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_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_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;

// 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)
  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;  // [
    12'h05d : ascii <= 8'h5c;  // \
    12'h05b : ascii <= 8'h5d;  // ]
    12'h136 : ascii <= 8'h5e;  // ^
    12'h14e : ascii <= 8'h5f;  // _    
    12'h00e : ascii <= 8'h60;  // `
    12'h01c : ascii <= 8'h61;  // a
    12'h032 : ascii <= 8'h62;  // b
    12'h021 : ascii <= 8'h63;  // c
    12'h023 : ascii <= 8'h64;  // d
    12'h024 : ascii <= 8'h65;  // e
    12'h02b : ascii <= 8'h66;  // f
    12'h034 : ascii <= 8'h67;  // g
    12'h033 : ascii <= 8'h68;  // h
    12'h043 : ascii <= 8'h69;  // i
    12'h03b : ascii <= 8'h6a;  // j
    12'h042 : ascii <= 8'h6b;  // k
    12'h04b : ascii <= 8'h6c;  // l
    12'h03a : ascii <= 8'h6d;  // m
    12'h031 : ascii <= 8'h6e;  // n
    12'h044 : ascii <= 8'h6f;  // o
    12'h04d : ascii <= 8'h70;  // p
    12'h015 : ascii <= 8'h71;  // q
    12'h02d : ascii <= 8'h72;  // r
    12'h01b : ascii <= 8'h73;  // s
    12'h02c : ascii <= 8'h74;  // t
    12'h03c : ascii <= 8'h75;  // u
    12'h02a : ascii <= 8'h76;  // v
    12'h01d : ascii <= 8'h77;  // w
    12'h022 : ascii <= 8'h78;  // x
    12'h035 : ascii <= 8'h79;  // y
    12'h01a : ascii <= 8'h7a;  // z
    12'h154 : ascii <= 8'h7b;  // {
    12'h15d : ascii <= 8'h7c;  // |
    12'h15b : ascii <= 8'h7d;  // }
    12'h10e : ascii <= 8'h7e;  // ~
    12'h?71 : ascii <= 8'h7f;  // (Delete OR DEL on numeric keypad)
    default : ascii <= 8'h2e;  // '.' used for unlisted characters.
  endcase
end


endmodule

//`undefine TOTAL_BITS
//`undefine EXTEND_CODE
//`undefine RELEASE_CODE
//`undefine LEFT_SHIFT
//`undefine RIGHT_SHIFT