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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.

//

//

//

//

//

// 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

`define EXTEND_CODE  16'hE0

`define RELEASE_CODE 16'hF0

`define LEFT_SHIFT   16'h12

`define RIGHT_SHIFT  16'h59

`define CTRL_KEY     16'h1D

module ps2_keyboard_interface (

  clk,

  reset,

  ps2_clk,

  ps2_data,

  rx_extended,

  rx_released,

  rx_shift_key_on,

  rx_ctrl_key_on,

  rx_scan_code,

  rx_ascii,

  rx_data_ready,       // rx_read_o

  rx_read,             // rx_read_ack_i

  tx_data,

  tx_write,

  tx_write_ack_o,

  tx_error_no_keyboard_ack

  );

// Parameters

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

parameter TIMER_60USEC_VALUE_PP = 1920; // Number of sys_clks for 60usec.

parameter TIMER_60USEC_BITS_PP  = 11;   // Number of bits needed for timer

parameter TIMER_5USEC_VALUE_PP = 186;   // Number of sys_clks for debounce

parameter TIMER_5USEC_BITS_PP  = 8;     // Number of bits needed for timer

parameter TRAP_SHIFT_KEYS_PP = 0;       // Default: No shift key trap.

// 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 = 13;

parameter m1_rx_rising_edge_marker = 14;

parameter m1_tx_force_clk_l = 3;

parameter m1_tx_first_wait_clk_h = 10;

parameter m1_tx_first_wait_clk_l = 11;

parameter m1_tx_reset_timer = 12;

parameter m1_tx_wait_clk_h = 2;

parameter m1_tx_clk_h = 4;

parameter m1_tx_clk_l = 5;

parameter m1_tx_wait_keyboard_ack = 6;

parameter m1_tx_done_recovery = 7;

parameter m1_tx_error_no_keyboard_ack = 8;

parameter m1_tx_rising_edge_marker = 9;

parameter m2_rx_data_ready = 1;

parameter m2_rx_data_ready_ack = 0;

// I/O declarations

input clk;

input reset;

inout ps2_clk;

inout ps2_data;

output rx_extended;

output rx_released;

output rx_shift_key_on;

output rx_ctrl_key_on;

output [7:0] rx_scan_code;

output [7:0] rx_ascii;

output rx_data_ready;

input rx_read;

input [7:0] tx_data;

input tx_write;

output tx_write_ack_o;

output tx_error_no_keyboard_ack;

reg rx_extended;

reg rx_released;

reg [7:0] rx_scan_code;

reg [7:0] rx_ascii;

reg rx_data_ready;

reg tx_error_no_keyboard_ack;

// Internal signal declarations

wire timer_60usec_done;

wire timer_5usec_done;

wire extended;

wire released;

wire shift_key_on;

                         // 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_event;    // Used only to clear: hold_released, hold_extended

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

wire tx_parity_bit;

wire rx_shifting_done;

wire tx_shifting_done;

wire [11:0] shift_key_plus_code;

reg [`TOTAL_BITS-1:0] q;

reg [3:0] m1_state;

reg [3:0] m1_next_state;

reg m2_state;

reg m2_next_state;

reg [3:0] bit_count;

reg enable_timer_60usec;

reg enable_timer_5usec;

reg [TIMER_60USEC_BITS_PP-1:0] timer_60usec_count;

reg [7:0] timer_5usec_count;

reg [7:0] ascii;      // "REG" type only because a case statement is used.

reg left_shift_key;

reg right_shift_key;

reg ctrl_key;

reg hold_extended;    // Holds prior value, cleared at rx_output_strobe

reg hold_released;    // Holds prior value, cleared at rx_output_strobe

reg ps2_clk_s;        // Synchronous version of this input

reg ps2_data_s;       // Synchronous version of this input

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.

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

// Module code

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

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

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

// machine clock, thus avoiding errors related to

// spurious state machine transitions.

//

// Since the initial state of registers is zero, and the idle state

// of the ps2_clk and ps2_data lines is "1" (due to pullups), the

// "sense" of the ps2_clk_s signal is inverted from the true signal.

// This allows the state machine to "come up" in the correct

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 tx_shifting_done

         or tx_write

         or ps2_clk_s

         or ps2_data_s

         or timer_60usec_done

         or timer_5usec_done

         )

begin : m1_state_logic

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

  ps2_clk_hi_z <= 1;

  ps2_data_hi_z <= 1;

  tx_error_no_keyboard_ack <= 0;

  enable_timer_60usec <= 0;

  enable_timer_5usec <= 0;

  case (m1_state)

    m1_rx_clk_h :

      begin

        enable_timer_60usec <= 1;

        if (tx_write) m1_next_state <= m1_tx_reset_timer;

        else 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 (tx_write) m1_next_state <= m1_tx_reset_timer;

        else if (ps2_clk_s) m1_next_state <= m1_rx_rising_edge_marker;

        else m1_next_state <= m1_rx_clk_l;

      end

    m1_tx_reset_timer:

      begin

        enable_timer_60usec <= 0;

        m1_next_state <= m1_tx_force_clk_l;

      end

    m1_tx_force_clk_l :

      begin

        enable_timer_60usec <= 1;

        ps2_clk_hi_z <= 0;  // Force the ps2_clk line low.

        if (timer_60usec_done) m1_next_state <= m1_tx_first_wait_clk_h;

        else m1_next_state <= m1_tx_force_clk_l;

      end

    m1_tx_first_wait_clk_h :

      begin

        enable_timer_5usec <= 1;

        ps2_data_hi_z <= 0;        // Start bit.

        if (~ps2_clk_s && timer_5usec_done)

          m1_next_state <= m1_tx_clk_l;

        else

          m1_next_state <= m1_tx_first_wait_clk_h;

      end

    // This state must be included because the device might possibly

    // delay for up to 10 milliseconds before beginning its clock pulses.

    // During that waiting time, we cannot drive the data (q[0]) because it

    // is possibly 1, which would cause the keyboard to abort its receive

    // and the expected clocks would then never be generated.

    m1_tx_first_wait_clk_l :

      begin

        ps2_data_hi_z <= 0;

        if (~ps2_clk_s) m1_next_state <= m1_tx_clk_l;

        else m1_next_state <= m1_tx_first_wait_clk_l;

      end

    m1_tx_wait_clk_h :

      begin

        enable_timer_5usec <= 1;

        ps2_data_hi_z <= q[0];

        if (ps2_clk_s && timer_5usec_done)

          m1_next_state <= m1_tx_rising_edge_marker;

        else

          m1_next_state <= m1_tx_wait_clk_h;

      end

    m1_tx_rising_edge_marker :

      begin

        ps2_data_hi_z <= q[0];

        m1_next_state <= m1_tx_clk_h;

      end

    m1_tx_clk_h :

      begin

        ps2_data_hi_z <= q[0];

        if (tx_shifting_done) m1_next_state <= m1_tx_wait_keyboard_ack;

        else if (~ps2_clk_s) m1_next_state <= m1_tx_clk_l;

        else m1_next_state <= m1_tx_clk_h;

      end

    m1_tx_clk_l :

      begin

        ps2_data_hi_z <= q[0];

        if (ps2_clk_s) m1_next_state <= m1_tx_wait_clk_h;

        else m1_next_state <= m1_tx_clk_l;

      end

    m1_tx_wait_keyboard_ack :

      begin

        if (~ps2_clk_s && ps2_data_s)

          m1_next_state <= m1_tx_error_no_keyboard_ack;

        else if (~ps2_clk_s && ~ps2_data_s)

          m1_next_state <= m1_tx_done_recovery;

        else m1_next_state <= m1_tx_wait_keyboard_ack;

      end

    m1_tx_done_recovery :

      begin

        if (ps2_clk_s && ps2_data_s) m1_next_state <= m1_rx_clk_h;

        else m1_next_state <= m1_tx_done_recovery;

      end

    m1_tx_error_no_keyboard_ack :

      begin

        tx_error_no_keyboard_ack <= 1;

        if (ps2_clk_s && ps2_data_s) m1_next_state <= m1_rx_clk_h;

        else m1_next_state <= m1_tx_error_no_keyboard_ack;

      end

    default : m1_next_state <= m1_rx_clk_h;

  endcase

end

// State register

always @(posedge clk)

begin : m2_state_register

  if (reset) m2_state <= m2_rx_data_ready_ack;

  else m2_state <= m2_next_state;

end

// State transition logic

always @(m2_state or rx_output_strobe or rx_read)

begin : m2_state_logic

  case (m2_state)

    m2_rx_data_ready_ack:

          begin

            rx_data_ready <= 1'b0;

            if (rx_output_strobe) m2_next_state <= m2_rx_data_ready;

            else m2_next_state <= m2_rx_data_ready_ack;

          end

    m2_rx_data_ready:

          begin

            rx_data_ready <= 1'b1;

            if (rx_read) m2_next_state <= m2_rx_data_ready_ack;

            else m2_next_state <= m2_rx_data_ready;

          end

    default : m2_next_state <= m2_rx_data_ready_ack;

  endcase

end

// This is the bit counter

always @(posedge clk)

begin

  if (   reset

      || rx_shifting_done

      || (m1_state == m1_tx_wait_keyboard_ack)        // After tx is 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_clk_h) && ~ps2_clk_s)  // increment for rx

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

           ||(m1_state == m1_tx_rising_edge_marker)   // increment for tx

           )

    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);

assign tx_shifting_done = (bit_count == `TOTAL_BITS-1);

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

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

assign tx_write_ack_o = (  (tx_write && (m1_state == m1_rx_clk_h))

                         ||(tx_write && (m1_state == m1_rx_clk_l))

                         );

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

assign tx_parity_bit = ~^tx_data;

// This is the shift register

always @(posedge clk)

begin

  if (reset) q <= 0;

  else if (tx_write_ack_o) q <= {1'b1,tx_parity_bit,tx_data,1'b0};

//  else if (((m1_state == m1_rx_clk_h) && ~ps2_clk_s)

  else if ( (m1_state == m1_rx_falling_edge_marker)

           ||(m1_state == m1_tx_rising_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));

// This is the 5usec timer counter

always @(posedge clk)

begin

  if (~enable_timer_5usec) timer_5usec_count <= 0;

  else if (~timer_5usec_done) timer_5usec_count <= timer_5usec_count + 1;

end

assign timer_5usec_done = (timer_5usec_count == TIMER_5USEC_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.

always @(posedge clk)

begin

  if (reset || rx_output_event)

  begin

    hold_extended <= 0;

    hold_released <= 0;

  end

  else

  begin

    if (rx_shifting_done && extended) hold_extended <= 1;

    if (rx_shifting_done && released) hold_released <= 1;

  end

end

// These bits contain the status of the two shift keys

always @(posedge clk)

begin

  if (reset) left_shift_key <= 0;

  else if ((q[8:1] == `LEFT_SHIFT) && rx_shifting_done && ~hold_released)

    left_shift_key <= 1;

  else if ((q[8:1] == `LEFT_SHIFT) && rx_shifting_done && hold_released)

    left_shift_key <= 0;

end

always @(posedge clk)

begin

  if (reset) right_shift_key <= 0;

  else if ((q[8:1] == `RIGHT_SHIFT) && rx_shifting_done && ~hold_released)

    right_shift_key <= 1;

  else if ((q[8:1] == `RIGHT_SHIFT) && rx_shifting_done && hold_released)

    right_shift_key <= 0;

end

assign rx_shift_key_on = left_shift_key || right_shift_key;

// This bit contains the status of the CTRL key

always @(posedge clk)

begin

  if (reset) ctrl_key <= 0;

  else if ((q[8:1] == `CTRL_KEY) && rx_shifting_done && ~hold_released)

    ctrl_key <= 1;

  else if ((q[8:1] == `CTRL_KEY) && rx_shifting_done && hold_released)

    ctrl_key <= 0;

end

assign rx_ctrl_key_on = ctrl_key;

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

always @(posedge clk)

begin

  if (reset)

  begin

    rx_extended <= 0;

    rx_released <= 0;

    rx_scan_code <= 0;

    rx_ascii <= 0;

  end

  else if (rx_output_strobe && q[8:1])

  begin

    rx_extended <= hold_extended;

    rx_released <= hold_released;

    rx_scan_code <= q[8:1];

    rx_ascii <= ascii;

  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_event  = (rx_shifting_done

                          && ~extended

                          && ~released

                          );

assign rx_output_strobe = (rx_shifting_done

                          && ~extended

                          && ~released

                          && ( (TRAP_SHIFT_KEYS_PP == 0)

                               || ( (q[8:1] != `RIGHT_SHIFT)

                                    &&(q[8:1] != `LEFT_SHIFT)

                                  )

                             )

                          );

// 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.

assign shift_key_plus_code = {3'b0,rx_shift_key_on,q[8:1]};

always @(shift_key_plus_code)

begin

  casez (shift_key_plus_code)

    12'h?66 : ascii <= 8'h08;  // Backspace ("backspace" key)

    12'h?0d : ascii <= 8'h09;  // Horizontal Tab

    12'h?5a : ascii <= 8'h0d;  // Carriage return ("enter" key)

    12'h?76 : ascii <= 8'h1b;  // Escape ("esc" key)

    12'h?29 : ascii <= 8'h20;  // Space

    12'h116 : ascii <= 8'h21;  // !

    12'h152 : ascii <= 8'h22;  // "

    12'h126 : ascii <= 8'h23;  // #

    12'h125 : ascii <= 8'h24;  // $

    12'h12e : ascii <= 8'h25;  // %

    12'h13d : ascii <= 8'h26;  // &

    12'h052 : ascii <= 8'h27;  // '

    12'h146 : ascii <= 8'h28;  // (

    12'h145 : ascii <= 8'h29;  // )

    12'h13e : ascii <= 8'h2a;  // *

    12'h155 : ascii <= 8'h2b;  // +

    12'h041 : ascii <= 8'h2c;  // ,

    12'h04e : ascii <= 8'h2d;  // -

    12'h049 : ascii <= 8'h2e;  // .

    12'h04a : ascii <= 8'h2f;  // /

    12'h045 : ascii <= 8'h30;  // 0

    12'h016 : ascii <= 8'h31;  // 1

    12'h01e : ascii <= 8'h32;  // 2

    12'h026 : ascii <= 8'h33;  // 3

    12'h025 : ascii <= 8'h34;  // 4

    12'h02e : ascii <= 8'h35;  // 5

    12'h036 : ascii <= 8'h36;  // 6

    12'h03d : ascii <= 8'h37;  // 7

    12'h03e : ascii <= 8'h38;  // 8

    12'h046 : ascii <= 8'h39;  // 9

    12'h14c : ascii <= 8'h3a;  // :

    12'h04c : ascii <= 8'h3b;  // ;

    12'h141 : ascii <= 8'h3c;  // <

    12'h055 : ascii <= 8'h3d;  // =

    12'h149 : ascii <= 8'h3e;  // >

    12'h14a : ascii <= 8'h3f;  // ?

    12'h11e : ascii <= 8'h40;  // @

    12'h11c : ascii <= 8'h41;  // A

    12'h132 : ascii <= 8'h42;  // B

    12'h121 : ascii <= 8'h43;  // C

    12'h123 : ascii <= 8'h44;  // D

    12'h124 : ascii <= 8'h45;  // E

    12'h12b : ascii <= 8'h46;  // F

    12'h134 : ascii <= 8'h47;  // G

    12'h133 : ascii <= 8'h48;  // H

    12'h143 : ascii <= 8'h49;  // I

    12'h13b : ascii <= 8'h4a;  // J

    12'h142 : ascii <= 8'h4b;  // K

    12'h14b : ascii <= 8'h4c;  // L

    12'h13a : ascii <= 8'h4d;  // M

    12'h131 : ascii <= 8'h4e;  // N

    12'h144 : ascii <= 8'h4f;  // O

    12'h14d : ascii <= 8'h50;  // P

    12'h115 : ascii <= 8'h51;  // Q

    12'h12d : ascii <= 8'h52;  // R

    12'h11b : ascii <= 8'h53;  // S

    12'h12c : ascii <= 8'h54;  // T

    12'h13c : ascii <= 8'h55;  // U

    12'h12a : ascii <= 8'h56;  // V

    12'h11d : ascii <= 8'h57;  // W

    12'h122 : ascii <= 8'h58;  // X

    12'h135 : ascii <= 8'h59;  // Y

    12'h11a : ascii <= 8'h5a;  // Z

    12'h054 : ascii <= 8'h5b;  // [