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--===========================================================================--
--                                                                           --
--  ps2_keyboard.vhd - Synthesizable PS/2 Keyboard Interface                 --
--                                                                           --
--===========================================================================--
--
--  File name      : ps2_keyboard.vhd
--
--  Purpose        : Implements a PS/2 Keyboard Interface
--                  
--  Dependencies   : ieee.std_logic_1164
--                   ieee.std_logic_unsigned
--                   ieee.std_logic_arith
--                   ieee.numeric_std
--                   unisim.vcomponents
--
--  Author         : Original Verilog version by John Clayton
--                   Converted to VHDL by John E. Kent
--
--  Email          : dilbert57@opencores.org      
--
--  Web            : http://opencores.org/project,system09
--
--  Description     :
--
-- This is a state-machine driven serial-to-parallel and parallel-to-serial
-- interface to the ps2 style keyboard interface.  The details of the operation
-- of the keyboard interface were obtained from the following website:
--
--   http://www.beyondlogic.org/keyboard/keybrd.htm
--
-- Some aspects of the keyboard interface are not implemented (e.g, parity
-- checking for the receive side, and recognition of the various commands
-- which the keyboard sends out, such as "power on selt test passed," "Error"
-- and "Resend.")  However, if the user wishes to recognize these reply
-- messages, the scan code output can always be used to extend functionality
-- as desired.
--
-- Note that the "Extended" (0xE0) and "Released" (0xF0) codes are recognized.
-- The rx interface provides separate indicator flags for these two conditions
-- with every valid character scan code which it provides.  The shift keys are
-- also trapped by the interface, in order to provide correct uppercase ASCII
-- characters at the ascii output, although the scan codes for the shift keys
-- are still provided at the scan code output.  So, the left/right ALT keys
-- can be differentiated by the presence of the rx_entended signal, while the
-- left/right shift keys are differentiable by the different scan codes
-- received.
--
-- The interface to the ps2 keyboard uses ps2_clk clock rates of
-- 30-40 kHz, dependent upon the keyboard itself.  The rate at which the state
-- machine runs should be at least twice the rate of the ps2_clk, so that the
-- states can accurately follow the clock signal itself.  Four times 
-- oversampling is better.  Say 200kHz at least.  The upper limit for clocking
-- the state machine will undoubtedly be determined by delays in the logic 
-- which decodes the scan codes into ASCII equivalents.  The maximum speed
-- will be most likely many megahertz, depending upon target technology.
-- In order to run the state machine extremely fast, synchronizing flip-flops
-- have been added to the ps2_clk and ps2_data inputs of the state machine.
-- This avoids poor performance related to slow transitions of the inputs.
-- 
-- Because this is a bi-directional interface, while reading from the keyboard
-- the ps2_clk and ps2_data lines are used as inputs.  While writing to the
-- keyboard, however (which may be done at any time.  If writing interrupts a
-- read from the keyboard, the keyboard will buffer up its data, and send
-- it later) both the ps2_clk and ps2_data lines are occasionally pulled low,
-- and pullup resistors are used to bring the lines high again, by setting
-- the drivers to high impedance state.
--
-- The tx interface, for writing to the keyboard, does not provide any special
-- pre-processing.  It simply transmits the 8-bit command value to the
-- keyboard.
--
-- Pullups MUST BE USED on the ps2_clk and ps2_data lines for this design,
-- whether they be internal to an FPGA I/O pad, or externally placed.
-- If internal pullups are used, they may be fairly weak, causing bounces
-- due to crosstalk, etc.  There is a "debounce timer" implemented in order
-- to eliminate erroneous state transitions which would occur based on bounce.
-- 
-- Parameters are provided in order to configure and appropriately size the
-- counter of a 60 microsecond timer used in the transmitter, depending on
-- the clock frequency used.  The 60 microsecond period is guaranteed to be
-- more than one period of the ps2_clk_s signal.
--
-- Also, a smaller 5 microsecond timer has been included for "debounce".
-- This is used because, with internal pullups on the ps2_clk and ps2_data
-- lines, there is some bouncing around which occurs
--
-- A parameter TRAP_SHIFT_KEYS allows the user to eliminate shift keypresses
-- from producing scan codes (along with their "undefined" ASCII equivalents)
-- at the output of the interface.  If TRAP_SHIFT_KEYS is non-zero, the shift
-- key status will only be reported by rx_shift_on.  No ascii or scan
-- codes will be reported for the shift keys.  This is useful for those who
-- wish to use the ASCII data stream, and who don't want to have to "filter
-- out" the shift key codes.
-- 
--  Copyright (C) 2001 - 2010 John Clayton and John Kent
--
--  This program is free software: you can redistribute it and/or modify
--  it under the terms of the GNU General Public License as published by
--  the Free Software Foundation, either version 3 of the License, 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 more details.
--
--  You should have received a copy of the GNU General Public License
--  along with this program.  If not, see <http://www.gnu.org/licenses/>.
--
--===========================================================================--
--                                                                           --
--                              Revision  History                            --
--                                                                           --
--===========================================================================--
--
-- Author: John Clayton
-- 2001-04-30 copied this file from lcd_2.v (pared down).
-- 2001-05-24 changed the first module from "ps2_keyboard_receiver"
--            to "ps2_keyboard_interface"
-- 2001-05-29 Added input synchronizing flip-flops.  Changed state
--            encoding (m1) for good operation after part config.
-- 2001-05-31 Added low drive strength and slow transitions to ps2_clk
--            and ps2_data in the constraints file.  Added the signal
--            "tx_shifting_done" as distinguished from "rx_shifting_done."
--            Debugged the transmitter portion in the lab.
-- 2001-06-01 Added horizontal tab to the ascii output.
-- 2001-06-01 Added parameter TRAP_SHIFT_KEYS.
-- 2001-06-05 Debugged the "debounce" timer functionality.
--            Used 60usec timer as a "watchdog" timeout during
--            receive from the keyboard.  This means that a keyboard
--            can now be "hot plugged" into the interface, without
--            messing up the bit_count, since the bit_count is reset
--            to zero during periods of inactivity anyway.  This was
--            difficult to debug.  I ended up using the logic analyzer,
--            and had to scratch my head quite a bit.
-- 2001-06-06 Removed extra comments before the input synchronizing
--            flip-flops.  Used the correct parameter to size the
--            5usec_timer_count.  Changed the name of this file from
--            ps2.v to ps2_keyboard.v
-- 2001-06/06 Removed "&& q[7:0]" in output_strobe logic.  Removed extra
--            commented out "else" condition in the shift register and
--            bit counter.
-- 2001-06-07 Changed default values for 60usec timer parameters so that
--            they correspond to 60usec for a 49.152MHz clock.
--
-- Author: John Kent
--	2001-02-10 Converted to VHDL
-- 2004-09-11 Added ctrl key 
--            Changed undefined key codes to x"ff"
--            Reversed clock polarity
-- 2004-10-18 Added ctrl keys to ASCII ROM
--            Added CAPS Lock toggle.
-- 2007-02-06 Added Generic Clock parameter
-- 2010-05-31 Revised header, added GPL
-- 2010-06-17 Change some signal names for consistancy
--
--
--
---------------------------------------------------------------------------------------
 
library ieee;
   use ieee.std_logic_1164.all;
   use ieee.std_logic_arith.all;
   use ieee.std_logic_unsigned.all;
   use ieee.numeric_std.all;
library unisim;
   use unisim.vcomponents.all;
 
entity ps2_keyboard is
  generic (
  CLK_FREQ_MHZ   : integer
  );
 
  port(
  clk             : in    std_logic;
  reset           : in    std_logic;
  rx_data         : out   std_logic_vector(7 downto 0);
  rx_read         : in    std_logic;
  rx_data_ready   : out   std_logic;
  rx_extended     : out   std_logic;
  rx_released     : out   std_logic;
  rx_shift_on     : out   std_logic;
  tx_data         : in    std_logic_vector(7 downto 0);
  tx_write        : in    std_logic;
  tx_data_empty   : out   std_logic;
  tx_error	      : out   std_logic;
  ps2_clk         : inout std_logic;
  ps2_data        : inout std_logic
  );
end ps2_keyboard;
 
-------------------------------------------------------------------------------
-- Architecture for ps2 keyboard interface
-------------------------------------------------------------------------------
architecture rtl of ps2_keyboard is
  -----------------------------------------------------------------------------
 
 
constant TOTAL_BITS   : integer := 11;
constant EXTEND_CODE  : integer := 16#E0#;
constant RELEASE_CODE : integer := 16#F0#;
constant LEFT_SHIFT   : integer := 16#12#;
constant RIGHT_SHIFT  : integer := 16#59#;
constant CTRL_CODE    : integer := 16#14#;
constant CAPS_CODE    : integer := 16#58#;
 
 
-- constants
 
-- The timer value can be up to (2^bits) inclusive.
-- Values for 49.152 MHz clock
--constant TIMER_60USEC_VALUE_PP : integer := 2950; -- Number of sys_clks for 60usec.
--constant TIMER_60USEC_BITS_PP  : integer := 12;   -- Number of bits needed for timer
--constant TIMER_5USEC_VALUE_PP  : integer := 186;   -- Number of sys_clks for debounce
--constant TIMER_5USEC_BITS_PP   : integer := 8;     -- Number of bits needed for timer
 
-- Values for  12.5 MHz Clock
--constant TIMER_60USEC_VALUE_PP : integer := 750; -- Number of sys_clks for 60usec.
--constant TIMER_60USEC_BITS_PP  : integer := 10;  -- Number of bits needed for timer
--constant TIMER_5USEC_VALUE_PP  : integer := 62;  -- Number of sys_clks for debounce
--constant TIMER_5USEC_BITS_PP   : integer := 6;   -- Number of bits needed for timer
 
-- Values for  25 MHz Clock
--constant TIMER_60USEC_VALUE_PP : integer := 1500; -- Number of sys_clks for 60usec.
--constant TIMER_60USEC_BITS_PP  : integer := 11;   -- Number of bits needed for timer
--constant TIMER_5USEC_VALUE_PP  : integer := 125;  -- Number of sys_clks for debounce
--constant TIMER_5USEC_BITS_PP   : integer := 7;    -- Number of bits needed for timer
 
-- Values for  generic Clock up to 50 MHz
constant TIMER_60USEC_VALUE_PP : integer := CLK_FREQ_MHZ * 60;  -- Number of sys_clks for 60usec.
constant TIMER_60USEC_BITS_PP  : integer := 12;                 -- Number of bits needed for timer
constant TIMER_5USEC_VALUE_PP  : integer := CLK_FREQ_MHZ * 5;   -- Number of sys_clks for debounce
constant TIMER_5USEC_BITS_PP   : integer := 8;                  -- Number of bits needed for timer
 
constant TRAP_SHIFT_KEYS_PP    : integer := 1;   -- Default: No shift key trap.
 
-- State encodings, provided as constants
-- for flexibility to the one instantiating the module.
-- In general, the default values need not be changed.
 
-- State "m1_rx_clk_l" has been chosen on purpose.  Since the input
-- synchronizing flip-flops initially contain zero, it takes one clk
-- for them to update to reflect the actual (idle = high) status of
-- the I/O lines from the keyboard.  Therefore, choosing 0 for m1_rx_clk_l
-- allows the state machine to transition to m1_rx_clk_h when the true
-- values of the input signals become present at the outputs of the
-- synchronizing flip-flops.  This initial transition is harmless, and it
-- eliminates the need for a "reset" pulse before the interface can operate.
 
type m1_type is ( m1_rx_clk_h, m1_rx_clk_l,
                  m1_tx_wait_clk_h, m1_tx_force_clk_l,
						m1_tx_clk_h, m1_tx_clk_l,
						m1_tx_wait_keyboard_ack, m1_tx_done_recovery,
						m1_tx_error, m1_tx_rising_edge_marker,
						m1_tx_first_wait_clk_h, m1_tx_first_wait_clk_l, m1_tx_reset_timer, 
                  m1_rx_falling_edge_marker, m1_rx_rising_edge_marker );
 
-- Internal signal declarations
signal timer_60usec_done  : std_logic;
signal timer_5usec_done   : std_logic;
signal extended           : std_logic;
signal released           : std_logic;
signal shift_key_on       : std_logic;
signal ctrl_key_on        : std_logic;
signal caps_key_on        : std_logic;
 
                         -- NOTE: These two signals used to be one.  They
                         --       were split into two signals because of
                         --       shift key trapping.  With shift key
                         --       trapping, no event is generated externally,
                         --       but the "hold" data must still be cleared
                         --       anyway regardless, in preparation for the
                         --       next scan codes.
signal rx_output_event    : std_logic;    -- Used only to clear: hold_released, hold_extended
signal rx_output_strobe   : std_logic;   -- Used to produce the actual output.
 
signal tx_parity_bit      : std_logic;
signal rx_shifting_done   : std_logic;
signal tx_shifting_done   : std_logic;
signal shift_key_plus_code: std_logic_vector(8 downto 0);
 
signal q                  : std_logic_vector(TOTAL_BITS-1 downto 0);
signal m1_state           : m1_type;
signal m1_next_state      : m1_type;
signal bit_count          : std_logic_vector(3 downto 0);
signal enable_timer_60usec: std_logic;
signal enable_timer_5usec : std_logic;
signal timer_60usec_count : std_logic_vector(TIMER_60USEC_BITS_PP-1 downto 0);
signal timer_5usec_count  : std_logic_vector(TIMER_5USEC_BITS_PP-1 downto 0);
signal ascii              : std_logic_vector(7 downto 0);      -- "REG" type only because a case statement is used.
signal left_shift_key     : std_logic;
signal right_shift_key    : std_logic;
signal hold_extended      : std_logic;  -- Holds prior value, cleared at rx_output_strobe
signal hold_released      : std_logic;  -- Holds prior value, cleared at rx_output_strobe
signal ps2_clk_s          : std_logic;  -- Synchronous version of this input
signal ps2_data_s         : std_logic;  -- Synchronous version of this input
signal ps2_clk_hi_z       : std_logic;  -- Without keyboard, high Z equals 1 due to pullups.
signal ps2_data_hi_z      : std_logic;  -- Without keyboard, high Z equals 1 due to pullups.
signal tx_data_empty_o	  : std_logic;
 
--
-- key lookup table
--
component keymap_rom
    Port (
    clk      : in  std_logic;
    rst      : in  std_logic;
    cs       : in  std_logic;
    rw       : in  std_logic;
    addr     : in  std_logic_vector (8 downto 0);
    data_in  : in  std_logic_vector (7 downto 0);
    data_out : out std_logic_vector (7 downto 0)
    );
end component;
 
begin
 
my_key_map : keymap_rom
     Port map (
	  clk      => clk,
	  rst      => reset,
	  cs       => '1',
	  rw       => '1',
	  addr     => shift_key_plus_code,
	  data_in  => "00000000",
	  data_out => ascii
	  );
 
----------------------------------------------------------------------------
-- Module code
-- assign ps2_clk = ps2_clk_hi_z?1'bZ:1'b0;
-- assign ps2_data = ps2_data_hi_z?1'bZ:1'b0;
--
ps2_direction : process( ps2_clk_hi_z, ps2_data_hi_z )
begin
 
  if( ps2_clk_hi_z = '1' ) then
     ps2_clk <= 'Z';
  else
     ps2_clk <= '0';
  end if;
 
  if( ps2_data_hi_z = '1' ) then
     ps2_data <= 'Z';
  else
     ps2_data <= '0';
  end if;
 
end process;
 
-- Input "synchronizing" logic -- synchronizes the inputs to the state
-- machine clock, thus avoiding errors related to
-- spurious state machine transitions.
ps2_synch : process(clk, ps2_clk, ps2_data)
begin
  if clk'event and clk='0' then
    ps2_clk_s <= ps2_clk;
    ps2_data_s <= ps2_data;
  end if;
end process;
 
-- State register
m1_state_register : process( clk, reset, m1_state )
begin
  if clk'event and clk='0' then
    if (reset = '1') then
	    m1_state <= m1_rx_clk_h;
    else 
	    m1_state <= m1_next_state;
    end if;
  end if;
end process;
 
m1_state_logic : process( m1_state, q,
                          tx_shifting_done, tx_write, 
								  ps2_clk_s, ps2_data_s,
								  timer_60usec_done, timer_5usec_done )
begin
  -- Output signals default to this value, unless changed in a state condition.
  ps2_clk_hi_z             <= '1';
  ps2_data_hi_z            <= '1';
  tx_error                 <= '0';
  enable_timer_60usec      <= '0';
  enable_timer_5usec       <= '0';
 
  case (m1_state) is
  when m1_rx_clk_h =>
        enable_timer_60usec <= '1';
        if (tx_write = '1') then
		      m1_next_state <= m1_tx_reset_timer;
        elsif (ps2_clk_s = '0') then
		      m1_next_state <= m1_rx_falling_edge_marker;
        else 
		      m1_next_state <= m1_rx_clk_h;
        end if;
 
  when m1_rx_falling_edge_marker =>
        enable_timer_60usec <= '0';
        m1_next_state <= m1_rx_clk_l;
 
  when m1_rx_clk_l =>
        enable_timer_60usec <= '1';
        if (tx_write = '1') then
		     m1_next_state <= m1_tx_reset_timer;
        elsif (ps2_clk_s = '1') then
		     m1_next_state <= m1_rx_rising_edge_marker;
        else 
		     m1_next_state <= m1_rx_clk_l;
        end if;
 
  when m1_rx_rising_edge_marker =>
        enable_timer_60usec <= '0';
        m1_next_state <= m1_rx_clk_h;
 
  when m1_tx_reset_timer =>
        enable_timer_60usec <= '0';
        m1_next_state <= m1_tx_force_clk_l;
 
  when m1_tx_force_clk_l =>
        enable_timer_60usec <= '1';
        ps2_clk_hi_z <= '0';  -- Force the ps2_clk line low.
        if (timer_60usec_done = '1') then
		     m1_next_state <= m1_tx_first_wait_clk_h;
        else 
		     m1_next_state <= m1_tx_force_clk_l;
        end	if;
 
  when m1_tx_first_wait_clk_h =>
        enable_timer_5usec <= '1';
        ps2_data_hi_z <= '0';        -- Start bit.
        if (ps2_clk_s = '0') and (timer_5usec_done = '1') then
          m1_next_state <= m1_tx_clk_l;
        else
          m1_next_state <= m1_tx_first_wait_clk_h;
        end	if;
 
    -- This state must be included because the device might possibly
    -- delay for up to 10 milliseconds before beginning its clock pulses.
    -- During that waiting time, we cannot drive the data (q[0]) because it
    -- is possibly 1, which would cause the keyboard to abort its receive
    -- and the expected clocks would then never be generated.
  when m1_tx_first_wait_clk_l =>
        ps2_data_hi_z <= '0';
        if (ps2_clk_s = '0') then
		     m1_next_state <= m1_tx_clk_l;
        else 
		     m1_next_state <= m1_tx_first_wait_clk_l;
        end	if;
 
  when m1_tx_wait_clk_h =>
        enable_timer_5usec <= '1';
        ps2_data_hi_z <= q(0);
        if (ps2_clk_s = '1') and (timer_5usec_done = '1') then
          m1_next_state <= m1_tx_rising_edge_marker;
        else
          m1_next_state <= m1_tx_wait_clk_h;
        end	if;
 
  when m1_tx_rising_edge_marker =>
        ps2_data_hi_z <= q(0);
        m1_next_state <= m1_tx_clk_h;
 
  when m1_tx_clk_h =>
        ps2_data_hi_z <= q(0);
        if (tx_shifting_done = '1') then
		     m1_next_state <= m1_tx_wait_keyboard_ack;
        elsif (ps2_clk_s = '0') then
		     m1_next_state <= m1_tx_clk_l;
        else
		     m1_next_state <= m1_tx_clk_h;
        end	if;
 
  when m1_tx_clk_l =>
        ps2_data_hi_z <= q(0);
        if (ps2_clk_s = '1') then
		     m1_next_state <= m1_tx_wait_clk_h;
        else
		     m1_next_state <= m1_tx_clk_l;
        end	if;
 
  when m1_tx_wait_keyboard_ack =>
        if (ps2_clk_s = '0') and (ps2_data_s = '1') then
           m1_next_state <= m1_tx_error;
        elsif (ps2_clk_s = '0') and (ps2_data_s = '0') then
           m1_next_state <= m1_tx_done_recovery;
        else 
		     m1_next_state <= m1_tx_wait_keyboard_ack;
        end	if;
 
  when m1_tx_done_recovery =>
        if (ps2_clk_s = '1') and (ps2_data_s = '1') then
		     m1_next_state <= m1_rx_clk_h;
        else 
		     m1_next_state <= m1_tx_done_recovery;
        end	if;
 
  when m1_tx_error =>
        tx_error <= '1';
        if (ps2_clk_s = '1') and (ps2_data_s ='1') then
		     m1_next_state <= m1_rx_clk_h;
        else 
		     m1_next_state <= m1_tx_error;
        end	if;
 
  when others =>
	     m1_next_state <= m1_rx_clk_h;
  end case;
end process;
 
-- This is the bit counter
bit_counter: process(clk, reset, m1_state, bit_count )
begin
  if clk'event and clk = '0' then
    if ( reset = '1' ) or
       ( rx_shifting_done = '1' ) or
       (m1_state = m1_tx_wait_keyboard_ack) then       -- After tx is done.
       bit_count <= "0000";  -- normal reset
    elsif (timer_60usec_done = '1' ) and
          (m1_state = m1_rx_clk_h)	and
          (ps2_clk_s = '1') then
       bit_count <= "0000";  -- rx watchdog timer reset
    elsif (m1_state = m1_rx_falling_edge_marker) or  -- increment for rx
          (m1_state = m1_tx_rising_edge_marker) then  -- increment for tx
       bit_count <= bit_count + 1;
    end if;
  end if;
end process;
 
assign: process( bit_count, tx_write, tx_data_empty_o, m1_state )
begin
  if (bit_count = TOTAL_BITS) then
     rx_shifting_done <= '1';
  else
     rx_shifting_done <= '0';
  end if;
 
  if (bit_count = (TOTAL_BITS-1)) then
     tx_shifting_done <= '1';
  else
     tx_shifting_done <= '0';
  end if;
 
-- This is the signal which enables loading of the shift register.
-- It also indicates "ack" to the device writing to the transmitter.
  if ((tx_write = '1') and (m1_state = m1_rx_clk_h)) or
     ((tx_write = '1') and (m1_state = m1_rx_clk_l)) then
     tx_data_empty_o <= '1';
  else 
     tx_data_empty_o <= '0';
  end if;
  tx_data_empty <= tx_data_empty_o;
end process;
 
-- This is the ODD parity bit for the transmitted word.
-- assign tx_parity_bit = ~^tx_data;
--
tx_parity_bit <= not( tx_data(7) xor tx_data(6) xor tx_data(5) xor tx_data(4) xor
                      tx_data(3) xor tx_data(2) xor tx_data(1) xor tx_data(0) );
 
-- This is the shift register
q_shift : process(clk, tx_data_empty_o, tx_parity_bit, tx_data,
                  m1_state, q, ps2_data_s, rx_shifting_done )
begin
  if clk'event and clk='0' then
    if (reset = '1') then
	    q <= "00000000000";
    elsif (tx_data_empty_o = '1') then
	    q <= "1" & tx_parity_bit & tx_data & "0";
    elsif ( (m1_state = m1_rx_falling_edge_marker)	or
            (m1_state = m1_tx_rising_edge_marker) ) then
       q <= ps2_data_s & q((TOTAL_BITS-1) downto 1);
    end if;
  end if;
 
-- Create the signals which indicate special scan codes received.
-- These are the "unlatched versions."
  if (q(8 downto 1) = EXTEND_CODE) and (rx_shifting_done = '1') then
    extended <= '1';
  else
    extended <= '0';
  end if;
  if (q(8 downto 1) = RELEASE_CODE) and (rx_shifting_done = '1') then
    released <= '1';
  else
    released <= '0';
  end if;
end process;
 
-- This is the 60usec timer counter
timer60usec: process(clk, enable_timer_60usec, timer_60usec_count)
begin
  if clk'event and clk = '0' then
    if (enable_timer_60usec = '0') then
	    timer_60usec_count <= (others => '0');
    elsif (timer_60usec_done = '0') then
	    timer_60usec_count <= timer_60usec_count + 1;
    end if;
  end if;
 
  if (timer_60usec_count = (TIMER_60USEC_VALUE_PP - 1)) then
	 timer_60usec_done <= '1';
  else
    timer_60usec_done <= '0';
  end if;
end process;
 
-- This is the 5usec timer counter
timer5usec : process(clk, enable_timer_5usec, timer_5usec_count )
begin
  if clk'event and clk = '0' then
    if (enable_timer_5usec = '0') then
	   timer_5usec_count <= (others => '0');
    elsif (timer_5usec_done = '0') then
	   timer_5usec_count <= timer_5usec_count + 1;
    end if;
  end if;
 
  if( timer_5usec_count = (TIMER_5USEC_VALUE_PP - 1)) then
	 timer_5usec_done <= '1';
  else
	 timer_5usec_done <= '0';
  end if;
end process;
 
 
-- Store the special scan code status bits
-- Not the final output, but an intermediate storage place,
-- until the entire set of output data can be assembled.
special_scan : process(clk, reset, rx_output_event, rx_shifting_done, extended, released )
begin
  if clk'event and clk='0' then
    if (reset = '1') or (rx_output_event = '1')	then
      hold_extended <= '0';
      hold_released <= '0';
    else
      if (rx_shifting_done = '1') and (extended = '1') then
	     hold_extended <= '1';
      end if;
      if (rx_shifting_done = '1') and (released = '1') then
	     hold_released <= '1';
      end if;
    end	if;
  end if;
end process;
 
 
-- These bits contain the status of the two shift keys
left_shift_proc : process(clk, reset, q, rx_shifting_done, hold_released )
begin
  if clk'event and clk = '0' then
    if (reset = '1') then
	   left_shift_key <= '0';
    elsif (q(8 downto 1) = LEFT_SHIFT) and 
	       (rx_shifting_done = '1') and 
			 (hold_released = '0') then
      left_shift_key <= '1';
    elsif (q(8 downto 1) = LEFT_SHIFT) and
	       (rx_shifting_done = '1') and
			 (hold_released = '1') then
      left_shift_key <= '0';
    end if;
  end if;
end process;
 
right_shift_proc : process(clk, reset, q, rx_shifting_done, hold_released )
begin
  if clk'event and clk = '0' then
    if (reset = '1') then
	   right_shift_key <= '0';
    elsif (q(8 downto 1) = RIGHT_SHIFT) and
	       (rx_shifting_done = '1') and
			 (hold_released = '0') then
      right_shift_key <= '1';
    elsif (q(8 downto 1) = RIGHT_SHIFT) and
	       (rx_shifting_done = '1') and
			 (hold_released = '1') then
      right_shift_key <= '0';
    end if;
  end if;
end process;
 
shift_key_on <= left_shift_key or right_shift_key;
rx_shift_on <= shift_key_on;
 
--
-- Control keys
--
ctrl_proc : process(clk, reset, q, rx_shifting_done, hold_released )
begin
  if clk'event and clk = '0' then
    if (reset = '1') then
	   ctrl_key_on <= '0';
    elsif (q(8 downto 1) = CTRL_CODE) and
	       (rx_shifting_done = '1') and
			 (hold_released = '0') then
      ctrl_key_on <= '1';
    elsif (q(8 downto 1) = CTRL_CODE) and
	       (rx_shifting_done = '1') and
			 (hold_released = '1') then
      ctrl_key_on <= '0';
    end if;
  end if;
end process;
 
--
-- Caps lock
--
caps_proc : process(clk, reset, q, rx_shifting_done, hold_released, caps_key_on )
begin
  if clk'event and clk = '0' then
    if (reset = '1') then
	   caps_key_on <= '0';
    elsif (q(8 downto 1) = CAPS_CODE) and
	       (rx_shifting_done = '1') and
			 (hold_released = '0') then
      caps_key_on <= not caps_key_on;
    end if;
  end if;
end process;
 
-- Output the special scan code flags, the scan code and the ascii
special_scan_proc : process(clk, reset,
									 hold_extended, hold_released, 
									 q, ascii, ctrl_key_on )
begin
  if clk'event and clk = '0' then
    if (reset = '1')	then
      rx_extended <= '0';
      rx_released <= '0';
--      rx_scan_code <= "00000000";
      rx_data <= "00000000";
    elsif (rx_output_strobe = '1') then
      rx_extended <= hold_extended;
      rx_released <= hold_released;
--      rx_scan_code <= q(8 downto 1);
    elsif ctrl_key_on = '1' then
	   rx_data <= ascii and x"1f";
    else
      rx_data <= ascii;
    end if;
  end if;
end process;
 
-- Store the final rx output data only when all extend and release codes
-- are received and the next (actual key) scan code is also ready.
-- (the presence of rx_extended or rx_released refers to the
-- the current latest scan code received, not the previously latched flags.)
 
rx_output_proc : process( clk, reset, 
                          rx_shifting_done, rx_output_strobe,
                          extended, released, 
								  q, ascii, rx_read )
begin
  if (rx_shifting_done = '1') and (extended = '0') and (released = '0') then
    rx_output_event <= '1';
  else
    rx_output_event <= '0';
  end if;
 
  if clk'event and clk = '0' then
    if reset = '1' then
	   rx_output_strobe <= '0';
    elsif (rx_shifting_done = '1') and
          (rx_output_strobe = '0') and 
	       (extended = '0') and
			 (released = '0') and
			 (hold_released = '0' ) and
          (ascii /= x"00" ) then
--	  ((TRAP_SHIFT_KEYS_PP = 0) or
--	    ( (q(8 downto 1) /= RIGHT_SHIFT) and 
--		   (q(8 downto 1) /= LEFT_SHIFT) and
--			(q(8 downto 1) /= CTRL_CODE) ) )then
      rx_output_strobe <= '1';
    elsif rx_read = '1' then
      rx_output_strobe <= '0';
    end if;
  end if;
  rx_data_ready <= rx_output_strobe;      
end process;
 
 
-- This part translates the scan code into an ASCII value...
-- Only the ASCII codes which I considered important have been included.
-- if you want more, just add the appropriate case statement lines...
-- (You will need to know the keyboard scan codes you wish to assign.)
-- The entries are listed in ascending order of ASCII value.
shift_key_plus_code <= shift_key_on & caps_key_on & q(7 downto 1);
 
--shift_map : process( shift_key_plus_code )
--begin
--  case shift_key_plus_code is
--  when x"066" => ascii <= x"08";  -- Backspace ("backspace" key)
--  when x"166" => ascii <= x"08";  -- Backspace ("backspace" key)
--  when x"00d" => ascii <= x"09";  -- Horizontal Tab
--  when x"10d" => ascii <= x"09";  -- Horizontal Tab
--  when x"05a" => ascii <= x"0d";  -- Carriage return ("enter" key)
--  when x"15a" => ascii <= x"0d";  -- Carriage return ("enter" key)
--  when x"076" => ascii <= x"1b";  -- Escape ("esc" key)
--  when x"176" => ascii <= x"1b";  -- Escape ("esc" key)
--  when x"029" => ascii <= x"20";  -- Space
--  when x"129" => ascii <= x"20";  -- Space
--  when x"116" => ascii <= x"21";  -- !
--  when x"152" => ascii <= x"22";  -- "
--  when x"126" => ascii <= x"23";  -- #
--  when x"125" => ascii <= x"24";  -- $
--  when x"12e" => ascii <= x"25";  --
--  when x"13d" => ascii <= x"26";  --
--  when x"052" => ascii <= x"27";  --
--  when x"146" => ascii <= x"28";  --
--  when x"145" => ascii <= x"29";  --
--  when x"13e" => ascii <= x"2a";  -- *
--  when x"155" => ascii <= x"2b";  -- +
--  when x"041" => ascii <= x"2c";  -- ,
--  when x"04e" => ascii <= x"2d";  -- -
--  when x"049" => ascii <= x"2e";  -- .
--  when x"04a" => ascii <= x"2f";  -- /
--  when x"045" => ascii <= x"30";  -- 0
--  when x"016" => ascii <= x"31";  -- 1
--  when x"01e" => ascii <= x"32";  -- 2
--  when x"026" => ascii <= x"33";  -- 3
--  when x"025" => ascii <= x"34";  -- 4
--  when x"02e" => ascii <= x"35";  -- 5
--  when x"036" => ascii <= x"36";  -- 6
--  when x"03d" => ascii <= x"37";  -- 7
--  when x"03e" => ascii <= x"38";  -- 8
--  when x"046" => ascii <= x"39";  -- 9
--  when x"14c" => ascii <= x"3a";  -- :
--  when x"04c" => ascii <= x"3b";  -- ;
--  when x"141" => ascii <= x"3c";  -- <
--  when x"055" => ascii <= x"3d";  -- =
--  when x"149" => ascii <= x"3e";  -- >
--  when x"14a" => ascii <= x"3f";  -- ?
--  when x"11e" => ascii <= x"40";  -- @
--  when x"11c" => ascii <= x"41";  -- A
--  when x"132" => ascii <= x"42";  -- B
--  when x"121" => ascii <= x"43";  -- C
--  when x"123" => ascii <= x"44";  -- D
--  when x"124" => ascii <= x"45";  -- E
--  when x"12b" => ascii <= x"46";  -- F
--  when x"134" => ascii <= x"47";  -- G
--  when x"133" => ascii <= x"48";  -- H
--  when x"143" => ascii <= x"49";  -- I
--  when x"13b" => ascii <= x"4a";  -- J
--  when x"142" => ascii <= x"4b";  -- K
--  when x"14b" => ascii <= x"4c";  -- L
--  when x"13a" => ascii <= x"4d";  -- M
--  when x"131" => ascii <= x"4e";  -- N
--  when x"144" => ascii <= x"4f";  -- O
--  when x"14d" => ascii <= x"50";  -- P
--  when x"115" => ascii <= x"51";  -- Q
--  when x"12d" => ascii <= x"52";  -- R
--  when x"11b" => ascii <= x"53";  -- S
--  when x"12c" => ascii <= x"54";  -- T
--  when x"13c" => ascii <= x"55";  -- U
--  when x"12a" => ascii <= x"56";  -- V
--  when x"11d" => ascii <= x"57";  -- W
--  when x"122" => ascii <= x"58";  -- X
--  when x"135" => ascii <= x"59";  -- Y
--  when x"11a" => ascii <= x"5a";  -- Z
--  when x"054" => ascii <= x"5b";  -- [
--  when x"05d" => ascii <= x"5c";  -- \
--  when x"05b" => ascii <= x"5d";  -- ]
--  when x"136" => ascii <= x"5e";  -- ^
--  when x"14e" => ascii <= x"5f";  -- _    
--  when x"00e" => ascii <= x"60";  -- `
--  when x"01c" => ascii <= x"61";  -- a
--  when x"032" => ascii <= x"62";  -- b
--  when x"021" => ascii <= x"63";  -- c
--  when x"023" => ascii <= x"64";  -- d
--  when x"024" => ascii <= x"65";  -- e
--  when x"02b" => ascii <= x"66";  -- f
--  when x"034" => ascii <= x"67";  -- g
--  when x"033" => ascii <= x"68";  -- h
--  when x"043" => ascii <= x"69";  -- i
--  when x"03b" => ascii <= x"6a";  -- j
--  when x"042" => ascii <= x"6b";  -- k
--  when x"04b" => ascii <= x"6c";  -- l
--  when x"03a" => ascii <= x"6d";  -- m
--  when x"031" => ascii <= x"6e";  -- n
--  when x"044" => ascii <= x"6f";  -- o
--  when x"04d" => ascii <= x"70";  -- p
--  when x"015" => ascii <= x"71";  -- q
--  when x"02d" => ascii <= x"72";  -- r
--  when x"01b" => ascii <= x"73";  -- s
--  when x"02c" => ascii <= x"74";  -- t
--  when x"03c" => ascii <= x"75";  -- u
--  when x"02a" => ascii <= x"76";  -- v
--  when x"01d" => ascii <= x"77";  -- w
--  when x"022" => ascii <= x"78";  -- x
--  when x"035" => ascii <= x"79";  -- y
--  when x"01a" => ascii <= x"7a";  -- z
--  when x"154" => ascii <= x"7b";  -- {
--  when x"15d" => ascii <= x"7c";  -- |
--  when x"15b" => ascii <= x"7d";  -- }
--  when x"10e" => ascii <= x"7e";  -- ~
--  when x"071" => ascii <= x"7f";  -- (Delete OR DEL on numeric keypad)
--  when x"171" => ascii <= x"7f";  -- (Delete OR DEL on numeric keypad)
--  when others => ascii <= x"ff";  -- 0xff used for unlisted characters.
--  end case;
--end process;
 
end rtl;