//Legal Notice: (C)2005 Altera Corporation. All rights reserved.  Your
//use of Altera Corporation's design tools, logic functions and other
//software and tools, and its AMPP partner logic functions, and any
//output files any of the foregoing (including device programming or
//simulation files), and any associated documentation or information are
//expressly subject to the terms and conditions of the Altera Program
//License Subscription Agreement or other applicable license agreement,
//including, without limitation, that your use is for the sole purpose
//of programming logic devices manufactured by Altera and sold by Altera
//or its authorized distributors.  Please refer to the applicable
//agreement for further details.

// synthesis translate_off
`timescale 1ns / 100ps
// synthesis translate_on
module sys_clk_timer (
                       // inputs:
                        address,
                        chipselect,
                        clk,
                        reset_n,
                        write_n,
                        writedata,

                       // outputs:
                        irq,
                        readdata
                     );

  output           irq;
  output  [ 15: 0] readdata;
  input   [  2: 0] address;
  input            chipselect;
  input            clk;
  input            reset_n;
  input            write_n;
  input   [ 15: 0] writedata;

  wire             clk_en;
  wire             control_continuous;
  wire             control_interrupt_enable;
  reg     [  3: 0] control_register;
  wire             control_wr_strobe;
  reg              counter_is_running;
  wire             counter_is_zero;
  wire    [ 31: 0] counter_load_value;
  reg     [ 31: 0] counter_snapshot;
  reg              delayed_unxcounter_is_zeroxx0;
  wire             do_start_counter;
  wire             do_stop_counter;
  reg              force_reload;
  reg     [ 31: 0] internal_counter;
  wire             irq;
  reg     [ 15: 0] period_h_register;
  wire             period_h_wr_strobe;
  reg     [ 15: 0] period_l_register;
  wire             period_l_wr_strobe;
  wire    [ 15: 0] read_mux_out;
  reg     [ 15: 0] readdata;
  wire             snap_h_wr_strobe;
  wire             snap_l_wr_strobe;
  wire    [ 31: 0] snap_read_value;
  wire             snap_strobe;
  wire             start_strobe;
  wire             status_wr_strobe;
  wire             stop_strobe;
  wire             timeout_event;
  reg              timeout_occurred;
  assign clk_en = 1;
  always @(posedge clk or negedge reset_n)
    begin
      if (reset_n == 0)
          internal_counter <= 32'hCF850;
      else if (counter_is_running || force_reload)
          if (counter_is_zero    || force_reload)
              internal_counter <= counter_load_value;
          else 
            internal_counter <= internal_counter - 1;
    end


  assign counter_is_zero = internal_counter == 0;
  assign counter_load_value = {period_h_register,
    period_l_register};

  always @(posedge clk or negedge reset_n)
    begin
      if (reset_n == 0)
          force_reload <= 0;
      else if (clk_en)
          force_reload <= period_h_wr_strobe || period_l_wr_strobe;
    end


  assign do_start_counter = start_strobe;
  assign do_stop_counter = (stop_strobe                            ) ||
    (force_reload                           ) ||
    (counter_is_zero && ~control_continuous );

  always @(posedge clk or negedge reset_n)
    begin
      if (reset_n == 0)
          counter_is_running <= 1'b0;
      else if (clk_en)
          if (do_start_counter)
              counter_is_running <= -1;
          else if (do_stop_counter)
              counter_is_running <= 0;
    end


  //delayed_unxcounter_is_zeroxx0, which is an e_register
  always @(posedge clk or negedge reset_n)
    begin
      if (reset_n == 0)
          delayed_unxcounter_is_zeroxx0 <= 0;
      else if (clk_en)
          delayed_unxcounter_is_zeroxx0 <= counter_is_zero;
    end


  assign timeout_event = (counter_is_zero) & ~(delayed_unxcounter_is_zeroxx0);
  always @(posedge clk or negedge reset_n)
    begin
      if (reset_n == 0)
          timeout_occurred <= 0;
      else if (clk_en)
          if (status_wr_strobe)
              timeout_occurred <= 0;
          else if (timeout_event)
              timeout_occurred <= -1;
    end


  assign irq = timeout_occurred && control_interrupt_enable;
  //s1, which is an e_avalon_slave
  assign read_mux_out = ({16 {(address == 2)}} & period_l_register) |
    ({16 {(address == 3)}} & period_h_register) |
    ({16 {(address == 4)}} & snap_read_value[15 : 0]) |
    ({16 {(address == 5)}} & snap_read_value[31 : 16]) |
    ({16 {(address == 1)}} & control_register) |
    ({16 {(address == 0)}} & {counter_is_running,
    timeout_occurred});

  always @(posedge clk or negedge reset_n)
    begin
      if (reset_n == 0)
          readdata <= 0;
      else if (clk_en)
          readdata <= read_mux_out;
    end


  assign period_l_wr_strobe = chipselect && ~write_n && (address == 2);
  assign period_h_wr_strobe = chipselect && ~write_n && (address == 3);
  always @(posedge clk or negedge reset_n)
    begin
      if (reset_n == 0)
          period_l_register <= 63568;
      else if (period_l_wr_strobe)
          period_l_register <= writedata;
    end


  always @(posedge clk or negedge reset_n)
    begin
      if (reset_n == 0)
          period_h_register <= 12;
      else if (period_h_wr_strobe)
          period_h_register <= writedata;
    end


  assign snap_l_wr_strobe = chipselect && ~write_n && (address == 4);
  assign snap_h_wr_strobe = chipselect && ~write_n && (address == 5);
  assign snap_strobe = snap_l_wr_strobe || snap_h_wr_strobe;
  always @(posedge clk or negedge reset_n)
    begin
      if (reset_n == 0)
          counter_snapshot <= 0;
      else if (snap_strobe)
          counter_snapshot <= internal_counter;
    end


  assign snap_read_value = counter_snapshot;
  assign control_wr_strobe = chipselect && ~write_n && (address == 1);
  always @(posedge clk or negedge reset_n)
    begin
      if (reset_n == 0)
          control_register <= 0;
      else if (control_wr_strobe)
          control_register <= writedata[3 : 0];
    end


  assign stop_strobe = writedata[3] && control_wr_strobe;
  assign start_strobe = writedata[2] && control_wr_strobe;
  assign control_continuous = control_register[1];
  assign control_interrupt_enable = control_register;
  assign status_wr_strobe = chipselect && ~write_n && (address == 0);


endmodule