Oeka Kids tablet

The Oeka Kids tablet is a Famicom accessory from Bandai that resembles a drawing tablet, and plugs into the expansion port. There are two known games that use this peripheral, both of which contain a variety of activities, such as painting, drawing lessons, hiragana lessons, and a variety of minigames such as mazes and sliding puzzles.

Very little information about this accessory is available, and only basic reverse engineering has been performed so far. As such, this information may be incorrect, but it seems to be acceptable for the two commercial games that use it.

Input ($4016 write)

7  bit  0
---- ----
xxxx xxAS
       ||
       |+- Strobe (0 = Latch, 1 = Read back)
       +-- Advance to next bit

To advance to the next bit, set A to 0 and then to 1. Bits can only be read while S is set to 1.

Output ($4017 read)

7  bit  0
---- ----
xxxx xDSx
      ||
      |+-- 0 if strobe is 1, 1 otherwise.
      +--- (Inverted) Serial data if strobe is 1, 0 if strobe is 0.

The serial data is returned most significant bit first, and inverted.

$4017.2
   ^
   |  <-- <-- <-- <--
   XXXXXXXXYYYYYYYYBA
   ||||||||||||||||||
   |||||||||||||||||+- Click
   ||||||||||||||||+-- Stylus is touching tablet
   ||||||||++++++++--- Stylus Y, scaled to 0-255
   ++++++++----------- Stylus X, scaled to 0-239