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-- #################################################################################################

-- # << NEORV32 - Bus Keeper (BUSKEEPER) >>                                                        #

-- # ********************************************************************************************* #

-- # This unit monitors the processor-internal bus. If the accessed module does not respond within #

-- # the defined number of cycles (VHDL package: max_proc_int_response_time_c) or issues an ERROR  #

-- # condition, the BUS KEEPER asserts the error signal to inform the CPU.                         #

-- # ********************************************************************************************* #

-- # BSD 3-Clause License                                                                          #

-- #                                                                                               #

-- # Copyright (c) 2022, Stephan Nolting. All rights reserved.                                     #

-- #                                                                                               #

-- # Redistribution and use in source and binary forms, with or without modification, are          #

-- # permitted provided that the following conditions are met:                                     #

-- #                                                                                               #

-- # 1. Redistributions of source code must retain the above copyright notice, this list of        #

-- #    conditions and the following disclaimer.                                                   #

-- #                                                                                               #

-- # 2. Redistributions in binary form must reproduce the above copyright notice, this list of     #

-- #    conditions and the following disclaimer in the documentation and/or other materials        #

-- #    provided with the distribution.                                                            #

-- #                                                                                               #

-- # 3. Neither the name of the copyright holder nor the names of its contributors may be used to  #

-- #    endorse or promote products derived from this software without specific prior written      #

-- #    permission.                                                                                #

-- #                                                                                               #

-- # THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS" AND ANY EXPRESS   #

-- # OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF               #

-- # MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE    #

-- # COPYRIGHT HOLDER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL,     #

-- # EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE #

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-- # OF THE POSSIBILITY OF SUCH DAMAGE.                                                            #

-- # ********************************************************************************************* #

-- # The NEORV32 Processor - https://github.com/stnolting/neorv32              (c) Stephan Nolting #

-- #################################################################################################

library ieee;

use ieee.std_logic_1164.all;

use ieee.numeric_std.all;

library neorv32;

use neorv32.neorv32_package.all;

entity neorv32_bus_keeper is

  port (

    -- host access --

    clk_i      : in  std_ulogic; -- global clock line

    rstn_i     : in  std_ulogic; -- global reset, low-active, async

    addr_i     : in  std_ulogic_vector(31 downto 0); -- address

    rden_i     : in  std_ulogic; -- read enable

    wren_i     : in  std_ulogic; -- write enable

    data_i     : in  std_ulogic_vector(31 downto 0); -- data in

    data_o     : out std_ulogic_vector(31 downto 0); -- data out

    ack_o      : out std_ulogic; -- transfer acknowledge

    err_o      : out std_ulogic; -- transfer error

    -- bus monitoring --

    bus_addr_i : in  std_ulogic_vector(31 downto 0); -- address

    bus_rden_i : in  std_ulogic; -- read enable

    bus_wren_i : in  std_ulogic; -- write enable

    bus_ack_i  : in  std_ulogic; -- transfer acknowledge from bus system

    bus_err_i  : in  std_ulogic; -- transfer error from bus system

    bus_tmo_i  : in  std_ulogic; -- transfer timeout (external interface)

    bus_ext_i  : in  std_ulogic; -- external bus access

    bus_xip_i  : in  std_ulogic  -- pending XIP access

  );

end neorv32_bus_keeper;

architecture neorv32_bus_keeper_rtl of neorv32_bus_keeper is

  -- IO space: module base address --

  constant hi_abb_c : natural := index_size_f(io_size_c)-1; -- high address boundary bit

  constant lo_abb_c : natural := index_size_f(buskeeper_size_c); -- low address boundary bit

  -- Control register --

  constant ctrl_err_type_c     : natural :=  0; -- r/-: error type LSB: 0=device error, 1=access timeout

  constant ctrl_nul_check_en_c : natural := 16; -- r/w: enable NULL address check

  constant ctrl_err_flag_c     : natural := 31; -- r/c: bus error encountered, sticky; cleared by writing zero

  --

  signal ctrl_null_check_en : std_ulogic;

  -- error codes --

  constant err_device_c  : std_ulogic := '0'; -- device access error

  constant err_timeout_c : std_ulogic := '1'; -- timeout error

  -- sticky error flags --

  signal err_flag : std_ulogic;

  signal err_type : std_ulogic;

  -- NULL address check --

  signal null_check : std_ulogic;

  -- access control --

  signal acc_en : std_ulogic; -- module access enable

  signal wren   : std_ulogic; -- word write enable

  signal rden   : std_ulogic; -- read enable

  -- controller --

  type control_t is record

    pending  : std_ulogic;

    timeout  : std_ulogic_vector(index_size_f(max_proc_int_response_time_c) downto 0);

    err_type : std_ulogic;

    bus_err  : std_ulogic;

  end record;

  signal control : control_t;

begin

  -- Sanity Check --------------------------------------------------------------------------

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

  assert not (max_proc_int_response_time_c < 2) report "NEORV32 PROCESSOR CONFIG ERROR! Processor-internal bus timeout <max_proc_int_response_time_c> has to >= 2." severity error;

  -- Access Control -------------------------------------------------------------------------

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

  acc_en <= '1' when (addr_i(hi_abb_c downto lo_abb_c) = buskeeper_base_c(hi_abb_c downto lo_abb_c)) else '0';

  wren   <= acc_en and wren_i;

  rden   <= acc_en and rden_i;

  -- Read/Write Access ----------------------------------------------------------------------

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

  rw_access: process(rstn_i, clk_i)

  begin

    if (rstn_i = '0') then

      ack_o              <= '-';

      data_o             <= (others => '-');

      ctrl_null_check_en <= '0'; -- required

      err_flag           <= '0'; -- required

      err_type           <= '0';

    elsif rising_edge(clk_i) then

      -- bus handshake --

      ack_o <= wren or rden;

      -- write access --

      if (wren = '1') then

        ctrl_null_check_en <= data_i(ctrl_nul_check_en_c);

      end if;

      -- read access --

      data_o <= (others => '0');

      if (rden = '1') then

        data_o(ctrl_err_type_c)     <= err_type;

        data_o(ctrl_nul_check_en_c) <= ctrl_null_check_en;

        data_o(ctrl_err_flag_c)     <= err_flag;

      end if;

      --

      if (control.bus_err = '1') then -- sticky error flag

        err_flag <= '1';

        err_type <= control.err_type;

      else

        if ((wren or rden) = '1') then -- clear on read or write access

          err_flag <= '0';

        end if;

      end if;

    end if;

  end process rw_access;

  -- Keeper ---------------------------------------------------------------------------------

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

  keeper_control: process(rstn_i, clk_i)

  begin

    if (rstn_i = '0') then

      control.pending  <= '0'; -- required

      control.bus_err  <= '0'; -- required

      control.err_type <= '-';

      control.timeout  <= (others => '-');

    elsif rising_edge(clk_i) then

      -- defaults --

      control.bus_err <= '0';

      -- access monitor: IDLE --

      if (control.pending = '0') then

        control.timeout <= std_ulogic_vector(to_unsigned(max_proc_int_response_time_c, index_size_f(max_proc_int_response_time_c)+1));

        if (bus_rden_i = '1') or (bus_wren_i = '1') then

          control.pending <= '1';

          if (null_check = '1') then -- invalid access to NULL address

            control.bus_err <= '1';

          end if;

        end if;

      -- access monitor: PENDING --

      else

        control.timeout <= std_ulogic_vector(unsigned(control.timeout) - 1); -- countdown timer

        if (bus_err_i = '1') or (control.bus_err = '1') then -- error termination by bus system

          control.err_type <= err_device_c; -- device error

          control.bus_err  <= '1';

          control.pending  <= '0';

        elsif ((or_reduce_f(control.timeout) = '0') and (bus_ext_i = '0') and (bus_xip_i = '0')) or -- valid internal access timeout

              (bus_tmo_i = '1') then -- external access timeout

          control.err_type <= err_timeout_c; -- timeout error

          control.bus_err  <= '1';

          control.pending  <= '0';

        elsif (bus_ack_i = '1') then -- normal termination by bus system

          control.err_type <= '0'; -- don't care

          control.bus_err  <= '0';

          control.pending  <= '0';

        end if;

      end if;

    end if;

  end process keeper_control;

  -- NULL address check --

  null_check <= '1' when (ctrl_null_check_en = '1') and (or_reduce_f(addr_i) = '0') else '0';

  -- signal bus error to CPU --

  err_o <= control.bus_err;

end neorv32_bus_keeper_rtl;