//-------------------------------------------------------------------------------------
//
// Author: John Clayton
// Date  : April 30, 2001
// Update: 6/06/01 copied this file from ps2.v (pared down).
// Update: 6/07/01 Finished initial coding efforts.
// Update: 6/09/01 Made minor changes to state machines during debugging.
//                 Fixed errors in state transitions.  Added state to m2
//                 so that "reset" causes the mouse to be initialized.
//                 Removed debug port.
//
//
//
//
//
// Description
//-------------------------------------------------------------------------------------
// This is a state-machine driven serial-to-parallel and parallel-to-serial
// interface to the ps2 style mouse.  The state diagram for part of the
// m2 state machine was obtained from the work of Rob Chapman, as published
// at: 
// www.ee.ualberta.ca/~elliott/ee552/studentAppNotes/1998_w/mouse_notes.html
//
//
// Some aspects of the mouse interface are not implemented (e.g, verifying
// the FA response code from the mouse when enabling streaming mode.)
// However, the mouse interface was designed so that "hot plugging" a mouse
// into the connector should cause the interface to send the F4 code to the
// mouse in order to enable streaming.  By this means, the mouse begins to
// operate, and no reset pulse should be needed.
//
// Similarly, there is a "watchdog" timer implemented, so that during periods
// of inactivity, the bit_count is cleared to zero.  Therefore, the effects of
// a bad count value are corrected, and internal errors of that type are not
// propagated into subsequent packet receive operations.
//
// To enable the streaming mode, F4 is sent to the mouse.
// The mouse responds with FA to acknowledge the command, and then enters
// streaming mode at the default rate of 100 packets per second (transmission
// of packets ceases when the activity at the mouse is not longer sensed.)
//
// There are additional commands to change the sampling rate and resolution
// of the mouse reported data.  Those commands are not implemented here.
// (E8,XX = set resolution 0,1,2,3)
// (E7 = set scaling 2:1)
// (E6 = reset scaling)
// (F3,XX = set sampling rate to XX packets per second.)
//
// At this time I do not know any of the command related to using the
// wheel of a "wheel mouse." 
//
// The packets consists of three bytes transmitted in sequence.  The interval
// between these bytes has been measured on two different mice, and found to
// be different.  On the slower (older) mouse it was approximately 345
// microseconds, while on a newer "wheel" mouse it was approximately 125
// microseconds.  The watchdog timer is designed to cause processing of a
// complete packet when it expires.  Therefore, the watchdog timer must last
// for longer than the "inter-byte delay" between bytes of the packet.
// I have set the default timer value to 400 usec, for my 49.152 MHz clock.
// The timer value and size of the timer counter is settable by parameters,
// so that other clock frequencies and settings may be used.  The setting for
// the watchdog timeout is not critical -- it only needs to be greater than
// the inter-byte delay as data is transmitted from the mouse, and no less
// than 60usec.
//
// Each "byte" of the packet is transmitted from the mouse as follows:
//
// 1 start bit, 8 data bits, 1 odd parity bit, 1 stop bit. == 11 bits total.
// (The data bits are sent LSB first)
//
// The data bits are formatted as follows:
//
// byte 0: YV, XV, YS, XS, 1, 0, R, L
// byte 1: X7..X0
// byte 2: Y7..Y0
//
// Where YV, XV are set to indicate overflow conditions.
//       XS, YS are set to indicate negative quantities (sign bits).
//        R,  L are set to indicate buttons pressed, left and right.
// 
//
//
// The interface to the ps2 mouse (like the keyboard) uses clock rates of
// 30-40 kHz, dependent upon the mouse itself.  The mouse generates the
// clock.
// The rate at which the state machine runs should be at least twice the 
// rate of the ps2_clk, so that the states can accurately follow the clock
// signal itself.  Four times oversampling is better.  Say 200kHz at least.
// In order to run the state machine extremely fast, synchronizing flip-flops
// have been added to the ps2_clk and ps2_data inputs of the state machine.
// This avoids poor performance related to slow transitions of the inputs.
//
// Because this is a bi-directional interface, while reading from the mouse
// the ps2_clk and ps2_data lines are used as inputs.  While writing to the
// mouse, however (which is done when a "packet" of less than 33 bits is
// received), both the ps2_clk and ps2_data lines are sometime pulled low by
// this interface.  As such, they are bidirectional, and pullups are used to
// return them to the "high" state, whenever the drivers are set to the
// high impedance state.
//
// Pullups MUST BE USED on the ps2_clk and ps2_data lines for this design,
// whether they be internal to an FPGA I/O pad, or externally placed.
// If internal pullups are used, they may be fairly weak, causing bounces
// due to crosstalk, etc.  There is a "debounce timer" implemented in order
// to eliminate erroneous state transitions which would occur based on bounce.
// Parameters are provided to configure the debounce timer for different
// clock frequencies.  2 or 3 microseconds of debounce should be plenty.
// You may possibly use much less, if your pullups are strong.
//
// A parameters is provided to configure a 60 microsecond period used while
// transmitting to the mouse.  The 60 microsecond period is guaranteed to be
// more than one period of the ps2_clk signal.
//
//
//-------------------------------------------------------------------------------------


`resetall
`timescale 1ns/100ps

`define TOTAL_BITS   33  // Number of bits in one full packet


module ps2_mouse_interface (
  clk,
  reset,
  ps2_clk,
  ps2_data,
  left_button,
  right_button,
  x_increment,
  y_increment,
  data_ready,       // rx_read_o
  read,             // rx_read_ack_i
  error_no_ack
  );

// Parameters

// The timer value can be up to (2^bits) inclusive.
parameter WATCHDOG_TIMER_VALUE_PP = 20000; // Number of sys_clks for 400usec.
parameter WATCHDOG_TIMER_BITS_PP  = 15;    // Number of bits needed for timer
parameter DEBOUNCE_TIMER_VALUE_PP = 186;   // Number of sys_clks for debounce
parameter DEBOUNCE_TIMER_BITS_PP  = 8;     // Number of bits needed for timer


// State encodings, provided as parameters
// for flexibility to the one instantiating the module.
// In general, the default values need not be changed.

// There are three state machines: m1, m2 and m3.
// States chosen as "default" states upon power-up and configuration:
//    "m1_clk_h"
//    "m2_wait"
//    "m3_data_ready_ack"

parameter m1_clk_h = 0;
parameter m1_falling_edge = 1;
parameter m1_falling_wait = 3;
parameter m1_clk_l = 2;
parameter m1_rising_edge = 6;
parameter m1_rising_wait = 4;

parameter m2_reset = 14;
parameter m2_wait = 0;
parameter m2_gather = 1;
parameter m2_verify = 3;
parameter m2_use = 2;
parameter m2_hold_clk_l = 6;
parameter m2_data_low_1 = 4;
parameter m2_data_high_1 = 5;
parameter m2_data_low_2 = 7;
parameter m2_data_high_2 = 8;
parameter m2_data_low_3 = 9;
parameter m2_data_high_3 = 11;
parameter m2_error_no_ack = 15;
parameter m2_await_response = 10;

parameter m3_data_ready = 1;
parameter m3_data_ready_ack = 0;

// I/O declarations
input clk;
input reset;
inout ps2_clk;
inout ps2_data;
output left_button;
output right_button;
output [8:0] x_increment;
output [8:0] y_increment;
output data_ready;
input read;
output error_no_ack;

reg left_button;
reg right_button;
reg [8:0] x_increment;
reg [8:0] y_increment;
reg data_ready;
reg error_no_ack;

// Internal signal declarations
wire watchdog_timer_done;
wire debounce_timer_done;
wire packet_good;

reg [`TOTAL_BITS-1:0] q;  // Shift register
reg [2:0] m1_state;
reg [2:0] m1_next_state;
reg [3:0] m2_state;
reg [3:0] m2_next_state;
reg m3_state;
reg m3_next_state;
reg [5:0] bit_count;      // Bit counter
reg [WATCHDOG_TIMER_BITS_PP-1:0] watchdog_timer_count;
reg [DEBOUNCE_TIMER_BITS_PP-1:0] debounce_timer_count;
reg ps2_clk_hi_z;     // Without keyboard, high Z equals 1 due to pullups.
reg ps2_data_hi_z;    // Without keyboard, high Z equals 1 due to pullups.
reg clean_clk;        // Debounced output from m1, follows ps2_clk.
reg rising_edge;      // Output from m1 state machine.
reg falling_edge;     // Output from m1 state machine.
reg output_strobe;    // Latches data data into the output registers

//--------------------------------------------------------------------------
// Module code

assign ps2_clk = ps2_clk_hi_z?1'bZ:1'b0;
assign ps2_data = ps2_data_hi_z?1'bZ:1'b0;

// State register
always @(posedge clk)
begin : m1_state_register
  if (!reset) m1_state <= m1_clk_h;
  else m1_state <= m1_next_state;
end

// State transition logic
always @(m1_state
         or ps2_clk
         or debounce_timer_done
         or watchdog_timer_done
         )
begin : m1_state_logic

  // Output signals default to this value, unless changed in a state condition.
  clean_clk <= 0;
  rising_edge <= 0;
  falling_edge <= 0;

  case (m1_state)
    m1_clk_h :
      begin
        clean_clk <= 1;
        if (~ps2_clk) m1_next_state <= m1_falling_edge;
        else m1_next_state <= m1_clk_h;
      end
      
    m1_falling_edge :
      begin
        falling_edge <= 1;
        m1_next_state <= m1_falling_wait;
      end
      
    m1_falling_wait :
      begin
        if (debounce_timer_done) m1_next_state <= m1_clk_l;
        else m1_next_state <= m1_falling_wait;
      end

    m1_clk_l :
      begin
        if (ps2_clk) m1_next_state <= m1_rising_edge;
        else m1_next_state <= m1_clk_l;
      end