// ***************************************************************************
// ***************************************************************************
// Copyright 2018 (c) Analog Devices, Inc. All rights reserved.
//
// In this HDL repository, there are many different and unique modules, consisting
// of various HDL (Verilog or VHDL) components. The individual modules are
// developed independently, and may be accompanied by separate and unique license
// terms.
//
// The user should read each of these license terms, and understand the
// freedoms and responsibilities that he or she has by using this source/core.
//
// This core 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.
//
// Redistribution and use of source or resulting binaries, with or without modification
// of this file, are permitted under one of the following two license terms:
//
//   1. The GNU General Public License version 2 as published by the
//      Free Software Foundation, which can be found in the top level directory
//      of this repository (LICENSE_GPL2), and also online at:
//      <https://www.gnu.org/licenses/old-licenses/gpl-2.0.html>
//
// OR
//
//   2. An ADI specific BSD license, which can be found in the top level directory
//      of this repository (LICENSE_ADIBSD), and also on-line at:
//      https://github.com/analogdevicesinc/hdl/blob/master/LICENSE_ADIBSD
//      This will allow to generate bit files and not release the source code,
//      as long as it attaches to an ADI device.
//
// ***************************************************************************
// ***************************************************************************

`timescale 1ns/100ps

module axi_slave #(
  parameter DATA_WIDTH = 32,
  parameter ACCEPTANCE = 3,
  parameter MIN_LATENCY = 16,
  parameter MAX_LATENCY = 32
) (
  input clk,
  input reset,

  input valid,
  output ready,
  input [31:0] addr,
  input [7:0] len,
  input [2:0] size,
  input [1:0] burst,
  input [2:0] prot,
  input [3:0] cache,

  output beat_stb,
  input beat_ack,
  output [31:0] beat_addr,
  output beat_last
);

reg [31:0] timestamp = 'h00;

always @(posedge clk) begin
  if (reset == 1'b1) begin
    timestamp <= 'h00;
  end else begin
    timestamp <= timestamp + 1'b1;
  end
end

reg [32+32+8-1:0] req_fifo[0:15];
reg [3:0] req_fifo_rd = 'h00;
reg [3:0] req_fifo_wr = 'h00;
wire [3:0] req_fifo_level = req_fifo_wr - req_fifo_rd;

assign ready = req_fifo_level < ACCEPTANCE;

always @(posedge clk) begin
  if (reset == 1'b1) begin
    req_fifo_wr <= 'h00;
  end else begin
    if (valid == 1'b1 && ready == 1'b1) begin
      req_fifo[req_fifo_wr][71:40] <= timestamp + {$random} % (MAX_LATENCY - MIN_LATENCY + 1) + MIN_LATENCY;
      req_fifo[req_fifo_wr][39:0] <= {addr,len};
      req_fifo_wr <= req_fifo_wr + 1'b1;
    end
  end
end

reg [7:0] beat_counter = 'h00;

assign beat_stb = req_fifo_level != 0 && timestamp > req_fifo[req_fifo_rd][71:40];
assign beat_last = beat_stb ? beat_counter == req_fifo[req_fifo_rd][0+:8] : 1'b0;
assign beat_addr = req_fifo[req_fifo_rd][8+:32] + beat_counter * DATA_WIDTH / 8;

always @(posedge clk) begin
  if (reset == 1'b1) begin
    beat_counter <= 'h00;
    req_fifo_rd <= 'h00;
  end else begin
    if (beat_ack == 1'b1) begin
      if (beat_last == 1'b1) begin
        beat_counter <= 'h00;
        req_fifo_rd <= req_fifo_rd + 1'b1;
      end else begin
        beat_counter <= beat_counter + 1'b1;
      end
    end
  end
end

endmodule