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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 OCRP-1

--

-- File name      : RxUnit.vhd

--

-- Purpose        : Implements an miniUART device for communication purposes 

--                  between the OR1K processor and the Host computer through

--                  an RS-232 communication protocol.

--                  

-- Library        : uart_lib.vhd

--

-- Dependencies   : IEEE.Std_Logic_1164

--

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

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

-- Revision list

-- Version   Author                 Date                        Changes

--

-- 0.1      Ovidiu Lupas     15 January 2000                   New model

-- 2.0      Ovidiu Lupas     17 April   2000  samples counter cleared for bit 0

--        olupas@opencores.org

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

-- Description    : Implements the receive unit of the miniUART core. Samples

--                  16 times the RxD line and retain the value in the middle of

--                  the time interval. 

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

-- Entity for Receive Unit - 9600 baudrate                                  --

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

library ieee;

   use ieee.std_logic_1164.all;

   use ieee.numeric_std.all;

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

-- Receive unit

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

entity RxUnit is

  port (

     Clk    : in  Std_Logic;  -- system clock signal

     Reset  : in  Std_Logic;  -- Reset input

     Enable : in  Std_Logic;  -- Enable input

     RxD    : in  Std_Logic;  -- RS-232 data input

     ReadD  : in  Std_Logic;  -- Read data signal

     FRErr  : out Std_Logic;  -- Status signal

     ORErr  : out Std_Logic;  -- Status signal

     DARdy  : out Std_Logic;  -- Status signal

     DataI  : out Std_Logic_Vector(7 downto 0));

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

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

-- Architecture for receive Unit

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

architecture Behaviour of RxUnit is

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

  -- Signals

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

  signal Start     : Std_Logic;             -- Syncro signal

  signal tmpRxD    : Std_Logic;             -- RxD buffer

  signal tmpDRdy   : Std_Logic;             -- Data ready buffer

  signal outErr    : Std_Logic;             -- 

  signal frameErr  : Std_Logic;             -- 

  signal BitCnt    : Unsigned(3 downto 0);  -- 

  signal SampleCnt : Unsigned(3 downto 0);  -- samples on one bit counter

  signal ShtReg    : Std_Logic_Vector(7 downto 0);  --

  signal DOut      : Std_Logic_Vector(7 downto 0);  --

begin

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

  -- Receiver process

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

  RcvProc : process(Clk,Reset,Enable,RxD)

      constant CntOne : Unsigned(3 downto 0):="0001";

  begin

     if Clk'event and Clk='1' then

        if Reset = '1' then

           BitCnt <= "0000";

           SampleCnt <= "0000";

           Start <= '0';

           tmpDRdy <= '0';

           frameErr <= '0';

           outErr <= '0';

           ShtReg <= "00000000";  --

           DOut   <= "00000000";  --

        else

           if ReadD = '1' then

              tmpDRdy <= '0';      -- Data was read

           end if;

           if Enable = '1' then

              if Start = '0' then

                 if RxD = '0' then -- Start bit, 

                    SampleCnt <= SampleCnt + CntOne;

                    Start <= '1';

                 end if;

              else

                 if SampleCnt = "1000" then  -- reads the RxD line

                    tmpRxD <= RxD;

                    SampleCnt <= SampleCnt + CntOne;

                 elsif SampleCnt = "1111" then

                    case BitCnt is

                         when "0000" =>

                                if tmpRxD = '1' then -- Start Bit

                                   Start <= '0';

                                else

                                   BitCnt <= BitCnt + CntOne;

                                end if;

                                SampleCnt <= SampleCnt + CntOne;

                         when "1001" =>

                                if tmpRxD = '0' then  -- stop bit expected

                                   frameErr <= '1';

                                else

                                   frameErr <= '0';

                                end if;

                                if tmpDRdy = '1' then -- 

                                   outErr <= '1';

                                else

                                   outErr <= '0';

                                end if;

                                tmpDRdy <= '1';

                                DOut <= ShtReg;

                                BitCnt <= "0000";

                                Start <= '0';

                         when others =>

                                BitCnt <= BitCnt + CntOne;

                                SampleCnt <= SampleCnt + CntOne;

                                ShtReg <= tmpRxD & ShtReg(7 downto 1);

                    end case;

                 else

                    SampleCnt <= SampleCnt + CntOne;

                 end if;

              end if;

           end if;

        end if;

     end if;

  end process;

  DARdy <= tmpDRdy;

  DataI <= DOut;

  FRErr <= frameErr;

  ORErr <= outErr;

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