Service Manual Part 2

CIRCUIT DESCRIPTION
4-12
September 2001
Part No. 001-5100-001
4.5.3 MICROCONTROLLER
The microcontroller is a Motorola
M68HC08XL36 chip. It includes 28K bytes of internal
ROM memory and 1K byte of internal SRAM. It does
not have an external bus and therefore cannot access
any external program memory.
The clock to the microcontroller is provided by
Y1 and an internal oscillator. The frequency of the
clock can be slightly offset by polarizing the base of
Q3 through software control. This prevents RF inter-
ference on some channels caused by the clock.
The microcontroller contains an SPI-compatible
synchronous serial bus. This bus consists of pins
MISO (U1-53), MOSI (U1-52), SPSCK (U1-50), and
a chip enable for each device with which it communi-
cates. The devices which communicate with the
microcontroller through this bus are as follows:
• Top Display driver chip (Top Display board)
• Front Display driver chip (Digital board)
• ADSIC chip (Digital board)
• Reference Oscillator (RF Board)
• Front-End DAC (RF Board)
• Synthesizer chip (RF Board)
• Optional DES board.
The microcontroller communicates with the DSP
chip (Digital board) through a custom serial bus. This
serial port includes pins PTA3 (U1-8), PTA4 (U1-9),
PTA5 (U1-10), PTA6 (U1-11), and PTA7 (U1-12).
The microcontroller uses its SCI asynchronous
serial bus for external communication with a computer
running programming or tuning software. The SCI
pins RxD (U1-42) and TxD (U1-43) are connected to
RS232 driver receiver U5. The other signals of a stan-
dard RS232 computer port (DSR, DTR, CTS, RTS)
are generated using microcontroller input/outputs.
The RS232 driver U5 converts signals from a
logic level of 0 and 5 V to a logic level of –10 and
+10V. The chip contains an internal charge pump to
generate –10V and +10V from the 5V power supply.
The RS232 chip can be put in standby mode by
leaving the line K/F-RS232* floating. This line is
connected to the side connector which allows it to turn
on U5 only when a computer is connected to the radio.
The keypad interfaces with the microcontroller
through eight lines (4 rows x 4 columns). The micro-
controller regularly polls these lines to detect a key
closure.
Serial EEPROM U3 is used to store some impor-
tant radio parameters. The EEPROM is read to or
written from using I/O lines PTC6 and PTC7 of the
microcontroller. PTC6 is used for the Data line, and
PTC7 is used as a clock line.
Shift register U14 expands the number of I/O
lines of the microcontroller. It uses the same data and
clock as the EEPROM plus an additional line (U1-45)
to control the latch. Other user interface inputs such as
the PTT and toggle switches are directly connected to
an I/O line of the microcontroller.
4.5.4 LOW VOLTAGE DETECT
Voltage comparator U4 detects a low voltage
condition and communicates this information to the
microcontroller through the pin PTC5 (U1-30). The
microcontroller can also detect through I/O IRQ2*
(U1-62) that a battery is connected.
4.5.5 LCD DISPLAYS AND DISPLAY DRIVERS
The radio has two displays which each have eight
characters, several icons and a backlight. One display
is located on the Keypad Board while the other display
is located on the Top Display Board. Each display is
driven by its own driver. The drivers are programmed
by the microcontroller through the SPI bus. Both
display drivers are connected to the common PI bus
but are individually addressable so that the displays
may show different information at the same time.
4.5.6 AUDIO CIRCUITS
The audio circuits on the Keypad/Display Board
consist of four op amps, two audio power amplifiers,
and an analog switch.
In receive mode, the analog receive waveform
created by the ADSIC (on the Digital Board) is fed to
an op amp summing amplifier (U9B). This amplifier
sums in the audio tones that are generated by the
microcontroller. The output of the summing amplifier
KEYPAD BOARD (CONT’D)