Commodore Amiga A1000, Hardware Reference Manual

Page: 243 / 380

A somewhat simplified method of determining the truth of the light pen value involves
instructing the system software to read the register only during the internal vertical
blanking period of 0<V20:
1. Read (long) VPOSR once, during the period of 0<V20.
2. Mask off the upper 15 bits of the 32-bit word and compare it with the hex value of
$10500 (V=261).
3. If the VPOSR value is greater than $10500, the light pen has not triggered since the
last top-of-screen. If the value is less, the light pen has triggered and the value read is
the screen position of the light pen.
Note that when the light pen latch is enabled, the VPOSR register may be latched at any
time, and cannot be relied on as a counter. This behavior may cause problems with
software that attempts to derive timing based on VPOSR ticks.
DIGITAL INPUT/OUTPUT ON THE CONTROLLER PORT
The Amiga can read and interpret many different and nonstandard controllers. The control
lines built into the POTGO register (address $DFF034) can redefine the functions of some
of the controller port pins.
Table 8-4 is the POTGO register bit description. POTGO ($DFF034) is the write-only
address for the pot control register. POTINP ($DFF016) is the read-only address for the
pot control register. The pot-control register controls a four-bit bidirectional VO port that
shares the same four pins as the four pot inputs.
Table 8-4: POTGO ($DFF034) and POTINP ($DFF016) Registers
Bit
Number Name Function
15 OUTRY Output enable for bit 14 (l=output)
14 DATRY data for port 2, pin 9
13 OUTRX Output enable for bit 12
12 DATRX data for port 2, pin 5
11 OUTLY Output enable for bit 10
10 DATLY data for port 1, pin 9 (right mouse button)
09 OUTLX Output enable for bit 8
08 DATLX data for port 1, pin 5 (middle mouse button)
07-01 X chip revision identification number
00 START Start pots (dump capacitors, start counters)
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