//////////////////////////////////////////////////////////////////////////////
// SPDX-FileCopyrightText: 2021 , Dinesh Annayya                          
// 
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
//      http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.
// SPDX-License-Identifier: Apache-2.0
// SPDX-FileContributor: Created by Dinesh Annayya <dinesha@opencores.org>
//
//////////////////////////////////////////////////////////////////////
////                                                              ////
////  OMS 8051 cores common library Module                        ////
////                                                              ////
////  This file is part of the OMS 8051 cores project             ////
////  https://github.com/dineshannayya/yifive_r0.git              ////
////  http://www.opencores.org/cores/oms8051mini/                 ////
////                                                              ////
////  Description                                                 ////
////    Async Fifo with threshold tracking/status                 ////
////                                                              ////
////  To Do:                                                      ////
////    nothing                                                   ////
////                                                              ////
////  Author(s):                                                  ////
////      - Dinesh Annayya, dinesha@opencores.org                 ////
////                                                              ////
////  Revision : Nov 26, 2016                                      //// 
////                                                              ////
//////////////////////////////////////////////////////////////////////
////                                                              ////
//// 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:: async FIFO
//    Following two ports are newly added
//        1. At write clock domain:
//           wr_total_free_space -->  Indicate total free transfer available 
//        2. At read clock domain:
//           rd_total_aval       -->  Indicate total no of transfer available
//-----------------------------------------------

`timescale  1ns/1ps
module async_fifo_th (
	           wr_clk,
                   wr_reset_n,
                   wr_en,
                   wr_data,
                   full,                 // sync'ed to wr_clk
                   afull,                 // sync'ed to wr_clk
                   wr_total_free_space,
                   rd_clk,
                   rd_reset_n,
                   rd_en,
                   empty,                // sync'ed to rd_clk
                   aempty,                // sync'ed to rd_clk
                   rd_total_aval,
                   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
   output   [AW:0]   wr_total_free_space; // Total Number of free space aval 
                                               // w.r.t write clk
                                               // note: Without accounting byte enables
   output   [AW:0]   rd_total_aval;       // Total Number of words avaialble 
                                               // w.r.t rd clock, 
                                              // note: Without accounting byte enables
   // 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;

   //--------------------------
   // Shows total number of words 
   // of free space available w.r.t write clock
   //--------------------------- 
   assign wr_total_free_space = FULL_DP - wr_cnt;

   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;
   //--------------------------
   // Shows total number of words 
   // space available w.r.t write clock
   //--------------------------- 
   assign rd_total_aval = rd_cnt;

   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;

    assign rd_data = mem[rd_ptr[AW-1:0]];

    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

// gray code monitor
reg [AW:0] last_gwr_ptr;
always @(posedge wr_clk or negedge wr_reset_n) begin
   if (!wr_reset_n) begin
      last_gwr_ptr <= #1 0;
   end
   else if (last_gwr_ptr !== grey_wr_ptr) begin
      check_ptr_chg(last_gwr_ptr, grey_wr_ptr);
      last_gwr_ptr <= #1 grey_wr_ptr;
   end 	
end

reg [AW:0] last_grd_ptr;
always @(posedge rd_clk or negedge rd_reset_n) begin
   if (!rd_reset_n) begin
     last_grd_ptr <= #1 0;
   end
   else if (last_grd_ptr !== grey_rd_ptr) begin
      check_ptr_chg(last_grd_ptr, grey_rd_ptr);
      last_grd_ptr <= #1 grey_rd_ptr;
   end 	
end

task check_ptr_chg;
input [AW:0] last_ptr;
input [AW:0] cur_ptr;
integer i;
integer ptr_diff;
begin
   ptr_diff = 0;
   for (i=0; i<= AW; i=i+ 1'b1) begin
      if (last_ptr[i] != cur_ptr[i]) begin
         ptr_diff = ptr_diff + 1'b1;
      end
   end
   if (ptr_diff !== 1) begin
      $display($time, "%m, ERROR! async fifo ptr has changed more than noe bit, last=%h, cur=%h",
				last_ptr, cur_ptr);
      $stop;
   end
end
endtask 	
   // synopsys translate_on

endmodule