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-- $Id: serport_uart_autobaud.vhd 314 2010-07-09 17:38:41Z mueller $

--

-- Copyright 2007-2010 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_uart_autobaud - syn

-- Description:    serial port UART - autobauder

--

-- Dependencies:   -

-- Test bench:     tb/tb_serport_autobaud

-- Target Devices: generic

-- Tool versions:  xst 8.1, 8.2, 9.1, 9.2, 11.4; ghdl 0.18-0.26

-- Revision History: 

-- Date         Rev Version  Comment

-- 2010-04-18   279   1.0.3  change ccnt start value to -3, better rounding

-- 2007-10-14    89   1.0.2  all instantiation with CDINIT=0

-- 2007-10-12    88   1.0.1  avoid ieee.std_logic_unsigned, use cast to unsigned

-- 2007-06-30    62   1.0    Initial version 

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

library ieee;

use ieee.std_logic_1164.all;

use ieee.std_logic_arith.all;

use work.slvtypes.all;

entity serport_uart_autobaud is         -- serial port uart: autobauder

  generic (

    CDWIDTH : positive := 13;           -- clk divider width

    CDINIT: natural := 15);             -- clk divider initial/reset setting

  port (

    CLK : in slbit;                     -- clock

    CE_MSEC : in slbit;                 -- 1 msec clock enable

    RESET : in slbit;                   -- reset

    RXSD : in slbit;                    -- receive serial data (uart view)

    CLKDIV : out slv(CDWIDTH-1 downto 0); -- clock divider setting

    ACT : out slbit;                    -- active; if 1 clkdiv is invalid

    DONE : out slbit                    -- resync done

  );

end serport_uart_autobaud;

architecture syn of serport_uart_autobaud is

  type state_type is (

    s_idle,

    s_break,

    s_wait,

    s_sync

  );

  type regs_type is record

    ccnt : slv(CDWIDTH-1+3 downto 0);   -- clock divider counter

    mcnt : slv7;                        -- msec counter

    seen1 : slbit;                      -- seen a '1' in this msec

    state : state_type;                 -- state

  end record regs_type;

  -- Note on initialization of ccnt:

  -- - in the current logic ccnt is incremented n-1 times when n is number

  --   clock cycles with a RXD of '0'. When running at 50 MBaud, ccnt will

  --   be incremented 7 (not 8!) times.

  -- - the three LSBs of ccnt should be at 100 under perfect conditions, this

  --   gives the best rounded estimate of CLKDIV.

  -- - therefore ccnt is inititialized with 111111.101: 101 + 111 -> 1100 

  --   --> ccntinit = -3

  constant ccntinit : slv(CDWIDTH-1+3 downto 0) :=

    conv_std_logic_vector(2**(CDWIDTH+3)-3, CDWIDTH+3);

  constant mcntzero : slv7 := (others=>'0');

  constant mcntlast : slv7 := (others=>'1');

  constant regs_init : regs_type := (

    conv_std_logic_vector(CDINIT,CDWIDTH)&"000",

    (others=>'0'),

    '0',

    s_idle

  );

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

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

begin

  assert CDINIT <= 2**CDWIDTH-1

  report "assert(CDINIT <= 2**CDWIDTH-1): CDINIT too large for given CDWIDTH"

  severity FAILURE;

  proc_regs: process (CLK)

  begin

    if CLK'event and CLK='1' 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, CE_MSEC, RESET, RXSD)

    variable r : regs_type := regs_init;

    variable n : regs_type := regs_init;

    variable iact : slbit := '0';

    variable idone : slbit := '0';

  begin

    r := R_REGS;

    n := R_REGS;

    iact  := '1';

    idone := '0';

    case r.state is

      when s_idle =>                    -- s_idle: idle, detect break --------

        iact := '0';

        if CE_MSEC = '1' then             -- if end of msec

          if r.seen1 = '0' then             -- if no '1' seen on RXD

            n.mcnt := unsigned(r.mcnt) + 1;   -- up break timer counter

            if r.mcnt = mcntlast then         -- after 127 msec

              n.state := s_break;                -- break detected !

            end if;

          else                              -- otherwise if '1' seen

            n.mcnt := mcntzero;               -- clear break timer again

          end if;

          n.seen1 := RXSD;                  -- latch current RXD value

        else                              -- otherwise if not at end-of-msec

          n.seen1 := r.seen1 or RXSD;       -- remember whether RXS=1 seen

        end if;

      when s_break =>                   -- s_break: detect end of break ------

        if RXSD = '1' then                -- if end of break seen 

          n.state := s_wait;                -- to s_wait to wait for sync char

          n.ccnt := ccntinit;               -- and initialize ccnt

        end if;                           -- otherwise stay in s_break

      when s_wait =>                    -- s_wait: wait for sync char --------

        if RXSD = '0' then                -- if start bit if sync char seen

          n.state := s_sync;                -- to s_sync to wait for end of '0'

        end if;                           -- otherwise stay in s_wait

      when s_sync =>                    -- s_sync: wait for end of '0' bits --

        if RXSD = '1' then                -- if end of '0' bits seen

          n.state := s_idle;                -- to s_idle, autobauding done

          idone := '1';                     -- emit done pulse

        else                              -- otherwise still in '0' of sync

          n.ccnt := unsigned(n.ccnt) + 1;   -- increment ccnt

        end if;

      when others => null;              -- -----------------------------------

    end case;

    N_REGS <= n;

    CLKDIV <= r.ccnt(CDWIDTH-1+3 downto 3);

    ACT    <= iact or RESET;

    DONE   <= idone;

  end process proc_next;

end syn;