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4
DISPLAY CIRCUIT BOARD
i
EEPROM
CIRCUIT
!
501,502,504
GATED
5-VOLT
REGULATOR
SWITCHED
CONTROL
DATA
ADORES
CRYSTAL
916
41-45
to!
FAST
SQUELCH
MICROCOMPUTER
31
(COPE)
CK,
DATA
LCD
J
FROM
MAIN
46-53
20,23
|
RADIOBD.
8
0506
BIAS
SWITCH
a
t
1
MKi
keveap
FROM
MAIN
RADIO BOARD
=
MAEPF-19702-0
SWITCHED
LCD BACKLIGHT
13
8-BIT
SERIAL
SHIFT REGISTER
LIQUID
DISPLA
DATA.
BUSY
24,37
DTMF
15
GENERATOR
Figure
3.
Display Circuitry Block Diagram
The
COPE
microcomputer communicates with the
radio board
over the DATA and
BUSY
lines. Both lines
are wired-or;
that is,
any processor can force the lines
to
a
logic low state (0 volts),
but not to
a
logic high (+5
volts).
This is
accomplished by using a 10kQ
"pull-up"
resistor
on
each
line.
These
resistors are located
on
the radio board and are connected to #2 regulated B+.
When
the
COPE
(or
any
other
processor) sends a
low
over the DATA
or
BUSY
line,
it
forces the
line to
the low state by sinking current through the line's out-
put pin. To
send a high, the processor switches the
output pin to the high
impedance state (open), and the
pull-up resistor
causes
the line to
go
high
(as
long
as
no
other
processor is forcing
it
low). Normally the
DATA
and
BUSY
lines will be
in
the high state.
Bus messages are indicated by 9600-baud data
on
the
DATA
line,
accompanied
by
a logic
low on the
BUSY
line.
A
constant
low on either line
indicates a
problem that could
be either hardware
or incorrect pro-
gramming of one of the microcomputers. To prevent
degradation
of
receiver performance, inductors
L501
and
L503,
and
capacitor C507 filter
out computer
"hash" interference
from
the
DATA
and
BUSY
lines.
The
COPE
gets
its
+5V
power (#2 regulated
5V
from
the radio board) through
pin
28. Inductor
L504
and
capacitor C501 provide filtering. The
COPE's
RESET
line,
pin
17, is
connected
to
the radio board's
RESET
line
via
filter
L502
and
pin
5 of the
LCD
inter-
connect flex. Whenever the
RESET
line
goes
low,
then high
again, the
COPE
reinitializes itself, briefly
turning on all
segments
of the display.
Components
Y502, C505, C506,
and
R528 are
the
external elements of the microcomputer
clock
circuit.
The
resulting
3.6864 MHz oscillator signal
is divided by
four inside the
COPE,
and becomes the internal clock.
14
Pins 30
through
40 are
control lines for the
EEP-
ROM, U501. These
lines are normally at
a
logic high
level unless the
COPE
is
accessing
data
from
the
EEPROM.
None
of
these lines should ever be at a
constant
low level.
Pin 32 is a power strobe for
EEPROM
U501;
power strobing is used to reduce current drain.
The
strobe signal controls the regulator circuit, which con-
sists
of
Q501, Q502, Q504, R503, and C502. When
pin
32 is high, the switched B+
on the emitter of
Q502
is regulated down
to
+5V
and applied
to pin 16
(Vdd)
of
EEPROM
U501. When
pin
32 is
low,
the voltage on
U501,
pin 16,
is reduced
to
OV.
Normally, pin
32
will
be
low;
it
goes
high only when data is being
accessed
from
the
EEPROM.
On power-up,
a series
of power
pulses, lasting
as
long
as a
second
or more,
are sent
from pin
32
as
the
COPE
reads and validates data
in
the
EEPROM.
Pins
9 through 16
make
up
a
bidirectional data bus
between the
COPE
and the
EEPROM. These
lines are
normally at
a
logic low unless data is being
accessed.
Pins
41
through 45 are output lines
from
the
COPE,
and
form the lower
five bits
of the
EEPROM
address. The upper eight
EEPROM
address
bits
(six
bits for
a 2k
board) come
from
U503,
an 8-bit serial-to-
parallel shift register.
These
address
bits are
sent
from
the
COPE
over
the serial peripheral interface
(SPI)
bus (pins 22, 25, and 26) at 57.6 kilobaud.
Pin
31
is the control line for the
LCD
backlight.
Two yellow-green
LEDs,
CR501
and
CR502,
make
up
the backlight.
These
LEDs
are driven by
a constant-
current
source consisting
of dual-diode
CR503,
resis-
tors R501 and
R504,
and transistor Q503.
The
current
through the
LEDs
(about 20 mA) is drawn
from
the
switched B+ supply.
The
current remains constant for
battery
voltages greater
than six volts.
B+
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