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-- $Id: serport_xonrx.vhd 476 2013-01-26 22:23:53Z mueller $

--

-- Copyright 2011- by Walter F.J. Mueller <W.F.J.Mueller@gsi.de>

--

-- This program is free software; you may redistribute and/or modify it under

-- the terms of the GNU General Public License as published by the Free

-- Software Foundation, either version 2, or at your option any later version.

--

-- This program is distributed in the hope that it will be useful, but

-- WITHOUT ANY WARRANTY, without even the implied warranty of MERCHANTABILITY

-- or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License

-- for complete details.

--

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

-- Module Name:    serport_xonrx - syn

-- Description:    serial port: xon/xoff logic rx path

--

-- Dependencies:   -

-- Test bench:     -

-- Target Devices: generic

-- Tool versions:  xst 13.1; ghdl 0.29

-- Revision History: 

-- Date         Rev Version  Comment

-- 2011-10-22   417   1.0    Initial version 

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

library ieee;

use ieee.std_logic_1164.all;

use ieee.numeric_std.all;

use work.slvtypes.all;

use work.serportlib.all;

entity serport_xonrx is                 -- serial port: xon/xoff logic rx path

  port (

    CLK : in slbit;                     -- clock

    RESET : in slbit;                   -- reset

    ENAXON : in slbit;                  -- enable xon/xoff handling

    ENAESC : in slbit;                  -- enable xon/xoff escaping

    UART_RXDATA : in slv8;              -- uart data out

    UART_RXVAL : in slbit;              -- uart data valid

    RXDATA : out slv8;                  -- user data out

    RXVAL : out slbit;                  -- user data valid

    RXHOLD : in slbit;                  -- user data hold

    RXOVR : out slbit;                  -- user data overrun

    TXOK : out slbit                    -- tx channel ok

  );

end serport_xonrx;

architecture syn of serport_xonrx is

  type regs_type is record

    txok : slbit;                       -- tx channel ok state

    escseen : slbit;                    -- escape seen

    rxdata : slv8;                      -- user rxdata

    rxval : slbit;                      -- user rxval

    rxovr : slbit;                      -- user rxovr

  end record regs_type;

  constant regs_init : regs_type := (

    '1',                                -- txok (startup default is ok !!)

    '0',                                -- escseen

    (others=>'0'),                      -- rxdata

    '0','0'                             -- rxval,rxovr

  );

  signal R_REGS : regs_type := regs_init;  -- state registers

  signal N_REGS : regs_type := regs_init;  -- next value state regs

begin

  proc_regs: process (CLK)

  begin

    if rising_edge(CLK) then

      if RESET = '1' then

        R_REGS <= regs_init;

      else

        R_REGS <= N_REGS;

      end if;

    end if;

  end process proc_regs;

  proc_next: process (R_REGS, ENAXON, ENAESC, UART_RXDATA, UART_RXVAL, RXHOLD)

    variable r : regs_type := regs_init;

    variable n : regs_type := regs_init;

  begin

    r := R_REGS;

    n := R_REGS;

    if ENAXON = '0' then

      n.txok := '1';

    end if;

    if ENAESC = '0' then

      n.escseen := '0';

    end if;

    n.rxovr := '0';                     -- ensure single clock pulse

    if UART_RXVAL = '1' then

      if ENAXON='1' and UART_RXDATA=c_serport_xon then

        n.txok := '1';

      elsif ENAXON='1' and UART_RXDATA=c_serport_xoff then

        n.txok := '0';

      elsif ENAESC='1' and UART_RXDATA=c_serport_xesc then

        n.escseen := '1';

      else

        if r.escseen = '1' then

          n.escseen := '0';

        end if;

        if r.rxval = '0' then

          n.rxval := '1';

          if r.escseen = '1' then

            n.rxdata := not UART_RXDATA;

          else

            n.rxdata := UART_RXDATA;

          end if;

        else

          n.rxovr := '1';

        end if;

      end if;

    end if;

    if r.rxval='1' and RXHOLD='0' then

      n.rxval := '0';

    end if;

    N_REGS <= n;

    RXDATA <= r.rxdata;

    RXVAL  <= r.rxval;

    RXOVR  <= r.rxovr;

    TXOK   <= r.txok;

  end process proc_next;

end syn;