Instead of using the pot pins as variable-resistive inputs, you can use these pins as a
four-bit input/output port. This provides you with two additional pins on each of the two
controller ports for general purpose I/O.
If you set the output enable for any pin to a 1, the Amiga disconnects the potentiometer
control circuitry from the port, and configures the pin for output. The state of the data bit
controls the logic level on the output pin. This register must be written to at the POTGO
address, and read from the POTINP address. There are large capacitors on these lines,
and it can take up to 300 microseconds for the line to change state.
To use the entire register as an input, sensing the current state of the pot pins, write all
0s to POTGO. Thereafter you can read the current state by using read-only address
POTINP. Note that bits set as inputs will be connected to the proportional counters (See
the description of the START bit in POTGO).
These lines can also be used for button inputs. A button is a normally open switch that
shorts to ground. The Amiga must provide a pull-up resistance on the sense pin. To do
this, set the proper pin to output, and drive the line high (set both OUT... and DAT... to
1). Reading POTINP will produce a 0 if the button is pressed, a 1 if it is not.
The joystick fire buttons can also be configured as outputs. CIAADDRA ($BFE201) contains
a mask that corresponds one-to-one with the data read register, CIAAPRA ($BFE001).
Setting a 1 in the direction position makes the corresponding bit an output. See the 8520
appendix for more details.
FLOPPY DISK CONTROLLER
The built-in disk controller in the system can handle up to four MFM-type devices.
Typically these are double-sided, double-density, 3.5" (9Omm) or 5.25" disk drives. One
3.5" drive is installed in the basic unit.
The controller is extremely flexible. It can DMA an entire track of raw MFM data into
memory in a single disk revolution. Special registers allow the CPU to synchronize with
specific data, or read input a byte at a time. The controller can read and write virtually
any double-density MFM encoded disk, including the Amiga V1.0 format, IBM PC (MS-
DOS) 5.25", IBM PC (MS-DOS) 3.5" and most CP/:/ITM formatted disks. The controller
has provisions for reading and writing most disk using the Group Coded Recording (GCR)
method, including Apple II disks. With motor speed tricks, the controller can read and
write Commodore 1541/1571 format diskettes.
- Interface Hardware 235 -
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