Tandy 1000
Technical Reference Manual
One half of
U1
(a
dual D-type flip-flop)
is
used
in
a "divide by
2"
configuration.
The 14.32 MHz oscillator signal from the 1000 interface bus
is
divided
in
half
for a clock speed of 7.14 M
Hz,
to be used by the 8042 processor.
The
7.14
MHz clock
is
at
Pin
5 of U1.
The other half of
U1
is
used as a reset latch. A reset signal
is
provided for
the 8042 processor by ORing together the System Reset signal from the com-
puter interface bus and the signal from the reset latch
at
Pin
11
of
U2.
The
8042 processor can be reset by a system reset or a write to I/O port 2FF. A
write to I/O port 2FD clears the reset signal.
8042 Processor
In
the heart of the option board
is
the 8042 processor
(U3).
This processor
acts
as
an
input port for serial information from the DIGI-Mouse and the
Clock/Calendar chip
(U8).
It
then translates this information to a parallel for-
mat and controls
its
transfer to the computer interface bus.
The
Clock/
Calendar option board uses interrupt request
IR3
to inform the 1000 when
it
is
ready to transfer data.
DIGI-Mouse Buffers and Filters
A 9-Pin DB jack
at
J2 connects the DIGI-Mouse to the clock/calendar board.
RC
filtering
is
used to reduce noise
in
the inputs. U4
(a
CMOS hex schmitt
trigger) provides further buffering and waveshaping of the DIGI-Mouse inputs,
which are interfaced to the processor chip through its peripheral port bits
P1
0
to P16.
Clock Chip
The Phillips-Signetics Clock/Calendar Chip (U8) interfaces directly to U3
at
its
peripheral port bits P17, P20-P23, and P27. The time base for the clock ship
is
a 32.768KHz crystal, which
is
similar to that found
in
watches. The battery
MUST be installed for the clock function to work. When a power failure oc-
curs, the chip indicates this by sending a low-battery signal
(POWF)
to the 8042
processor.
Theory
of
Operation (Software)
User specifications:
• The motion sensor
is
oriented with the connecting cable and buttons
pointing away from the user, the positive x axis extends to the right and
the positive y axis extends toward the user.
• Maximum motion allowed before
it
is
detected
is
1/80 of
an
inch along
either axis. This distance
is
called a mickey.
• The minimum unit of time for the Clock/Calendar chip
is
1 minute. Max-
imum accuracy
is
±
1/2
minute. The units of time accepted by the chip
are minutes,
(24)
hours, day of month, and month.
92
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