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//-------------------------------------------------------------------------------------

//

// Author: John Clayton

// Date  : April 30, 2001

// Update: 4/30/01 copied this file from lcd_2.v (pared down).

// Update: 5/24/01 changed the first module from "ps2_keyboard_receiver"

//                 to "ps2_keyboard_interface"

// Update: 5/29/01 Added input synchronizing flip-flops.  Changed state

//                 encoding (m1) for good operation after part config.

// Update: 5/31/01 Added low drive strength and slow transitions to ps2_clk

//                 and ps2_data in the constraints file.  Added the signal

//                 "tx_shifting_done" as distinguished from "rx_shifting_done."

//                 Debugged the transmitter portion in the lab.

// Update: 6/01/01 Added horizontal tab to the ascii output.

// Update: 6/01/01 Added parameter TRAP_SHIFT_KEYS.

// Update: 6/05/01 Debugged the "debounce" timer functionality.

//                 Used 60usec timer as a "watchdog" timeout during

//                 receive from the keyboard.  This means that a keyboard

//                 can now be "hot plugged" into the interface, without

//                 messing up the bit_count, since the bit_count is reset

//                 to zero during periods of inactivity anyway.  This was

//                 difficult to debug.  I ended up using the logic analyzer,

//                 and had to scratch my head quite a bit.

// Update: 6/06/01 Removed extra comments before the input synchronizing

//                 flip-flops.  Used the correct parameter to size the

//                 5usec_timer_count.  Changed the name of this file from

//                 ps2.v to ps2_keyboard.v

// Update: 6/06/01 Removed "&& q[7:0]" in output_strobe logic.  Removed extra

//                 commented out "else" condition in the shift register and

//                 bit counter.

// Update: 6/07/01 Changed default values for 60usec timer parameters so that

//                 they correspond to 60usec for a 49.152MHz clock.

//

//

//

//

//

// Description

//-------------------------------------------------------------------------------------

// This is a state-machine driven serial-to-parallel and parallel-to-serial

// interface to the ps2 style keyboard interface.  The details of the operation

// of the keyboard interface were obtained from the following website:

//

//   http://www.beyondlogic.org/keyboard/keybrd.htm

//

// Some aspects of the keyboard interface are not implemented (e.g, parity

// checking for the receive side, and recognition of the various commands

// which the keyboard sends out, such as "power on selt test passed," "Error"

// and "Resend.")  However, if the user wishes to recognize these reply

// messages, the scan code output can always be used to extend functionality

// as desired.

//

// Note that the "Extended" (0xE0) and "Released" (0xF0) codes are recognized.

// The rx interface provides separate indicator flags for these two conditions

// with every valid character scan code which it provides.  The shift keys are

// also trapped by the interface, in order to provide correct uppercase ASCII

// characters at the ascii output, although the scan codes for the shift keys

// are still provided at the scan code output.  So, the left/right ALT keys

// can be differentiated by the presence of the rx_entended signal, while the

// left/right shift keys are differentiable by the different scan codes

// received.

//

// The interface to the ps2 keyboard uses ps2_clk clock rates of

// 30-40 kHz, dependent upon the keyboard itself.  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.  The upper limit for clocking

// the state machine will undoubtedly be determined by delays in the logic 

// which decodes the scan codes into ASCII equivalents.  The maximum speed

// will be most likely many megahertz, depending upon target technology.

// 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 keyboard

// the ps2_clk and ps2_data lines are used as inputs.  While writing to the

// keyboard, however (which may be done at any time.  If writing interrupts a

// read from the keyboard, the keyboard will buffer up its data, and send

// it later) both the ps2_clk and ps2_data lines are occasionally pulled low,

// and pullup resistors are used to bring the lines high again, by setting

// the drivers to high impedance state.

//

// The tx interface, for writing to the keyboard, does not provide any special

// pre-processing.  It simply transmits the 8-bit command value to the

// keyboard.

//

// 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 in order to configure and appropriately size the

// counter of a 60 microsecond timer used in the transmitter, depending on

// the clock frequency used.  The 60 microsecond period is guaranteed to be

// more than one period of the ps2_clk_s signal.

//

// Also, a smaller 5 microsecond timer has been included for "debounce".

// This is used because, with internal pullups on the ps2_clk and ps2_data

// lines, there is some bouncing around which occurs

//

// A parameter TRAP_SHIFT_KEYS allows the user to eliminate shift keypresses

// from producing scan codes (along with their "undefined" ASCII equivalents)

// at the output of the interface.  If TRAP_SHIFT_KEYS is non-zero, the shift

// key status will only be reported by rx_shift_key_on.  No ascii or scan

// codes will be reported for the shift keys.  This is useful for those who

// wish to use the ASCII data stream, and who don't want to have to "filter

// out" the shift key codes.

//

//-------------------------------------------------------------------------------------

`resetall

`timescale 1ns/100ps

`define TOTAL_BITS   11          //total data bits of the data package

`define EXTEND_CODE  8'hE0      //extend code

`define RELEASE_CODE 8'hF0      //release code

module ps2_keyboard(

  clk,

  reset,

  ps2_clk,

  ps2_data,

  interrupt,

  rx_scan_code

  );

// Parameters

// The timer value can be up to (2^bits) inclusive.

parameter TIMER_60USEC_VALUE_PP = 840; // Number of sys_clks for 60usec. (for a 14Mhz clock)

parameter TIMER_60USEC_BITS_PP  = 10;   // 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.

// State "m1_rx_clk_l" has been chosen on purpose.  Since the input

// synchronizing flip-flops initially contain zero, it takes one clk

// for them to update to reflect the actual (idle = high) status of

// the I/O lines from the keyboard.  Therefore, choosing 0 for m1_rx_clk_l

// allows the state machine to transition to m1_rx_clk_h when the true

// values of the input signals become present at the outputs of the

// synchronizing flip-flops.  This initial transition is harmless, and it

// eliminates the need for a "reset" pulse before the interface can operate.

parameter m1_rx_clk_h = 1;

parameter m1_rx_clk_l = 0;

parameter m1_rx_falling_edge_marker = 3;

parameter m1_rx_rising_edge_marker = 4;

// I/O declarations

input clk;

input reset;

input ps2_clk;

input ps2_data;

output interrupt;

output [7:0] rx_scan_code;

//output [7:0] rx_ascii;

reg rx_extended;

reg rx_released;

reg [7:0] rx_scan_code;

reg interrupt;

// Internal signal declarations

wire timer_60usec_done;

wire extended;

wire released;

                         // NOTE: These two signals used to be one.  They

                         //       were split into two signals because of

                         //       shift key trapping.  With shift key

                         //       trapping, no event is generated externally,

                         //       but the "hold" data must still be cleared

                         //       anyway regardless, in preparation for the

                         //       next scan codes.

wire rx_output_strobe;   // Used to produce the actual output.

wire rx_shifting_done;

reg [`TOTAL_BITS-1:0] q;

reg [3:0] m1_state;

reg [3:0] m1_next_state;

reg [3:0] bit_count;

reg enable_timer_60usec;

reg [TIMER_60USEC_BITS_PP-1:0] timer_60usec_count;

reg ps2_clk_s;        // Synchronous version of this input

reg ps2_data_s;       // Synchronous version of this input

//--------------------------------------------------------------------------

// Module code

// Input "synchronizing" logic -- synchronizes the inputs to the state

// machine clock, thus avoiding errors related to

// spurious state machine transitions.

always @(posedge clk)

begin

  ps2_clk_s <= ps2_clk;

  ps2_data_s <= ps2_data;

end

// State register

always @(posedge clk)

begin : m1_state_register

  if (reset) m1_state <= m1_rx_clk_h;

  else m1_state <= m1_next_state;

end

// State transition logic

always @(m1_state

         or q

         or ps2_clk_s

         or ps2_data_s

         or timer_60usec_done

         )

begin : m1_state_logic

  // Output signals default to this value, unless changed in a state condition.

  enable_timer_60usec <= 0;

  case (m1_state)

    m1_rx_clk_h :

      begin

        enable_timer_60usec <= 1;

                  if (~ps2_clk_s) m1_next_state <= m1_rx_falling_edge_marker;

        else m1_next_state <= m1_rx_clk_h;

      end

    m1_rx_falling_edge_marker :

      begin

        enable_timer_60usec <= 0;

        m1_next_state <= m1_rx_clk_l;

      end

    m1_rx_rising_edge_marker :

      begin

        enable_timer_60usec <= 0;

        m1_next_state <= m1_rx_clk_h;

      end

    m1_rx_clk_l :

      begin

        enable_timer_60usec <= 1;

                  if (ps2_clk_s) m1_next_state <= m1_rx_rising_edge_marker;

        else m1_next_state <= m1_rx_clk_l;

      end

    default : m1_next_state <= m1_rx_clk_h;

  endcase

end

// This is the bit counter

always @(posedge clk)

begin

  if ( reset

        || rx_shifting_done

      )

                bit_count <= 0;  // normal reset

  else if (timer_60usec_done

           && (m1_state == m1_rx_clk_h)

           && (ps2_clk_s)

      )

                bit_count <= 0;  // rx watchdog timer reset

  else if ( (m1_state == m1_rx_falling_edge_marker)   // increment for rx

           )

    bit_count <= bit_count + 1;

end

// This signal is high for one clock at the end of the timer count.

assign rx_shifting_done = (bit_count == `TOTAL_BITS);

// This is the signal which enables loading of the shift register.

// It also indicates "ack" to the device writing to the transmitter.

// This is the ODD parity bit for the transmitted word.

// This is the shift register

always @(posedge clk)

begin

  if (reset) q <= 0;

  else if ( (m1_state == m1_rx_falling_edge_marker) )

    q <= {ps2_data_s,q[`TOTAL_BITS-1:1]};

end

// This is the 60usec timer counter

always @(posedge clk)

begin

  if (~enable_timer_60usec) timer_60usec_count <= 0;

  else if (~timer_60usec_done) timer_60usec_count <= timer_60usec_count + 1;

end

assign timer_60usec_done = (timer_60usec_count == (TIMER_60USEC_VALUE_PP - 1));

// Create the signals which indicate special scan codes received.

// These are the "unlatched versions."

assign extended = (q[8:1] == `EXTEND_CODE) && rx_shifting_done;

assign released = (q[8:1] == `RELEASE_CODE) && rx_shifting_done;

// Store the special scan code status bits

// Not the final output, but an intermediate storage place,

// until the entire set of output data can be assembled.

// Output the special scan code flags, the scan code and the ascii

always @(posedge clk)

begin

  if (reset)

  begin

    rx_scan_code <= 0;

    interrupt<=0;

  end

  else if (rx_output_strobe)      //if not extended, not relaeased,get the scan_code 

  begin

    rx_scan_code <= q[8:1];

    interrupt<=1;

  end

  else

  begin

  //rx_scan_code<=rx_scan_code;

  interrupt<=0;

  end

end

// Store the final rx output data only when all extend and release codes

// are received and the next (actual key) scan code is also ready.

// (the presence of rx_extended or rx_released refers to the

// the current latest scan code received, not the previously latched flags.)

assign rx_output_strobe = rx_shifting_done;

//                          && ~extended 

//                          && ~released

//                          );

// This part translates the scan code into an ASCII value...

// Only the ASCII codes which I considered important have been included.

// if you want more, just add the appropriate case statement lines...

// (You will need to know the keyboard scan codes you wish to assign.)

// The entries are listed in ascending order of ASCII value.

endmodule

//`undefine TOTAL_BITS

//`undefine EXTEND_CODE

//`undefine RELEASE_CODE

//`undefine LEFT_SHIFT

//`undefine RIGHT_SHIFT