 1/100 Sec
end TIMER; 

architecture TIMER_BEH of TIMER is

signal QTEMP3, QTEMP2, QTEMP1, QTEMP0, Qst3, Qst2, Qst1, Qst0: STD_LOGIC_VECTOR (3 downto 0);
signal CARRY3, CARRY2, CARRY1: STD_LOGIC;

begin
    D0: process (C, RST, RUN)
    begin
       if RST = '1' then              -- RESET the timer
          QTEMP0 <= "0000";
          CARRY1 <='0';
       elsif (C'event and C='0') then
          if RUN = '1' then           -- the RUN needs to be checked here only since this cell drives the next
              QTEMP0 <= QTEMP0 + 1;
          else
              null;
          end if;
          if QTEMP0 = "1001" then
             QTEMP0 <= "0000";
             CARRY1 <= '1';
          else
             CARRY1 <= '0';
          end if;
       end if;
    end process;

    D1: process (CARRY1, RST)
    begin
       if RST = '1' then	-- RESET the timer
          QTEMP1 <= "0000";
          CARRY2 <='0';
       elsif (CARRY1'event and CARRY1='1') then
          QTEMP1 <= QTEMP1 + 1;
          if QTEMP1 = "1001" then
             QTEMP1 <= "0000";
             CARRY2 <= '1';
          else
             CARRY2 <= '0';
          end if;
       end if;
    end process;

    D2: process (CARRY2, RST)
    begin
       if RST = '1' then	-- RESET the timer
          QTEMP2 <= "0000";
          CARRY3 <='0';
       elsif (CARRY2'event and CARRY2='1') then
          QTEMP2 <= QTEMP2 + 1;
          if QTEMP2 = "1001" then
             QTEMP2 <= "0000";
             CARRY3 <= '1';
          else
             CARRY3 <= '0';
          end if;
       end if;
    end process;

    D3: process (CARRY3, RST)
    begin
       if RST = '1' then
          QTEMP3 <= "0000";
       elsif (CARRY3'event and CARRY3='1') then
          QTEMP3 <= QTEMP3 + 1;
          if QTEMP3 = "0101" then
             QTEMP3 <= "0000";
          else
             null;
          end if;
       end if;
    end process;

    Qst3 <= "0000" when RST = '1'
    else QTEMP3 when EN = '1'
    else Qst3;
    
    Qst2 <= "0000" when RST = '1'
    else QTEMP2 when EN = '1'
    else Qst2;

    Qst1 <= "0000" when RST = '1'
    else QTEMP1 when EN = '1'
    else Qst1;

    Qst0 <= "0000" when RST = '1'
    else QTEMP0 when EN = '1'
    else Qst0;

    Q3 <= Qst3;
    Q2 <= Qst2;
    Q1 <= Qst1;
    Q0 <= Qst0;
   
end TIMER_BEH;