View source for JSX-C

== Protocol ==

Proposal for a protocol to handle joypads with a '''variable number of sticks and buttons''', plug in the MSX’s '''[https://www.msx.org/wiki/General_Purpose_port general purpose port]'''.

This protocol can manage joypads with up to '''15 analog axis''' (one stick requires 2 axis) and '''18 buttons'''.

Pin 8 is used to synchronize data transmission:
* Leaving Pin 8 LOW for long enough (>1 ms, about 3580 t-states) resets the joystick's internal counter,
* then successive pulses on Pin 8 (LOW > HIGH) are used for data transmission (6 bits per 6 bits).

Internal counter is used to send successive data: first the device ID, then each of the input data.

== States ==
=== Idle state (no-communication) ===

Write R#15
: Pin 8: LOW (for >1 ms)
: Reset internal data counter

Read R#14
: Get 0x0F (JSX idle fingerprint)
: <u>Note</u>: It could be any value between 00-3F not already return by [https://www.msx.org/wiki/MSX-HID#List_of_known_fingerprints other MSX devices])

=== Communication state ===

Write R#15
: Pin 8: LOW

Write R#15
: Pin 8: HIGH (request data)

Read R#14
: 6-bit data 

Pin binding for received data:
<syntaxhighlight>
Bits 7 6 5 4 3 2 1 0
–-–-–-–-–-–-–-–--–-–-
Pins x x 7 6 4 3 2 1
</syntaxhighlight>

==== Device ID ====

First received data is the JSX device ID:
<syntaxhighlight>
 7 6 5 4 3 2 1 0
–-–-–-–-–-–-–-–--
 0 0 A A A A B B
     │ │ │ │ └─┴── Number of button rows (0-3)
     │ │ │ │         0: No button
     │ │ │ │         1: Up to 6 buttons
     │ │ │ │         2: Up to 12 buttons
     │ │ │ │         3: Up to 18 buttons
     └─┴─┴─┴────── Number of axis (0-15)
</syntaxhighlight>
<u>Note</u>: Number of axis or number of button rows can be zero, but not both. So any device ID up to 0x3F is valid except 0x00.

Input number can be computed as:
* Axis number (<tt>Axies</tt>): <tt>ID >> 2</tt>,
* Button rows (<tt>BtnRow</tt>): <tt>ID & 0x03</tt>,
* Total input number (<tt>TotIn</tt>): <tt>Axies + BtnRow</tt>.

After first data read, the device must be run <tt>TotIn</tt> times to read all data (first all axies, then all button rows).

==== Axis data ====
Repeat this step for each axis (if any), then for each button row (if any). Number of axis and button rows is given by the device ID

Get 6-bit values
: For axis:		signed 6-bits value [-32 ; 31]
: For boutons:	bits field for 6 boutons (0: pressed, 1: released)

==== Button row data ====
Repeat this step for each axis (if any), then for each button row (if any). Number of axis and button rows is given by the device ID

Get 6-bit values
: For axis:		signed 6-bits value [-32 ; 31]
: For boutons:	bits field for 6 boutons (0: pressed, 1: released)

== Example ==
For a Joypad with a stick with X and Y axis (2 axis), and 4 buttons (1 row), the reading process would be:
* After starting communication, first read to R#14 return the ID.<br/>ID: 0x09 [00 0010 01] for 2 axis, and 1 buttons row,
* Next reading give X-axis value [-32 ; 31],
* Next reading give Y-axis value [-32 ; 31],
* Next and last reading give button states (for up to 6 boutons).

[[File:Joystick.png|512px]]

Note: [https://www.msx.org/wiki/MSX-HID MSX-HID] fingerprint would be: 0Fh, 09h, 0Fh

[[Category:Proposal]][[Category:Proposal/Protocol]]