Internal of HHKB pro
=====================
HHKB pro has MCU and some chips on separate two PCBs.

Controller PCB
--------------
    M38K07M4    Renesas MCU with USB function
                http://documentation.renesas.com/eng/products/mpumcu/rej03b0192_38k0ds.pdf

    (HHKB_controller.jpg)

Keyswitch PCB
-------------
    HC4051      Analog Multiplexer: select a row line.
                http://www.alldatasheet.com/datasheet-pdf/pdf/203989/KODENSHI/KK74HC4051A.html
    LS145       BCD Decoder: select a column line.
                http://www.alldatasheet.com/datasheet-pdf/pdf/27373/TI/SN74LS145D.html
    BU9831      Non-volatile electronic potentiometer: for calibration?
                http://www.alldatasheet.com/datasheet-pdf/pdf/36387/ROHM/BU9831.html
    TP1683/4    Capacitive Sensing controller: no datasheet available.

    (HHKB_keyswitch.jpg)

    Topre original chip?
    (HHKB_TP1684.jpg)


Connector Cable
---------------
Two PCBs are connected by 15 lines(13 in case of Pro2).
Vcc and GND use 3(2) lines each, other 9 lines are for keyboard signaling.

    Keyswitch PCB connector                                 Teensy++ pins
    -------------------------------------------------------------------------------
     1  Vcc(5V)   Not exist on Pro2                         5V
     2  Vcc(5V)                                             5V
     3  Vcc(5V)                                             5V
     4  TP1684    KEY: Low(0) when key pressed              PE6 input(with pullup)
     5  TP1684    KEY_PREV: assert previous key state???    PE7 output
     6  HC4051    A(bit0) select 8 rows(0 to 7)             PB0 output
     7  HC4051    B(bit1)                                   PB1 output
     8  HC4051    C(bit2)                                   PB2 output
     9  LS145     A(bit0) select 8 columns(0 to 7)          PB3 output
    10  LS145     B(bit1)                                   PB4 output
    11  LS145     C(bit2)                                   PB5 output
    12  LS145     D(enable) Low(0) enable selected column   PB6 output
    13  GND                                                 GND
    14  GND                                                 GND
    15  GND       Not exist on Pro2                         GND

    NOTE: guessing pin5(KEY_PREV) may work for hysteresis of capacitive sensing.

    (HHKB_connector.jpg)


Keyswitch matrix
----------------
60 keyswitches in 8*8 matrix. It is ghost-free and bounce-free.

      COL 0     1       2       3       4       5       6       7
    ROW ---------------------------------------------------------------
      0|  2     q       w       s       a       z       x       c
      1|  3     4       r       e       d       f       v       b
      2|  5     6       y       t       g       h       n       _NONE_
      3|  1     Esc     Tab     Control LShift  LAlt    LMeta   Space
      4|  7     8       u       i       k       j       m       _NONE_
      5|  \     `       Delete  Return  Fn      RShift  RAlt    RMeta
      6|  9     0       o       p       ;       l       ,       _NONE_
      7|  -     +       ]       [       '       /       .       _NONE_


Matrix diagram:

             +-------------------------+-+-+-+-+-+-+-+     Vcc
             |bias control?            - - - - - - - -     ---
             |                  3.9K*8 R R R R R R R R      |
    +--------^+      +--------+        - - - - - - - -      |  
    |        2|      | HC4051 <0-------|-|-|-|-|-|-|-|--|R|-+
    |         |capa. |        <1-------|-|-|-|-|-|-|-|--|R|-+
    | TP1684  |sense |        <2-------|-|-|-|-|-|-|-|--|R|-+
    |       11<------|        <3-------|-|-|-|-|-|-|-|--|R|-+
    |         |      |        <4-------|-|-|-|-|-|-|-|--|R|-+
    |         |      |        <5-------|-|-|-|-|-|-|-|--|R|-+
    |         <-+    |        <6-------|-|-|-|-|-|-|-|--|R|-+
    |   1   4 | |    |        <7-------|-|-|-|-|-|-|-|--|R|-+
    +---V---^-+ |    +-^-^-^--+        0 1 2 3 4 5 6 7  33K*8
       KEY PREV |      A B C         +-----------------+
        |   | +-^----+ | | |         |      LS145      |
    Vcc |   | |BU9831| | | |         +-^--^--^--^------+
    --- |   | +------+ | | |           A  B  C  D   +------+
     |  |   |          | | |           |  |  |  |   |      |
    1-3 4   5          6 7 8           9 10 11 12 13-15    |
    +--------------------------------------------------+   |
    |                connector                         |  ---
    +--------------------------------------------------+  GND
                    to controller
                                    

Signals charts
--------------
    While pressing space bar, watched HHKB original controller signals by logic analyzer.
    Row and column is looping between 0-7 each for selecting a key.
    A key is scaned every about 15ms, so scan rate is 66Hz.

    (HHKB_chart1.jpg)

    Space bar locate at ROW:3 COL:7. A key is selected by HC4051(C,B,A) and LS145(C,B,A).
    Key state can be read on TP1684(4/KEY) while asserting low on LS145(D). 

    Usage of TP1684(5) is not clear. Controller seemed to output previous key state on this line.
    However key state can be read without using this signal.

    (HHKB_chart2.jpg)


Matrix scan pseudo code
-----------------------
    for (row: 0-7) {
        SELECT_ROW(row);        // set HC4051(A,B,C)

        for (col: 0-7) {
            SELECT_COL(col);    // set LS145(A,B,C)

            _delay_us(40);

            if (prev_key_state(row, col)) {
                KEY_PREV_ON;
            }

            _delay_us(7);

            ENALBLE_COL();      // set LS145(D) to low

            _delay_us(10);

            if (KEY == 0) {     // read TP1684(KEY)
                // key pressed
            } else {
                // not pressed
            }

            KEY_PREV_OFF;
            UNALBLE_COL();      // set LS145(D) to high

            _delay_us(150);
        }
    }



EOF
