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/*********************************************************************

  ASYNC FIFO

  This file is part of the sdram controller project

  http://www.opencores.org/cores/sdr_ctrl/

  Description: ASYNC FIFO

  To Do:

    nothing

  Author(s):  Dinesh Annayya, dinesha@opencores.org

 Copyright (C) 2000 Authors and OPENCORES.ORG

 This source file may be used and distributed without

 restriction provided that this copyright statement is not

 removed from the file and that any derivative work contains

 the original copyright notice and the associated disclaimer.

 This source file is free software; you can redistribute it

 and/or modify it under the terms of the GNU Lesser General

 Public License as published by the Free Software Foundation;

 either version 2.1 of the License, or (at your option) any

later version.

 This source is distributed in the hope that it will be

 useful, but WITHOUT ANY WARRANTY; without even the implied

 warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR

 PURPOSE.  See the GNU Lesser General Public License for more

 details.

 You should have received a copy of the GNU Lesser General

 Public License along with this source; if not, download it

 from http://www.opencores.org/lgpl.shtml

*******************************************************************/

//-------------------------------------------

// async FIFO

//-----------------------------------------------

//`timescale  1ns/1ps

module async_fifo (wr_clk,

                   wr_reset_n,

                   wr_en,

                   wr_data,

                   full,                 // sync'ed to wr_clk

                   afull,                 // sync'ed to wr_clk

                   rd_clk,

                   rd_reset_n,

                   rd_en,

                   empty,                // sync'ed to rd_clk

                   aempty,                // sync'ed to rd_clk

                   rd_data);

   parameter W = 4'd8;

   parameter DP = 3'd4;

   parameter WR_FAST = 1'b1;

   parameter RD_FAST = 1'b1;

   parameter FULL_DP = DP;

   parameter EMPTY_DP = 1'b0;

   parameter AW = (DP == 2)   ? 1 :

                  (DP == 4)   ? 2 :

                  (DP == 8)   ? 3 :

                  (DP == 16)  ? 4 :

                  (DP == 32)  ? 5 :

                  (DP == 64)  ? 6 :

                  (DP == 128) ? 7 :

                  (DP == 256) ? 8 : 0;

   output [W-1 : 0]  rd_data;

   input [W-1 : 0]   wr_data;

   input             wr_clk, wr_reset_n, wr_en, rd_clk, rd_reset_n,

                     rd_en;

   output            full, empty;

   output            afull, aempty;       // about full and about to empty

   // synopsys translate_off

   initial begin

      if (AW == 0) begin

         $display ("%m : ERROR!!! Fifo depth %d not in range 2 to 256", DP);

      end // if (AW == 0)

   end // initial begin

   // synopsys translate_on

   reg [W-1 : 0]    mem[DP-1 : 0];

   /*********************** write side ************************/

   reg [AW:0] sync_rd_ptr_0, sync_rd_ptr_1;

   wire [AW:0] sync_rd_ptr;

   reg [AW:0] wr_ptr, grey_wr_ptr;

   reg [AW:0] grey_rd_ptr;

   reg full_q;

   wire full_c;

   wire afull_c;

   wire [AW:0] wr_ptr_inc = wr_ptr + 1'b1;

   wire [AW:0] wr_cnt = get_cnt(wr_ptr, sync_rd_ptr);

   assign full_c  = (wr_cnt == FULL_DP) ? 1'b1 : 1'b0;

   assign afull_c = (wr_cnt == FULL_DP-1) ? 1'b1 : 1'b0;

   always @(posedge wr_clk or negedge wr_reset_n) begin

        if (!wr_reset_n) begin

                wr_ptr <= 0;

                grey_wr_ptr <= 0;

                full_q <= 0;

        end

        else if (wr_en) begin

                wr_ptr <= wr_ptr_inc;

                grey_wr_ptr <= bin2grey(wr_ptr_inc);

                if (wr_cnt == (FULL_DP-1)) begin

                        full_q <= 1'b1;

                end

        end

        else begin

                if (full_q && (wr_cnt<FULL_DP)) begin

                        full_q <= 1'b0;

                end

        end

    end

    assign full  = (WR_FAST == 1) ? full_c : full_q;

    assign afull = afull_c;

    always @(posedge wr_clk) begin

        if (wr_en) begin

                mem[wr_ptr[AW-1:0]] <= wr_data;

        end

    end

    wire [AW:0] grey_rd_ptr_dly ;

    assign #1 grey_rd_ptr_dly = grey_rd_ptr;

    // read pointer synchronizer

    always @(posedge wr_clk or negedge wr_reset_n) begin

        if (!wr_reset_n) begin

                sync_rd_ptr_0 <= 0;

                sync_rd_ptr_1 <= 0;

        end

        else begin

                sync_rd_ptr_0 <= grey_rd_ptr_dly;

                sync_rd_ptr_1 <= sync_rd_ptr_0;

        end

    end

    assign sync_rd_ptr = grey2bin(sync_rd_ptr_1);

   /************************ read side *****************************/

   reg [AW:0] sync_wr_ptr_0, sync_wr_ptr_1;

   wire [AW:0] sync_wr_ptr;

   reg [AW:0] rd_ptr;

   reg empty_q;

   wire empty_c;

   wire aempty_c;

   wire [AW:0] rd_ptr_inc = rd_ptr + 1'b1;

   wire [AW:0] sync_wr_ptr_dec = sync_wr_ptr - 1'b1;

   wire [AW:0] rd_cnt = get_cnt(sync_wr_ptr, rd_ptr);

   assign empty_c  = (rd_cnt == 0) ? 1'b1 : 1'b0;

   assign aempty_c = (rd_cnt == 1) ? 1'b1 : 1'b0;

   always @(posedge rd_clk or negedge rd_reset_n) begin

      if (!rd_reset_n) begin

         rd_ptr <= 0;

         grey_rd_ptr <= 0;

         empty_q <= 1'b1;

      end

      else begin

         if (rd_en) begin

            rd_ptr <= rd_ptr_inc;

            grey_rd_ptr <= bin2grey(rd_ptr_inc);

            if (rd_cnt==(EMPTY_DP+1)) begin

               empty_q <= 1'b1;

            end

         end

         else begin

            if (empty_q && (rd_cnt!=EMPTY_DP)) begin

              empty_q <= 1'b0;

            end

         end

       end

    end

    assign empty  = (RD_FAST == 1) ? empty_c : empty_q;

    assign aempty = aempty_c;

    reg [W-1 : 0]  rd_data_q;

   wire [W-1 : 0] rd_data_c = mem[rd_ptr[AW-1:0]];

   always @(posedge rd_clk) begin

        rd_data_q <= rd_data_c;

   end

   assign rd_data  = (RD_FAST == 1) ? rd_data_c : rd_data_q;

    wire [AW:0] grey_wr_ptr_dly ;

    assign #1 grey_wr_ptr_dly =  grey_wr_ptr;

    // write pointer synchronizer

    always @(posedge rd_clk or negedge rd_reset_n) begin

        if (!rd_reset_n) begin

           sync_wr_ptr_0 <= 0;

           sync_wr_ptr_1 <= 0;

        end

        else begin

           sync_wr_ptr_0 <= grey_wr_ptr_dly;

           sync_wr_ptr_1 <= sync_wr_ptr_0;

        end

    end

    assign sync_wr_ptr = grey2bin(sync_wr_ptr_1);

/************************ functions ******************************/

function [AW:0] bin2grey;

input [AW:0] bin;

reg [8:0] bin_8;

reg [8:0] grey_8;

begin

        bin_8 = bin;

        grey_8[1:0] = do_grey(bin_8[2:0]);

        grey_8[3:2] = do_grey(bin_8[4:2]);

        grey_8[5:4] = do_grey(bin_8[6:4]);

        grey_8[7:6] = do_grey(bin_8[8:6]);

        grey_8[8] = bin_8[8];

        bin2grey = grey_8;

end

endfunction

function [AW:0] grey2bin;

input [AW:0] grey;

reg [8:0] grey_8;

reg [8:0] bin_8;

begin

        grey_8 = grey;

        bin_8[8] = grey_8[8];

        bin_8[7:6] = do_bin({bin_8[8], grey_8[7:6]});

        bin_8[5:4] = do_bin({bin_8[6], grey_8[5:4]});

        bin_8[3:2] = do_bin({bin_8[4], grey_8[3:2]});

        bin_8[1:0] = do_bin({bin_8[2], grey_8[1:0]});

        grey2bin = bin_8;

end

endfunction

function [1:0] do_grey;

input [2:0] bin;

begin

        if (bin[2]) begin  // do reverse grey

                case (bin[1:0])

                        2'b00: do_grey = 2'b10;

                        2'b01: do_grey = 2'b11;

                        2'b10: do_grey = 2'b01;

                        2'b11: do_grey = 2'b00;

                endcase

        end

        else begin

                case (bin[1:0])

                        2'b00: do_grey = 2'b00;

                        2'b01: do_grey = 2'b01;

                        2'b10: do_grey = 2'b11;

                        2'b11: do_grey = 2'b10;

                endcase

        end

end

endfunction

function [1:0] do_bin;

input [2:0] grey;

begin

        if (grey[2]) begin      // actually bin[2]

                case (grey[1:0])

                        2'b10: do_bin = 2'b00;

                        2'b11: do_bin = 2'b01;

                        2'b01: do_bin = 2'b10;

                        2'b00: do_bin = 2'b11;

                endcase

        end

        else begin

                case (grey[1:0])

                        2'b00: do_bin = 2'b00;

                        2'b01: do_bin = 2'b01;

                        2'b11: do_bin = 2'b10;

                        2'b10: do_bin = 2'b11;

                endcase

        end

end

endfunction

function [AW:0] get_cnt;

input [AW:0] wr_ptr, rd_ptr;

begin

        if (wr_ptr >= rd_ptr) begin

                get_cnt = (wr_ptr - rd_ptr);

        end

        else begin

                get_cnt = DP*2 - (rd_ptr - wr_ptr);

        end

end

endfunction

// synopsys translate_off

always @(posedge wr_clk) begin

   if (wr_en && full) begin

      $display($time, "%m Error! afifo overflow!");

      $stop;

   end

end

always @(posedge rd_clk) begin

   if (rd_en && empty) begin

      $display($time, "%m error! afifo underflow!");

      $stop;

   end

end

// synopsys translate_on

endmodule