ADB Protocol
============

Resources
---------
Apple IIgs Hardware Reference Second Edition [p80(Chapter6 p121)]
    ftp://ftp.apple.asimov.net/pub/apple_II/documentation/Apple%20IIgs%20Hardware%20Reference.pdf
ADB Keycode
    http://72.0.193.250/Documentation/macppc/adbkeycodes/
    http://m0115.web.fc2.com/m0115.jpg
ADB Signaling
    http://kbdbabel.sourceforge.net/doc/kbd_signaling_pcxt_ps2_adb.pdf
ADB Overview & History
    http://en.wikipedia.org/wiki/Apple_Desktop_Bus
Microchip Application Note: ADB device(with code for PIC16C)
    http://www.microchip.com/stellent/idcplg?IdcService=SS_GET_PAGE&nodeId=1824&appnote=en011062
AVR ATtiny2131 ADB to PS/2 converter(Japanese)
    http://hp.vector.co.jp/authors/VA000177/html/KeyBoardA5DEA5CBA5A2II.html


Pinouts
-------
    Female socket from the front

      4o  o3
     2o    o1
        ==

    1: Data
    2: Power SW(low when press Power key)
    3: Vcc(5V)
    4: GND


Commands
--------
    ADB command is 1byte and consists of 4bit-address, 2bit-command
    type and 2bit-register. The commands are always sent by Host.

    Command format:
    7 6 5 4 3 2 1 0
    | | | |------------ address
            | |-------- command type
                | |---- register

    bits                commands
    ------------------------------------------------------
    - - - - 0 0 0 0     Send Request(reset all devices)
    A A A A 0 0 0 1     Flush(reset a device)
    - - - - 0 0 1 0     Reserved
    - - - - 0 0 1 1     Reserved
    - - - - 0 1 - -     Reserved
    A A A A 1 0 R R     Listen(write to a device)
    A A A A 1 1 R R     Talk(read from a device)

    The command to read keycodes from keyboard is 0x2C which
    consist of keyboard address 2 and Talk against register 0. 

    Address:
    2:  keyboard
    3:  mice

    Registers:
    0: application(keyobard/mice use to store its data.)
    1: application
    2: application
    3: status and command


Communication
-------------
    This is a minimum information for keyboard communication.
    See "Resources" for detail.

    Signaling:

    ~~~~____________~~||||||||||||__~~~~~_~~|||||||||||||||__~~~~

        |800us     |  |7 Command 0|  |   |  |15-64  Data  0|Stopbit(0)
        +Attention |              |  |   +Startbit(1)
                   +Startbit(1)   |  +Tlt(140-260us)
                                  +stopbit(0)

    Bit cells:

    bit0: ______~~~
          65    :35us

    bit1: ___~~~~~~
          35 :65us

    bit0 low time: 60-70% of bit cell(42-91us)
    bit1 low time: 30-40% of bit cell(21-52us)
    bit cell time: 70-130us
    [from Apple IIgs Hardware Reference Second Edition]

    Criterion for bit0/1:
    After 55us if line is low/high then bit is 0/1.

    Attention & start bit:
    Host asserts low in 560-1040us then places start bit(1).

    Tlt(Stop to Start):
    Bus stays high in 140-260us then device places start bit(1).

    Global reset:
    Host asserts low in 2.8-5.2ms. All devices are forced to reset.

    Send request from device(Srq):
    Device can request to send at commad(Global only?) stop bit.
    keep low for 300us to request.


Keyboard data(register0)
    This 16bit data can contains 2 keycodes and 2 released flags.
    First keycode is palced in upper nibble. When one keyocode is sent,
    lower nibble is 0xFF.
    Release flag is 1 when key is released.

    15 14 . . . . . 8 7 6 . . . . . 0
    |  |keycode1      | |keycode2
    |released(1)      |released(1)

    Keycodes:
    Scancode consists of 7bit keycode and 1bit release flag.
    Device can send two keycodes at once. If just one keycode is sent
    keycode1 contains it and keyocode2 is 0xFF.

    Power switch:
    You can read the state from PSW line(active low) however
    the switch has a special scancode 0x7F7F, so you can
    also read from Data line. It uses 0xFFFF for release scancode.

END_OF_ADB