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//////////////////////////////////////////////////////////////////////

////                                                              ////

////  OMS 8051 Digital core Module                                ////

////                                                              ////

////  This file is part of the OMS 8051 cores project             ////

////  http://www.opencores.org/cores/oms8051mini/                 ////

////                                                              ////

////  Description                                                 ////

////   64K * 8 external data ram                                  ////

////                                                              ////

////  To Do:                                                      ////

////   nothing                                                    ////

////                                                              ////

////  Author(s):                                                  ////

////      - Simon Teran, simont@opencores.org                     ////

////      - Dinesh Annayya, dinesha@opencores.org                 ////

////  Revision : Nov 26, 2016                                     //// 

//////////////////////////////////////////////////////////////////////

//     v0.0 - Dinesh A, 8th Dec 2016, 

//          1. converted to 8bit RAM Mode

////                                                              ////

//////////////////////////////////////////////////////////////////////

////                                                              ////

//// 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                     ////

////                                                              ////

//////////////////////////////////////////////////////////////////////

module oc8051_xram (clk, rst, wr, addr, data_in, data_out, ack, stb);

//

// external data ram for simulation. part of oc8051_tb

// it's tehnology dependent

//

// clk          (in)  clock

// addr         (in)  addres

// data_in      (out) data input

// data_out     (in)  data output

// wr           (in)  write

// ack          (out) acknowlage

// stb          (in)  strobe

//

parameter DELAY=1;

input clk, wr, stb, rst;

input [7:0] data_in;

input [15:0] addr;

output [7:0] data_out;

output ack;

reg ackw, ackr;

reg [7:0] data_out;

reg [2:0] cnt;

integer i;

//

// buffer

reg [7:0] buff [65535:0];  //64kb

//reg [7:0] buff [8388607:0];  //8Mb

assign ack =  ackw || ackr;

// Intialise the memory

initial

begin

  for (i=0; i<65536; i=i+1)

    buff [i] = 8'h00;

end

//

// writing to ram

always @(posedge clk or posedge rst)

begin

  if (rst)

    ackw <= #1 1'b0;

  else if (wr && stb && ((DELAY==3'b000) || (cnt==3'b000))) begin

    buff[addr]   <= #1 data_in[7:0];

    ackw <= #1 1'b1;

  end else ackw <= #1 1'b0;

end

always @(posedge clk or posedge rst)

  if (rst)

    ackr <= #1 1'b0;

  else if (stb && !wr && ((DELAY==3'b000) || (cnt==3'b000))) begin

    data_out <= #1 buff [addr];

    ackr <= #1 1'b1;

  end else begin

    ackr <= #1 1'b0;

    data_out <= #1 8'h00;

  end

always @(posedge clk or posedge rst)

begin

  if (rst)

    cnt <= #1 DELAY;

  else if (cnt==3'b000)

    cnt <= #1 DELAY;

  else if (stb)

    cnt <= #1 cnt - 3'b001;

  else cnt <= #1 DELAY;

end

endmodule