SAM MOUSE TURBO wheel support
by Velesoft

The original SAM MOUSE interface can't support a wheel mouse (there are no free pins at the mouse
connector). But the SAM MOUSE TURBO supports a mouse with the PS/2 protocol, 5 buttons
and a wheel. All signals are linked and can be used. However a firmware in the
CPLD of the SAM MOUSE TURBO is based on the standard SAM MOUSE communication.

Here is an idea how to modify the firmware and how to add a wheel status:

Original SAM MOUSE communication protocol
-----------------------------------------------------------------
reading 9 times the port #FFFE means CPU will read:
1) keyboard state
2) keyboard state
3) mouse buttons (three buttons + 1 bit is unused/reserved)
4) X-AXIS counter (high 4 bits)
5) X-AXIS counter (middle 4 bits)
6) X-AXIS counter (low 4 bits)
7) Y-AXIS counter (high 4 bits)
8) Y-AXIS counter (middle 4 bits)
9) Y-AXIS counter (low 4 bits)

All 9 ports must be read during time 100ys.

Theoretically can be added a status with the wheel counter or 4th and 5th mouse buttons.
Here is a new protocol idea:

reading 11 times the port #FFFE means CPU will read:
1) keyboard state
2) keyboard state
3) mouse buttons (three buttons + 1 bit is unused/reserved)
4) X-AXIS counter (high 4 bits)
5) X-AXIS counter (middle 4 bits)
6) X-AXIS counter (low 4 bits)
7) Y-AXIS counter (high 4 bits)
8) Y-AXIS counter (middle 4 bits)
9) Y-AXIS counter (low 4 bits)
10) mouse wheel counter (high 4 bits)
11) mouse wheel counter (low 4 bits)

This protocol returns 8bit counter for wheel (-128 to 127).
It is not sure if there is a free space in the current CPLD chip for this counter.

The SAM MOUSE uses counters for X and Y axis which are too big. Each counter has a 12bit value
(2048 steps). We can reduce counters to 8bit as in the KEMPSTON MOUSE for the Sinclair ZX
Spectrum (256 steps). High 4 bits of X and Y axis are unused and only (hardware)
emulated for a backward compatibility. After such an optimalisation will be the protocol
the same (11 times read FFFE) and free space in CPLD can be used for wheel counter.
Up to 5 mouse buttons can be readable in the 12th reading of FFFE.