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--===========================================================================--

--

--  S Y N T H E Z I A B L E    miniUART   C O R E

--

--  www.OpenCores.Org - January 2000

--  This core adheres to the GNU public license  

-- Design units   : miniUART core for the System68

--

-- File name      : clkunit2.vhd

--

-- Purpose        : Implements an miniUART device for communication purposes 

--                  between the CPU68 processor and the Host computer through

--                  an RS-232 communication protocol.

--                  

-- Dependencies   : ieee.std_logic_1164

--                  ieee.numeric_std

--

--===========================================================================--

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

-- Revision list

-- Version   Author                 Date              Changes

--

-- 0.1      Ovidiu Lupas       15 January 2000        New model

--        olupas@opencores.org

--

-- 2.0      John Kent          10 November 2002       Added programmable baud rate

-- 3.0      John Kent          15 December 2002       Fix TX clock divider

-- 3.1      John kent          12 January  2003       Changed divide by 1 for 38.4Kbps

-- 3.3      John Kent          6  September 2003      Changed Clock Edge.

--        dilbert57@opencores.org

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

-- Description    : Generates the Baud clock and enable signals for RX & TX

--                  units. 

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

-- Entity for Baud rate generator Unit - 9600 baudrate                       --

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

library ieee;

   use ieee.std_logic_1164.all;

   use ieee.numeric_std.all;

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

-- Baud rate generator

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

entity ClkUnit is

  port (

     Clk      : in  Std_Logic;  -- System Clock

     Reset    : in  Std_Logic; -- Reset input

     EnableRx : out Std_Logic;  -- Control signal

     EnableTx : out Std_Logic;  -- Control signal

          BaudRate : in Std_Logic_Vector(1 downto 0));

end; --================== End of entity ==============================--

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

-- Architecture for Baud rate generator Unit

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

architecture Behaviour of ClkUnit is

signal tmpEnRx : std_logic;

begin

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

  -- Divides the system clock of 40 MHz    div 260 gives 153KHz for 9600bps

  --                             48 MHz    div 156 gives 306KHz for 19.2Kbps

  --                             24 MHz    div 156 gives 153KHz for 9600bps

  --                             9.8304MHz div 32  gives 306KHz for 19.2Kbps

  --                             4.9152MHz div 32  gives 153KHz for 9600bps

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

  DivClk : process(Clk,Reset,tmpEnRx, BaudRate)

   variable Count  : unsigned(7 downto 0);

   constant CntOne : Unsigned(7 downto 0):="00000001";

   begin

     if Clk'event and Clk = '0' then

        if Reset = '1' then

           Count := "00000000";

           tmpEnRx <= '0';

        else

                          if Count = "00000000" then

                                 tmpEnRx <= '1';

                                 case BaudRate is

                                 when "00" =>

                                 -- 6850 divide by 1 ((1*2)-1) (synchronous)

                                 -- miniUart 9.83MHz div 16 = 38.4Kbps

                                   Count := "00001111";

                                 when "01" =>

                                 -- 6850 divide by 16 ((16*2)-1) (9600 Baud)

                                 -- miniUart 9.83MHz div 32 = 19.2Kbps

                                   Count := "00011111";

                                 when "10" =>

                                 -- 6850 divide by 64 ((64*2)-1) (2400 Baud)

                                 -- miniUart 9.83MHz div 128 = 4800bps

                                   Count := "01111111";

                                 when others =>

--                               when "11" => -- reset

                                   Count := "00000000";

                                   null;

                                 end case;

                          else

             tmpEnRx <= '0';

                       Count := Count - CntOne;

           end if;

        end if;

     end if;

     EnableRx <= tmpEnRx;

  end process;

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

  -- Provides the EnableTX signal, at 9.6 KHz

  -- Divide by 16

  -- Except it wasn't ... it counted up to "10010" (18)

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

  DivClk16 : process(Clk,Reset,tmpEnRX)

   variable Cnt16  : unsigned(4 downto 0);

   constant CntOne : Unsigned(4 downto 0):="00001";

   begin

    if Clk'event and Clk = '0' then

      if Reset = '1' then

        Cnt16 := "00000";

        EnableTX <= '0';

      else

        case Cnt16 is

          when "00000" =>

            if tmpEnRx = '1' then

              Cnt16 := "01111";

              EnableTx <='1';

                                else

                                  Cnt16 := Cnt16;

                                  EnableTx <= '0';

                                end if;

               when others =>

            if tmpEnRx = '1' then

              Cnt16 := Cnt16 - CntOne;

                                else

                                  Cnt16 := Cnt16;

                                end if;

            EnableTX <= '0';

        end case;

                end if;

    end if;

  end process;

end Behaviour; --==================== End of architecture ===================--