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

-- # << NEORV32 - Bus Switch >>                                                                    #

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

-- # Allows to access a single peripheral bus ("p_bus") by two controller busses. Controller port  #

-- # A ("ca_bus") has priority over controller port B ("cb_bus").                                  #

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

-- # BSD 3-Clause License                                                                          #

-- #                                                                                               #

-- # Copyright (c) 2020, 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    #

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

-- # 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_busswitch is

  generic (

    PORT_CA_READ_ONLY : boolean := false; -- set if controller port A is read-only

    PORT_CB_READ_ONLY : boolean := false  -- set if controller port B is read-only

  );

  port (

    -- global control --

    clk_i           : in  std_ulogic; -- global clock, rising edge

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

    -- controller interface a --

    ca_bus_addr_i   : in  std_ulogic_vector(data_width_c-1 downto 0); -- bus access address

    ca_bus_rdata_o  : out std_ulogic_vector(data_width_c-1 downto 0); -- bus read data

    ca_bus_wdata_i  : in  std_ulogic_vector(data_width_c-1 downto 0); -- bus write data

    ca_bus_ben_i    : in  std_ulogic_vector(03 downto 0); -- byte enable

    ca_bus_we_i     : in  std_ulogic; -- write enable

    ca_bus_re_i     : in  std_ulogic; -- read enable

    ca_bus_cancel_i : in  std_ulogic; -- cancel current bus transaction

    ca_bus_ack_o    : out std_ulogic; -- bus transfer acknowledge

    ca_bus_err_o    : out std_ulogic; -- bus transfer error

    -- controller interface b --

    cb_bus_addr_i   : in  std_ulogic_vector(data_width_c-1 downto 0); -- bus access address

    cb_bus_rdata_o  : out std_ulogic_vector(data_width_c-1 downto 0); -- bus read data

    cb_bus_wdata_i  : in  std_ulogic_vector(data_width_c-1 downto 0); -- bus write data

    cb_bus_ben_i    : in  std_ulogic_vector(03 downto 0); -- byte enable

    cb_bus_we_i     : in  std_ulogic; -- write enable

    cb_bus_re_i     : in  std_ulogic; -- read enable

    cb_bus_cancel_i : in  std_ulogic; -- cancel current bus transaction

    cb_bus_ack_o    : out std_ulogic; -- bus transfer acknowledge

    cb_bus_err_o    : out std_ulogic; -- bus transfer error

    -- peripheral bus --

    p_bus_addr_o    : out std_ulogic_vector(data_width_c-1 downto 0); -- bus access address

    p_bus_rdata_i   : in  std_ulogic_vector(data_width_c-1 downto 0); -- bus read data

    p_bus_wdata_o   : out std_ulogic_vector(data_width_c-1 downto 0); -- bus write data

    p_bus_ben_o     : out std_ulogic_vector(03 downto 0); -- byte enable

    p_bus_we_o      : out std_ulogic; -- write enable

    p_bus_re_o      : out std_ulogic; -- read enable

    p_bus_cancel_o  : out std_ulogic; -- cancel current bus transaction

    p_bus_ack_i     : in  std_ulogic; -- bus transfer acknowledge

    p_bus_err_i     : in  std_ulogic  -- bus transfer error

  );

end neorv32_busswitch;

architecture neorv32_busswitch_rtl of neorv32_busswitch is

  -- access buffer --

  signal ca_rd_req_buf : std_ulogic;

  signal ca_wr_req_buf : std_ulogic;

  signal cb_rd_req_buf : std_ulogic;

  signal cb_wr_req_buf : std_ulogic;

  -- access requests --

  signal ca_req_current  : std_ulogic;

  signal cb_req_current  : std_ulogic;

  signal ca_req_buffered : std_ulogic;

  signal cb_req_buffered : std_ulogic;

  -- internal bus lines --

  signal ca_bus_ack : std_ulogic;

  signal cb_bus_ack : std_ulogic;

  signal ca_bus_err : std_ulogic;

  signal cb_bus_err : std_ulogic;

  signal p_bus_we   : std_ulogic;

  signal p_bus_re   : std_ulogic;

  -- access arbiter --

  type arbiter_state_t is (IDLE, BUSY, RETIRE, BUSY_SWITCHED, RETIRE_SWITCHED);

  type arbiter_t is record

    state     : arbiter_state_t;

    state_nxt : arbiter_state_t;

    bus_sel   : std_ulogic;

    re_trig   : std_ulogic;

    we_trig   : std_ulogic;

  end record;

  signal arbiter : arbiter_t;

begin

  -- Access Buffer --------------------------------------------------------------------------

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

  access_buffer: process(rstn_i, clk_i)

  begin

    if (rstn_i = '0') then

      ca_rd_req_buf <= '0';

      ca_wr_req_buf <= '0';

      cb_rd_req_buf <= '0';

      cb_wr_req_buf <= '0';

    elsif rising_edge(clk_i) then

      -- controller A requests --

      if (ca_rd_req_buf = '0') and (ca_wr_req_buf = '0') then -- idle

        ca_rd_req_buf <= ca_bus_re_i;

        ca_wr_req_buf <= ca_bus_we_i;

      elsif (ca_bus_cancel_i = '1') or -- controller cancels access

            (ca_bus_err = '1') or -- peripheral cancels access

            (ca_bus_ack = '1') then -- normal termination

        ca_rd_req_buf <= '0';

        ca_wr_req_buf <= '0';

      end if;

      -- controller B requests --

      if (cb_rd_req_buf = '0') and (cb_wr_req_buf = '0') then

        cb_rd_req_buf <= cb_bus_re_i;

        cb_wr_req_buf <= cb_bus_we_i;

      elsif (cb_bus_cancel_i = '1') or -- controller cancels access

            (cb_bus_err = '1') or -- peripheral cancels access

            (cb_bus_ack = '1') then -- normal termination

        cb_rd_req_buf <= '0';

        cb_wr_req_buf <= '0';

      end if;

    end if;

  end process access_buffer;

  -- any current requests? --

  ca_req_current <= (ca_bus_re_i or ca_bus_we_i) when (PORT_CA_READ_ONLY = false) else ca_bus_re_i;

  cb_req_current <= (cb_bus_re_i or cb_bus_we_i) when (PORT_CB_READ_ONLY = false) else cb_bus_re_i;

  -- any buffered requests? --

  ca_req_buffered <= (ca_rd_req_buf or ca_wr_req_buf) when (PORT_CA_READ_ONLY = false) else ca_rd_req_buf;

  cb_req_buffered <= (cb_rd_req_buf or cb_wr_req_buf) when (PORT_CB_READ_ONLY = false) else cb_rd_req_buf;

  -- Access Arbiter Sync --------------------------------------------------------------------

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

  -- for registers that require a specific reset state --

  arbiter_sync: process(rstn_i, clk_i)

  begin

    if (rstn_i = '0') then

      arbiter.state <= IDLE;

    elsif rising_edge(clk_i) then

      arbiter.state <= arbiter.state_nxt;

    end if;

  end process arbiter_sync;

  -- Peripheral Bus Arbiter -----------------------------------------------------------------

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

  arbiter_comb: process(arbiter, ca_req_current, cb_req_current, ca_req_buffered, cb_req_buffered,

                        ca_rd_req_buf, ca_wr_req_buf, cb_rd_req_buf, cb_wr_req_buf,

                        ca_bus_cancel_i, cb_bus_cancel_i, p_bus_ack_i, p_bus_err_i)

  begin

    -- arbiter defaults --

    arbiter.state_nxt <= arbiter.state;

    arbiter.bus_sel   <= '0';

    arbiter.we_trig   <= '0';

    arbiter.re_trig   <= '0';

    -- state machine --

    case arbiter.state is

      when IDLE => -- Controller a has full bus access

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

        if (ca_req_current = '1') then -- current request?

          arbiter.bus_sel   <= '0';

          arbiter.state_nxt <= BUSY;

        elsif (ca_req_buffered = '1') then -- buffered request?

          arbiter.bus_sel   <= '0';

          arbiter.state_nxt <= RETIRE;

        elsif (cb_req_current = '1') then -- current request from controller b?

          arbiter.bus_sel   <= '1';

          arbiter.state_nxt <= BUSY_SWITCHED;

        elsif (cb_req_buffered = '1') then -- buffered request from controller b?

          arbiter.bus_sel   <= '1';

          arbiter.state_nxt <= RETIRE_SWITCHED;

        end if;

      when BUSY => -- transaction in progress

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

        arbiter.bus_sel <= '0';

        if (ca_bus_cancel_i = '1') or -- controller cancels access

           (p_bus_err_i = '1') or -- peripheral cancels access

           (p_bus_ack_i = '1') then -- normal termination

          arbiter.state_nxt <= IDLE;

        end if;

      when RETIRE => -- retire pending access

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

        arbiter.bus_sel <= '0';

        if (PORT_CA_READ_ONLY = false) then

          arbiter.we_trig <= ca_wr_req_buf;

        end if;

        arbiter.re_trig   <= ca_rd_req_buf;

        arbiter.state_nxt <= BUSY;

      when BUSY_SWITCHED => -- switched transaction in progress

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

        arbiter.bus_sel <= '1';

        if (cb_bus_cancel_i = '1') or -- controller cancels access

           (p_bus_err_i = '1') or -- peripheral cancels access

           (p_bus_ack_i = '1') then -- normal termination

          arbiter.state_nxt <= IDLE;

        end if;

      when RETIRE_SWITCHED => -- retire pending switched access

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

        arbiter.bus_sel <= '1';

        if (PORT_CB_READ_ONLY = false) then

          arbiter.we_trig <= cb_wr_req_buf;

        end if;

        arbiter.re_trig   <= cb_rd_req_buf;

        arbiter.state_nxt <= BUSY_SWITCHED;

    end case;

  end process arbiter_comb;

  -- Peripheral Bus Switch ------------------------------------------------------------------

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

  p_bus_addr_o   <= ca_bus_addr_i   when (arbiter.bus_sel = '0') else cb_bus_addr_i;

  p_bus_wdata_o  <= cb_bus_wdata_i  when (PORT_CA_READ_ONLY = true) else ca_bus_wdata_i  when (PORT_CB_READ_ONLY = true) else

                    ca_bus_wdata_i  when (arbiter.bus_sel = '0') else cb_bus_wdata_i;

  p_bus_ben_o    <= cb_bus_ben_i    when (PORT_CA_READ_ONLY = true) else ca_bus_ben_i    when (PORT_CB_READ_ONLY = true) else

                    ca_bus_ben_i    when (arbiter.bus_sel = '0') else cb_bus_ben_i;

  p_bus_we       <= ca_bus_we_i     when (arbiter.bus_sel = '0') else cb_bus_we_i;

  p_bus_re       <= ca_bus_re_i     when (arbiter.bus_sel = '0') else cb_bus_re_i;

  p_bus_cancel_o <= ca_bus_cancel_i when (arbiter.bus_sel = '0') else cb_bus_cancel_i;

  p_bus_we_o     <= (p_bus_we or arbiter.we_trig);

  p_bus_re_o     <= (p_bus_re or arbiter.re_trig);

  ca_bus_rdata_o <= p_bus_rdata_i;

  cb_bus_rdata_o <= p_bus_rdata_i;

  ca_bus_ack     <= p_bus_ack_i and (not arbiter.bus_sel);

  cb_bus_ack     <= p_bus_ack_i and (    arbiter.bus_sel);

  ca_bus_ack_o   <= ca_bus_ack;

  cb_bus_ack_o   <= cb_bus_ack;

  ca_bus_err     <= p_bus_err_i and (not arbiter.bus_sel);

  cb_bus_err     <= p_bus_err_i and (    arbiter.bus_sel);

  ca_bus_err_o   <= ca_bus_err;

  cb_bus_err_o   <= cb_bus_err;

end neorv32_busswitch_rtl;