Service manual

CIRCUIT DESCRIPTION
5-5
April 2001
Part No. 001-9800-203
Temporary data storage is provided by an internal
1K x 8 RAM in microcontroller U101 and with high
tier and data models, by external 8K x 8 RAM U107.
These devices are used as a “scratchpad” during
program execution.
Reset
The microcontroller resets when power is turned
on and also when the 5-volt supply drops below the
normal range. Reset clears several internal registers
and restarts the operating program. This prevents
improper operation which may result during low -
voltage conditions.
The microcontroller resets itself automatically for
4064 clock cycles when power is applied to the VCC
input. Low-voltage reset is triggered by low-voltage
sensor U100. When the 5-volt supply drops to approx-
imately 4.25 volts, the RESET output goes low. This
resets the microcontroller and also inhibits operation
for as long as it is low. The microcontroller also has
internal reset circuits which trigger reset if problems
occur with the clock signal, illegal op codes, or the
watchdog timer circuit.
Reset is also triggered when the transceiver is
Flash programmed. A low pulse is created by C363
and R421 when the MODA/MODB inputs of the
microcontroller are pulled low to initiate this program-
ming. This automatically places the microcontroller in
the flash programming mode. However, reset does not
occur when flash programming is complete, so power
must be turned off and then on again to resume normal
operation.
Data Bus
A bi-directional data bus consisting of D0-D7 is
used to transfer data in and out of the microcontroller.
It is used to transfer parallel data in and out of memory
chips U107 (high tier only) and U108, and also
program latches U110-U112. The logic level on the
R/W
pin determines the direction of data on the data
bus. If it is high, data is read into U101, and if it is low,
data is written out. The E output goes high to indicate
when data on the data bus is valid or when an external
device can place data on the data bus.
Address Bus
The address bus consisting of A0-A15 is used for
addressing the memory location in U107 or U108 that
data is being written to or read from. In addition, it
provides chip select signals to latches U110-U112.
Refer to Section 5.3.2 for more information on
memory and I/O addressing.
A/D Converter Inputs
VRH/VRL - These inputs provide the reference volt-
ages for the A/D converter circuitry. R115 and C105
attenuate noise present in the 5-volt supply applied to
VRH.
PE0 - RSSI (Receive Signal Strength Indicator) input
from limiter/detector U201 in the receiver. This signal
is used along with the squelch signal to determine
when valid data may be present and when to unmute
the receive audio.
PE1 - Battery voltage input. The switched 13.6-volt
supply is divided down by R161 and R168 to provide
a 0-5 volt input. If the battery voltage is excessively
high, the transmitter is disabled.
PE2 - Power amplifier temperature input from ther-
mistor R601 on the PA board. The DC voltage of this
signal decreases as temperature increases.
PE3 - Lock detect input from synthesizer IC U804. If
this signal is high (near 5 volts) the synthesizer is
locked on frequency (see Section 5.7.6).
PE4 - Power switch sense input. This input is high
when the power switch is on and low when it is off.
When the off condition is sensed, the microcontroller
saves the current settings and then powers down the
transceiver by turning Q110 off (see Section 5.2.1).
PE5 - Ignition switch sense input. This input is low
when the ignition switch is on and high when it is off.
The microcontroller senses the ignition switch to
control such features as the power-off delay and horn
alert.
PE6 - This input senses the voltage on the IN2 pin of
data modem connector J301 and Option 2 slot wire-out
W311.
AUDIO/LOGIC DESCRIPTION (ALL MODELS)