library ieee;
use ieee.std_logic_1164.all;
use ieee.numeric_std.all;
use work.wishbone_package.all;
use work.fifo_package.all;

entity clock_crossing is
   generic(
      sync_depth : natural := 2;
      log2fifo   : natural := 4);
   port(
      -- Common wishbone signals
      rst        : in  std_logic;
      -- Slave control port
      slave_clk  : in  std_logic;
      slave_i    : in  wishbone_slave_in;
      slave_o    : out wishbone_slave_out;
      -- Master reader port
      master_clk : in  std_logic;
      master_i   : in  wishbone_master_in;
      master_o   : out wishbone_master_out);
end clock_crossing;

architecture rtl of clock_crossing is
   constant mWE_start  : natural := 0;
   constant mWE_end    : natural := mWE_start;
   constant mADR_start : natural := mWE_end + 1;
   constant mADR_end   : natural := mADR_start + wishbone_address_width - 1;
   constant mDAT_start : natural := mADR_end + 1;
   constant mDAT_end   : natural := mDAT_start + wishbone_data_width - 1;
   constant mSEL_start : natural := mDAT_end + 1;
   constant mSEL_end   : natural := mSEL_start + (wishbone_data_width/8) - 1;
   constant mlen       : natural := mSEL_end + 1;
   
   signal msend, mrecv : wishbone_master_out;
   signal msend_vect, mrecv_vect : std_logic_vector(mlen-1 downto 0);
   signal mw_rdy, mw_en, mr_rdy, mr_en : std_logic;
   
   constant sACK_start : natural := 0;
   constant sACK_end   : natural := sACK_start;
   constant sRTY_start : natural := sACK_end + 1;
   constant sRTY_end   : natural := sRTY_start;
   constant sERR_start : natural := sRTY_end + 1;
   constant sERR_end   : natural := sERR_start;
   constant sDAT_start : natural := sERR_end + 1;
   constant sDAT_end   : natural := sDAT_start + wishbone_data_width - 1;
   constant slen       : natural := sDAT_end + 1;
   
   signal ssend, srecv : wishbone_slave_out;
   signal ssend_vect, srecv_vect : std_logic_vector(slen-1 downto 0);
   signal sw_rdy, sw_en, sr_rdy, sr_en, sa_rdy, sa_en : std_logic;
   
   signal master_o_STB : std_logic;
   signal slave_o_PUSH : std_logic;
begin
   mfifo : fifo
      generic map(addr_width => log2fifo, data_width => mlen, sync_depth => sync_depth, gray_code => true)
      port map(w_clk => slave_clk,  w_rdy => mw_rdy, w_en => mw_en, w_data => msend_vect,
               r_clk => master_clk, r_rdy => mr_rdy, r_en => mr_en, r_data => mrecv_vect,
               a_clk => '0', a_rdy => open, a_en => '0', rst => rst);

   msend_vect(mWE_start) <= msend.WE;
   msend_vect(mADR_end downto mADR_start) <= msend.ADR;
   msend_vect(mDAT_end downto mDAT_start) <= msend.DAT;
   msend_vect(mSEL_end downto mSEL_start) <= msend.SEL;
   
   mrecv.WE <= mrecv_vect(mWE_start);
   mrecv.ADR <= mrecv_vect(mADR_end downto mADR_start);
   mrecv.DAT <= mrecv_vect(mDAT_end downto mDAT_start);
   mrecv.SEL <= mrecv_vect(mSEL_end downto mSEL_start);
   
   sfifo : fifo
      generic map(addr_width => log2fifo, data_width => slen, sync_depth => sync_depth, gray_code => true)
      port map(w_clk => master_clk, w_rdy => open,   w_en => sw_en, w_data => ssend_vect,
               r_clk => slave_clk,  r_rdy => sr_rdy, r_en => sr_en, r_data => srecv_vect,
               a_clk => slave_clk,  a_rdy => sa_rdy, a_en => sa_en, rst => rst);
   
   ssend_vect(sACK_start) <= ssend.ACK;
   ssend_vect(sRTY_start) <= ssend.RTY;
   ssend_vect(sERR_start) <= ssend.ERR;
   ssend_vect(sDAT_end downto sDAT_start) <= ssend.DAT;
   
   srecv.ACK <= srecv_vect(sACK_start);
   srecv.RTY <= srecv_vect(sRTY_start);
   srecv.ERR <= srecv_vect(sERR_start);
   srecv.DAT <= srecv_vect(sDAT_end downto sDAT_start);

   -- Slave clock domain: slave -> mFIFO
   mw_en <= mw_rdy and sa_rdy and slave_i.CYC and slave_i.STB;
   sa_en <= mw_rdy and sa_rdy and slave_i.CYC and slave_i.STB;
   slave_o.STALL <= not mw_rdy or not sa_rdy;
   msend.ADR <= slave_i.ADR;
   msend.WE  <= slave_i.WE;
   msend.SEL <= slave_i.SEL;
   msend.DAT <= slave_i.DAT;
   
   -- Master clock domain: mFIFO -> master
   mr_en <= mr_rdy and (not master_o_STB or not master_i.STALL);
   master_o.CYC <= '1';
   master_o.STB <= master_o_STB;
   master_o.ADR <= mrecv.ADR;
   master_o.WE  <= mrecv.WE;
   master_o.SEL <= mrecv.SEL;
   master_o.DAT <= mrecv.DAT;
   
   drive_master_port : process(master_clk)
   begin
      if rising_edge(master_clk) then
         if rst = '1' then
            master_o_STB <= '0';
         else
            master_o_STB <= mr_en or (master_o_STB and master_i.STALL);
         end if;
      end if;
   end process;
   
   -- Master clock domain: master -> sFIFO
   sw_en <= master_i.ACK or master_i.ERR or master_i.RTY;
   ssend.ACK <= master_i.ACK;
   ssend.ERR <= master_i.ERR;
   ssend.RTY <= master_i.RTY;
   ssend.DAT <= master_i.DAT;
   
   -- Slave clock domain: sFIFO -> slave
   sr_en <= sr_rdy;
   slave_o.DAT <= srecv.DAT;
   slave_o.ACK <= srecv.ACK and slave_o_PUSH;
   slave_o.RTY <= srecv.RTY and slave_o_PUSH;
   slave_o.ERR <= srecv.ERR and slave_o_PUSH;
   
   drive_slave_port : process(slave_clk)
   begin
      if rising_edge(slave_clk) then
         if rst = '1' then
            slave_o_PUSH <= '0';
         else
            slave_o_PUSH <= sr_en;
         end if;
      end if;
   end process;
end rtl;