GAME CONTROLLER INTERFACE TO MOUSE/TRACKBALL QUADRATURE INPUTS
A mouse or trackball is a device that translates planar motion into
pulse trains. Quadrature techniques are employed to preserve the
direction as well as magnitude of displacement. The registers JOY0DAT
and JOY1DAT become counter registers, with y displacement in the high
byte and x in the low byte. Movement causes the following action:
Up: y decrements
Down: y increments
Right: x increments
Left: x decrements
To determine displacement, JOYxDAT is read twice with corresponding x
and y values subtracted (careful, modulo 128 arithmetic). Note that
if either count changes by more than 127, both distance and direction
become ambiguous. There is a relationship between the sampling
interval and the maximum speed (that is, change in distance) that
can be resolved as follows:
Velocity < Distance(max) / SampleTime
Velocity < SQRT(DeltaX**2 + DeltaY**2) / SampleTime
For an Amiga with a 200 count-per-inch mouse sampling during each
vertical blanking interval, the maximum velocity in either the X or Y
direction becomes:
Velocity < (128 Counts * 1 inch/200 Counts) / .017 sec = 38 in/sec
which should be sufficient for most users.
NOTE: The Amiga software is designed to do mouse update cycles during
vertical blanking. The horizontal and vertical counters are always
valid and may be read at any time.
CONNECTOR PIN USAGE FOR MOUSE/TRACKBALL QUADRATURE INPUTS
PIN MNEMONIC DESCRIPTION HARDWARE REGISTER/NOTES
--- -------- ----------- -----------------------
1 V Vertical pulses JOY[0/1]DAT<15:8>
2 H Horizontal pulses JOY[0/1]DAT(7:0>
3 VQ Vertical quadrature pulses JOY[0/1]DAT<15:8>
4 HQ Horizontal quadrature pulses JOY[0/1]DAT<7:0>
5 UBUT* Unused mouse button See Proportional Inputs.
6 LBUT* Left mouse button See Fire Button.
7 +5V +5V, current limited
8 Ground
9 RBUT* Right mouse button See Proportional Inputs.
- 306 Appendix E -
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