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

-- # << NEORV32 - External Bus Interface (WISHBONE) >>                                             #

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

-- # The interface provides registers for all outgoing and for all incoming signals. If the host   #

-- # cancels an activetransfer, the Wishbone arbiter still waits some time for the bus system to   #

-- # ACK/ERR the transfer before the arbiter forces termination.                                   #

-- #                                                                                               #

-- # Even when all processor-internal memories and IO devices are disabled, the EXTERNAL address   #

-- # space ENDS at address 0xffff0000 (begin of internal BOOTROM address space).                   #

-- #                                                                                               #

-- # All bus accesses from the CPU, which do not target the internal IO region / the internal      #

-- # bootlloader / the internal instruction or data memories (if implemented), are delegated via   #

-- # this Wishbone gateway to the external bus interface. Accessed peripherals can have a response #

-- # latency of up to BUS_TIMEOUT - 2 cycles.                                                      #

-- #                                                                                               #

-- # This interface supports classic/standard Wishbone transactions (WB_PIPELINED_MODE = false)    #

-- # and also pipelined transactions (WB_PIPELINED_MODE = true).                                   #

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

-- # BSD 3-Clause License                                                                          #

-- #                                                                                               #

-- # Copyright (c) 2021, 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 #

-- # GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED    #

-- # AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING     #

-- # NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED  #

-- # 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_wishbone is

  generic (

    WB_PIPELINED_MODE : boolean := false;  -- false: classic/standard wishbone mode, true: pipelined wishbone mode

    -- Internal instruction memory --

    MEM_INT_IMEM_EN   : boolean := true;   -- implement processor-internal instruction memory

    MEM_INT_IMEM_SIZE : natural := 8*1024; -- size of processor-internal instruction memory in bytes

    -- Internal data memory --

    MEM_INT_DMEM_EN   : boolean := true;   -- implement processor-internal data memory

    MEM_INT_DMEM_SIZE : natural := 4*1024; -- size of processor-internal data memory in bytes

    -- Bus Timeout --

    BUS_TIMEOUT       : natural := 63      -- cycles after an UNACKNOWLEDGED bus access triggers a bus fault exception

  );

  port (

    -- global control --

    clk_i    : in  std_ulogic; -- global clock line

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

    -- host access --

    src_i    : in  std_ulogic; -- access type (0: data, 1:instruction)

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

    rden_i   : in  std_ulogic; -- read enable

    wren_i   : in  std_ulogic; -- write enable

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

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

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

    cancel_i : in  std_ulogic; -- cancel current bus transaction

    excl_i   : in  std_ulogic; -- exclusive access request

    excl_o   : out std_ulogic; -- state of exclusiv access (set if failed)

    ack_o    : out std_ulogic; -- transfer acknowledge

    err_o    : out std_ulogic; -- transfer error

    priv_i   : in  std_ulogic_vector(01 downto 0); -- current CPU privilege level

    -- wishbone interface --

    wb_tag_o : out std_ulogic_vector(03 downto 0); -- request tag

    wb_adr_o : out std_ulogic_vector(31 downto 0); -- address

    wb_dat_i : in  std_ulogic_vector(31 downto 0); -- read data

    wb_dat_o : out std_ulogic_vector(31 downto 0); -- write data

    wb_we_o  : out std_ulogic; -- read/write

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

    wb_stb_o : out std_ulogic; -- strobe

    wb_cyc_o : out std_ulogic; -- valid cycle

    wb_tag_i : in  std_ulogic; -- response tag

    wb_ack_i : in  std_ulogic; -- transfer acknowledge

    wb_err_i : in  std_ulogic  -- transfer error

  );

end neorv32_wishbone;

architecture neorv32_wishbone_rtl of neorv32_wishbone is

  -- constants --

  constant xbus_timeout_c : natural := BUS_TIMEOUT/4;

  -- access control --

  signal int_imem_acc : std_ulogic;

  signal int_dmem_acc : std_ulogic;

  signal int_boot_acc : std_ulogic;

  signal xbus_access  : std_ulogic;

  -- bus arbiter

  type ctrl_state_t is (IDLE, BUSY, CANCELED, RESYNC);

  type ctrl_t is record

    state   : ctrl_state_t;

    we      : std_ulogic;

    rd_req  : std_ulogic;

    wr_req  : std_ulogic;

    adr     : std_ulogic_vector(31 downto 0);

    wdat    : std_ulogic_vector(31 downto 0);

    rdat    : std_ulogic_vector(31 downto 0);

    sel     : std_ulogic_vector(3 downto 0);

    ack     : std_ulogic;

    err     : std_ulogic;

    timeout : std_ulogic_vector(index_size_f(xbus_timeout_c)-1 downto 0);

    src     : std_ulogic;

    excl    : std_ulogic;

    exclr   : std_ulogic; -- response

    priv    : std_ulogic_vector(1 downto 0);

  end record;

  signal ctrl    : ctrl_t;

  signal stb_int : std_ulogic;

  signal cyc_int : std_ulogic;

begin

  -- Sanity Checks --------------------------------------------------------------------------

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

  -- max bus timeout latency lower than recommended --

  assert not (BUS_TIMEOUT <= 32) report "NEORV32 PROCESSOR CONFIG WARNING: Bus timeout should be >32 when using external bus interface." severity warning;

  -- external memory iterface protocol + max timeout latency notifier (warning) --

  assert not (wb_pipe_mode_c = false) report "NEORV32 PROCESSOR CONFIG NOTE: Implementing external memory interface using STANDARD Wishbone protocol." severity note;

  assert not (wb_pipe_mode_c = true) report "NEORV32 PROCESSOR CONFIG NOTE! Implementing external memory interface using PIEPLINED Wishbone protocol." severity note;

  -- endianness --

  assert not (xbus_big_endian_c = false) report "NEORV32 PROCESSOR CONFIG NOTE: Using LITTLE-ENDIAN byte order for external memory interface." severity note;

  assert not (xbus_big_endian_c = true)  report "NEORV32 PROCESSOR CONFIG NOTE: Using BIG-ENDIAN byte order for external memory interface." severity note;

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

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

  -- access to processor-internal IMEM or DMEM? --

  int_imem_acc <= '1' when (addr_i(31 downto index_size_f(MEM_INT_IMEM_SIZE)) = imem_base_c(31 downto index_size_f(MEM_INT_IMEM_SIZE))) and (MEM_INT_IMEM_EN = true) else '0';

  int_dmem_acc <= '1' when (addr_i(31 downto index_size_f(MEM_INT_DMEM_SIZE)) = dmem_base_c(31 downto index_size_f(MEM_INT_DMEM_SIZE))) and (MEM_INT_DMEM_EN = true) else '0';

  -- access to processor-internal BOOTROM or IO devices? --

  int_boot_acc <= '1' when (addr_i(31 downto 16) = boot_rom_base_c(31 downto 16)) else '0'; -- hacky!

  -- actual external bus access? --

  xbus_access <= (not int_imem_acc) and (not int_dmem_acc) and (not int_boot_acc);

  -- Bus Arbiter -----------------------------------------------------------------------------

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

  bus_arbiter: process(rstn_i, clk_i)

  begin

    if (rstn_i = '0') then

      ctrl.state   <= IDLE;

      ctrl.we      <= '0';

      ctrl.rd_req  <= '0';

      ctrl.wr_req  <= '0';

      ctrl.adr     <= (others => '0');

      ctrl.wdat    <= (others => '0');

      ctrl.rdat    <= (others => '0');

      ctrl.sel     <= (others => '0');

      ctrl.timeout <= (others => '0');

      ctrl.ack     <= '0';

      ctrl.err     <= '0';

      ctrl.src     <= '0';

      ctrl.excl    <= '0';

      ctrl.exclr   <= '0';

      ctrl.priv    <= "00";

    elsif rising_edge(clk_i) then

      -- defaults --

      ctrl.rdat    <= (others => '0');

      ctrl.ack     <= '0';

      ctrl.err     <= '0';

      ctrl.exclr   <= '0';

      ctrl.timeout <= std_ulogic_vector(to_unsigned(xbus_timeout_c, index_size_f(xbus_timeout_c)));

      -- state machine --

      case ctrl.state is

        when IDLE => -- waiting for host request

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

          ctrl.rd_req <= '0';

          ctrl.wr_req <= '0';

          -- buffer all outgoing signals --

          ctrl.we   <= wren_i;

          ctrl.adr  <= addr_i;

          if (xbus_big_endian_c = true) then -- endianness conversion

            ctrl.wdat <= data_i;

            ctrl.sel  <= ben_i;

          else

            ctrl.wdat <= bswap32_f(data_i);

            ctrl.sel  <= bit_rev_f(ben_i);

          end if;

          ctrl.src  <= src_i;

          ctrl.excl <= excl_i;

          ctrl.priv <= priv_i;

          -- valid new or buffered read/write request --

          if ((xbus_access and (wren_i or ctrl.wr_req or rden_i or ctrl.rd_req)) = '1') then

            ctrl.state <= BUSY;

          end if;

        when BUSY => -- transfer in progress

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

          ctrl.rdat  <= wb_dat_i;

          ctrl.exclr <= wb_tag_i; -- set if exclusive access success

          if (cancel_i = '1') then -- transfer canceled by host

            ctrl.state <= CANCELED;

          elsif (wb_err_i = '1') then -- abnormal bus termination

            ctrl.err   <= '1';

            ctrl.state <= CANCELED;

          elsif (wb_ack_i = '1') then -- normal bus termination

            ctrl.ack   <= '1';

            ctrl.state <= IDLE;

          end if;

        when CANCELED => -- wait for cycle to be completed either by peripheral or by timeout (ignore result of transfer)

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

          ctrl.wr_req <= ctrl.wr_req or wren_i; -- buffer new request

          ctrl.rd_req <= ctrl.rd_req or rden_i; -- buffer new request

          -- wait for bus.peripheral to ACK transfer (as "aborted" but still somehow "completed")

          -- or wait for a timeout and force termination

          ctrl.timeout <= std_ulogic_vector(unsigned(ctrl.timeout) - 1); -- timeout counter

          if (wb_ack_i = '1') or (or_all_f(ctrl.timeout) = '0') then

            ctrl.state <= RESYNC;

          end if;

        when RESYNC => -- make sure transfer is done!

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

          if (wb_ack_i = '0') then

            ctrl.state <= IDLE;

          end if;

        when others => -- undefined

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

          ctrl.state <= IDLE;

      end case;

    end if;

  end process bus_arbiter;

  -- host access --

  data_o <= ctrl.rdat when (xbus_big_endian_c = true) else bswap32_f(ctrl.rdat); -- endianness conversion

  ack_o  <= ctrl.ack;

  err_o  <= ctrl.err;

  excl_o <= ctrl.exclr;

  -- wishbone interface --

  wb_tag_o(0) <= '1' when (ctrl.priv = priv_mode_m_c) else '0'; -- privileged access when in machine mode

  wb_tag_o(1) <= '0'; -- 0 = secure, 1 = non-secure

  wb_tag_o(2) <= ctrl.src; -- 0 = data access, 1 = instruction access

  wb_tag_o(3) <= ctrl.excl; -- 1 = exclusive access request

  wb_adr_o  <= ctrl.adr;

  wb_dat_o  <= ctrl.wdat;

  wb_we_o   <= ctrl.we;

  wb_sel_o  <= ctrl.sel;

  wb_stb_o  <= stb_int when (WB_PIPELINED_MODE = true) else cyc_int;

  wb_cyc_o  <= cyc_int;

  stb_int <= '1' when (ctrl.state = BUSY) else '0';

  cyc_int <= '0' when (ctrl.state = IDLE) or (ctrl.state = RESYNC) else '1';

end neorv32_wishbone_rtl;