Timer; Joystick Interface - Tandy 1000 SL Technical Reference Manual

The Palette allows the 16 colors to be remapped in any desired
organization. Normally, the Palette is set for a 1:l mapping (red
= red, blue
= blue, and so on) for PC compatibility. However,
instantly changing the on-screen colors is a powerful tool for
animation or graphics programs.
After the Palette, the RGBI data is resynchronized in the Post
Palette register. The final logic before the RGBI data is
buffered off the chip in the Border MUX. This MUX allows the
Border to be replaced with any color selected by the border color
latch. This latch is normally disabled in PC modes, but it is
used in all PC jr modes.
Timer
The final Tandy 1000 SL function other than 1/0 is the timer
found in the KFIT custom IC (U30). This part is composed of three
independent programmable counters. The clock for all three
counters is 1.1931 MHz, which is derived from 14.318 MHz/12.
Counters 0 and 1 are permanently enabled. Counter 2 is controlled
by port Hex 0061, Bit 0. Counter 0 is connected to system
interrupt 0 and is used for software timing functions. Counter 1
is used for refresh function timing. Counter 2 is connected to
the sound circuit and its output can be read at port Hex 0062,
Bit 5.
Joystick
Interface
The joystick interface contained in the 8079021 custom IC (U40)
converts positional information from hand-held joysticks (1 or 2)
into CPU data. Each joystick provides one or two push-buttons and
X, Y position for a total of four bits each. Two joysticks can be
used.
The joystick handle is connected to two potentiometers mounted
perpendicular to each other; one for X position, one for Y
position. Through the cable, the main logic board applies +5 VDC
to one side and ground to the other of the pots. The pot wiper is
the position signal: a voltage between 0 and
+5
VDC. This signal
is applied to one input of a comparator HU2. The other comparator
input is the reference signal (a ramp between 0.0 to +5.0 volts).
When the position signal is equal to or less than the reference
signal, the comparator output goes true. This comparator output
is the X or Y position data bit. The ramp is reset to 0.0 VDC
whenever an
1/0
Write is made at Port 200/201 Hex. The joystick
information is "read" by the CPU at Port 200/201 Hex through U40.
16