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

-- Title      : axil2wb

-- Project    : 

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

-- File       : axil2wb.vhd

-- Author     : Wojciech M. Zabolotny  <wzab@ise.pw.edu.pl>

-- Company    : Institute of Electronic Systems, Warsaw University of Technology

-- Created    : 2016-04-24

-- Last update: 2016-05-15

-- License    : This is a PUBLIC DOMAIN code, published under

--              Creative Commons CC0 license

-- Platform   : 

-- Standard   : VHDL'93/02

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

-- Description: AXI Lite -> WB bridge

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

-- Copyright (c) 2016 

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

-- Revisions  :

-- Date        Version  Author  Description

-- 2016-05-13  1.0      WZab    Created

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

-- The AXI implementation is based on the description of AXI provided by

-- Rich Griffin in "Designing a Custom AXI-lite Slave Peripheral"

-- available at:

-- silica.com/wcsstore/Silica/Silica+Content+Library/Silica+Home/resources/71b10b18-9c9c-44c6-b62d-9a031b8f3df8/SILICA_Xilinx_Designing_a_custom_axi_slave_rev1.pdf

--

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

-- Implementation details

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

-- In the AXI bus the read and write accesses may be handled independently

-- In the IPbus they can't therefore we must provide an arbitration scheme.

-- We assume "Write before read"

-- 

-- We must avoid duplicated writes and reads (which may corruppt e.g.

-- FIFO slaves at IPbus!)

--

-- Additionally the IPbus uses the word adressing, while AXI uses the byte

-- addressing. That is handled by the function a_axi2ipb, which additionally

-- zeroes bits not used by the IPbus segment...

library IEEE;

use IEEE.STD_LOGIC_1164.all;

library work;

entity axil2wb is

  generic (

    ADRWIDTH  : integer := 15;

    DATAWIDTH : integer := 32);

  port (

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

    -- AXI Interface

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

    -- Clock and Reset

    S_AXI_ACLK    : in  std_logic;

    S_AXI_ARESETN : in  std_logic;

    -- Write Address Channel

    S_AXI_AWADDR  : in  std_logic_vector(ADRWIDTH-1 downto 0);

    S_AXI_AWVALID : in  std_logic;

    S_AXI_AWREADY : out std_logic;

    -- Write Data Channel

    S_AXI_WDATA   : in  std_logic_vector(31 downto 0);

    S_AXI_WSTRB   : in  std_logic_vector(3 downto 0);

    S_AXI_WVALID  : in  std_logic;

    S_AXI_WREADY  : out std_logic;

    -- Read Address Channel

    S_AXI_ARADDR  : in  std_logic_vector(ADRWIDTH-1 downto 0);

    S_AXI_ARVALID : in  std_logic;

    S_AXI_ARREADY : out std_logic;

    -- Read Data Channel

    S_AXI_RDATA   : out std_logic_vector(31 downto 0);

    S_AXI_RRESP   : out std_logic_vector(1 downto 0);

    S_AXI_RVALID  : out std_logic;

    S_AXI_RREADY  : in  std_logic;

    -- Write Response Channel

    S_AXI_BRESP   : out std_logic_vector(1 downto 0);

    S_AXI_BVALID  : out std_logic;

    S_AXI_BREADY  : in  std_logic;

    -- AWPROT and ARPROT - required by Altera

    --S_AXI_ARPROT  : in  std_logic_vector(2 downto 0);

    --S_AXI_AWPROT  : in  std_logic_vector(2 downto 0);

    -- Here we have the WB ports

    -- The clock and reset are comming from AXI!

    wb_clk_o      : out std_logic;

    wb_rst_o      : out std_logic;

    -- master_ipb_out - flattened due to Vivado inability to handle user types

    -- in BD

    wb_addr_o     : out std_logic_vector(31 downto 0);

    wb_dat_o      : out std_logic_vector(31 downto 0);

    wb_we_o       : out std_logic;

    wb_sel_o      : out std_logic_vector(3 downto 0);

    wb_stb_o      : out std_logic;

    wb_cyc_o      : out std_logic;

    -- master_ipb_in -  flattened due to Vivado inability to handle user types

    -- in BD

    wb_dat_i      : in  std_logic_vector(31 downto 0);

    wb_err_i      : in  std_logic;  -- Not used in figure 1-2 in specification!

    wb_ack_i      : in  std_logic

    );

end entity axil2wb;

architecture beh of axil2wb is

  function a_axi2wb (

    constant axi_addr : std_logic_vector(ADRWIDTH-1 downto 0))

    return std_logic_vector is

    variable wb_addr : std_logic_vector(31 downto 0);

  begin  -- function a_axi2wb

    wb_addr                      := (others => '0');

    -- Divide the address by 4 (we use word addresses, not the byte addresses)

    wb_addr(ADRWIDTH-3 downto 0) := axi_addr(ADRWIDTH-1 downto 2);

    return wb_addr;

  end function a_axi2wb;

  signal read_wait, read_wait_in, write_wait, write_wait_in : boolean                                := false;

  signal rdata, rdata_in, addr, addr_in, wdata, wdata_in    : std_logic_vector(DATAWIDTH-1 downto 0) := (others => '0');

  signal bresp, rresp, bresp_in, rresp_in                   : std_logic_vector(1 downto 0)           := "00";

  signal del_bresp, del_rresp, del_bresp_in, del_rresp_in   : boolean                                := false;

begin  -- architecture beh

  wb_clk_o <= S_AXI_ACLK;

  wb_rst_o <= S_AXI_ARESETN;

  wb_sel_o <= (others => '1');          -- We support only whole word accesses

  qq : process (S_AXI_ARADDR, S_AXI_ARVALID, S_AXI_AWADDR, S_AXI_AWVALID,

                S_AXI_BREADY, S_AXI_RREADY, S_AXI_WDATA, S_AXI_WSTRB,

                S_AXI_WVALID, addr, bresp, del_bresp, del_rresp, rdata,

                read_wait, rresp, wb_ack_i, wb_dat_i, wb_err_i, wdata,

                write_wait) is

    variable is_read, is_write : boolean := false;

  begin  -- process qq

    -- Defaults

    is_read       := false;

    is_write      := false;

    wb_stb_o      <= '0';

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

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

    wb_we_o       <= '0';

    wb_cyc_o      <= '0';

    -- Flags handling delayed acceptance of results

    del_bresp_in  <= del_bresp;

    del_rresp_in  <= del_rresp;

    -- Registers storing the results

    bresp_in      <= bresp;

    rresp_in      <= rresp;

    rdata_in      <= rdata;

    wdata_in      <= wdata;

    read_wait_in  <= read_wait;

    write_wait_in <= write_wait;

    addr_in       <= addr;

    S_AXI_BVALID  <= '0';

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

    S_AXI_ARREADY <= '0';

    S_AXI_RVALID  <= '0';

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

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

    S_AXI_AWREADY <= '0';

    S_AXI_WREADY  <= '0';

    -- Real processing

    -- Handling of delayed responses

    if del_bresp then

      S_AXI_BRESP  <= bresp;

      S_AXI_BVALID <= '1';

      if S_AXI_BREADY = '1' then

        del_bresp_in <= false;

      end if;

    elsif del_rresp then

      S_AXI_RRESP  <= rresp;

      S_AXI_RDATA  <= rdata;

      S_AXI_RVALID <= '1';

      if S_AXI_RREADY = '1' then

        del_rresp_in <= false;

      end if;

    -- Handling of new transactions

    elsif (S_AXI_AWVALID = '1' and S_AXI_WVALID = '1') or write_wait then

      is_write := true;

    elsif S_AXI_ARVALID = '1' or read_wait then

      is_read := true;

    end if;

    -- Set the IPbus signals

    if is_write then

      -- Check if this is a new transmission

      if S_AXI_AWVALID = '1' and S_AXI_WVALID = '1' and write_wait = false then

        -- This is a new transmission

        -- Check if this is a correct 32-bit write

        if S_AXI_WSTRB /= "1111" then

          -- Erroneouos write. If slave is ready to accept status, inform about it

          S_AXI_AWREADY <= '1';

          S_AXI_WREADY  <= '1';

          S_AXI_BRESP   <= "10";

          S_AXI_BVALID  <= '1';

          if S_AXI_BREADY = '0' then

            -- Prepare delayed response

            bresp_in     <= "10";

            del_bresp_in <= true;

          end if;

        else

          -- Correct write

          -- Write transaction on IPbus

          wb_addr_o     <= a_axi2wb(S_AXI_AWADDR);

          wb_dat_o      <= S_AXI_WDATA;

          wb_stb_o      <= '1';

          wb_cyc_o      <= '1';

          wb_we_o       <= '1';

          -- Store data for the next cycles

          addr_in       <= a_axi2wb(S_AXI_AWADDR);

          wdata_in      <= S_AXI_WDATA;

          S_AXI_AWREADY <= '1';

          S_AXI_WREADY  <= '1';

          write_wait_in <= true;

        end if;

      else

        -- This the next cycle of the write transmission

        wb_addr_o <= addr;

        wb_dat_o  <= wdata;

        wb_stb_o  <= '1';

        wb_cyc_o  <= '1';

        wb_we_o   <= '1';

      end if;

      -- Check the slave response

      if wb_err_i = '1' then

        write_wait_in <= false;

        S_AXI_BRESP   <= "10";

        S_AXI_BVALID  <= '1';

        if S_AXI_BREADY = '0' then

          -- Prepare delayed response

          bresp_in     <= "10";

          del_bresp_in <= true;

        end if;

      elsif wb_ack_i = '1' then

        write_wait_in <= false;

        S_AXI_BRESP   <= "00";

        S_AXI_BVALID  <= '1';

        if S_AXI_BREADY = '0' then

          -- Prepare delayed response

          bresp_in     <= "00";

          del_bresp_in <= true;

        end if;

      end if;

    elsif is_read then

      -- Read transaction on IPbus

      if S_AXI_ARVALID = '1' and read_wait = false then

        addr_in       <= a_axi2wb(S_AXI_ARADDR);

        wb_addr_o     <= a_axi2wb(S_AXI_ARADDR);

        S_AXI_ARREADY <= '1';

        -- Remember that we are in read

        read_wait_in  <= true;

      else

        wb_addr_o <= addr;

      end if;

      wb_stb_o <= '1';

      wb_cyc_o <= '1';

      wb_we_o  <= '0';

      -- Check the slave response

      if wb_err_i = '1' then

        S_AXI_RRESP  <= "10";

        S_AXI_RDATA  <= wb_dat_i;

        S_AXI_RVALID <= '1';

        read_wait_in <= false;

        if S_AXI_RREADY = '0' then

          -- Prepare delayed response

          rresp_in     <= "10";

          rdata_in     <= wb_dat_i;

          del_rresp_in <= true;

        end if;

      elsif wb_ack_i = '1' then

        S_AXI_RRESP  <= "00";

        S_AXI_RDATA  <= wb_dat_i;

        S_AXI_RVALID <= '1';

        read_wait_in <= false;

        if S_AXI_RREADY = '0' then

          -- Prepare delayed response

          rresp_in     <= "00";

          rdata_in     <= wb_dat_i;

          del_rresp_in <= true;

        end if;

      end if;

    end if;

  end process qq;

  process (S_AXI_ACLK) is

  begin  -- process

    if S_AXI_ACLK'event and S_AXI_ACLK = '1' then  -- rising clock edge

      if S_AXI_ARESETN = '0' then       -- synchronous reset (active low)

        del_rresp  <= false;

        del_bresp  <= false;

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

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

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

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

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

        read_wait  <= false;

        write_wait <= false;

      else

        del_rresp  <= del_rresp_in;

        del_bresp  <= del_bresp_in;

        addr       <= addr_in;

        rdata      <= rdata_in;

        wdata      <= wdata_in;

        rresp      <= rresp_in;

        bresp      <= bresp_in;

        read_wait  <= read_wait_in;

        write_wait <= write_wait_in;

      end if;

    end if;

  end process;

end architecture beh;