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The OPT B+ voltage powers external accessories
used with the radio.
C. B+ Routing and DC Voltage Distribution
(for an Open Architecture Controller and an
800 or 900MHz Transceiver)
This radio differs from previous Motorola portable
radios in that B+ from the battery is electrically switched
to most of the radio, rather than routed through the
on/off/volume switch, S403/R401. The electrical switch-
ing of B+ supports a "keep-alive" mode. Under software
control, even when the on/off switch has been turned to
the off position, power remains on.
Raw B+ (7.5V) from the battery (Batt B+) enters the
radio on the transceiver board through a 3-contact
spring pin arrangement (P404) as B+, where it is routed
through two ferrite beads (E102 and E101) and applied
to the RF PA and the ALC IC on pin 13. Battery B+ is
fused, and is then routed to the controller board, where
it enters on connecter J704 pins 1 and 20. From the
controller, BATT B+ fans out to three different areas: (1)
the secure or data option board via connector jack J702
pin 1, (2) the electrical switch IC, U712 pins 2 and 3,
and (3) the control-top flex via connector jack J703 pin
8. UNSW B+ is routed to the secure board so that it can
perform key management and other functions indepen-
dently of SW B+. UNSW B+ is routed to the electrical
switch IC, U712 (a P-channel FET in an SOIC-8 pack-
age), which connects it to SW B+ when the control
voltage at U712 pin 4 is low. SW B+ is then distributed
to the rest of the radio, including the transceiver board,
the display/keypad board, and the secure or data option
board, as well as other controller board circuitry. Finally,
UNSW B+ is routed to the mechanical ON/OFF switch
via connector jack J703 pin 8, and returns to the con-
troller as MECH SWB+ (J703 pin 10). This signal is
used to activate the electrical switch (U712), and also is
fed to a resistive divider so that the microprocessor
(U705) can monitor the battery voltage.
The electrical switch (U712) is activated by transis-
tor Q703, which in turn is driven by either the MECH
SWB+ or the B+ CNTL signals turning on one or both of
the diodes in CR704. Let us consider what happens
when the radio is initially off and all circuits are powered
down. When the user switches the ON/OFF switch to
the ON position, the MECH SWB+ signal will be con-
nected to UNSW B+ and transistor Q702 will then be
turned on. Transistor Q703 pin 3 will go low (< .3 V),
and this will turn on U712, which in turn connects
UNSW B+ to SW B+. The SW B+ will then be fed to all
the other radio circuitry, and the radio will begin its nor-
mal power-on sequence. In particular, the
microprocessor, U705, will initialize after regulated Vdd
from U708 reaches 5.0 V. It can then program the gate
array (U710) so that the B+ CNTL signal can be an out-
put high or low (initially this pin, U710-G8, is configured
as an input so that it does not drive diode CR704).
Recalling that SW B+ to the radio is controlled by
U712, which is activated by the B+ CNTL signal or
MECH SWB+ via CR704 and Q702, if the user turns off
the ON/OFF switch then MECH SWB+ drops to zero
volts. If the microprocessor has set B+ CNTL to logic
zero, then Q702's inverted output (pin 3) will be high,
and the power switch (U712) will turn off, and SW B+
will drop to zero. If, however, the controller is pro-
grammed to support power-down de-affiliation (typically
for a trunked system only), then it will have left B+ CNTL
at a logic high. In this case, when the ON/OFF switch is
turned off, SW B+ will continue to be supplied to the
radio, but the microprocessor will sense that the switch
has turned off by reading that the voltage on pin U705-
PE1 has fallen to zero. The microprocessor can then
key up the transmitter and send a de-affiliation ISW to
the trunking system. After receiving and verifying an
acknowledgement, the microprocessor then shuts down
SW B+ (and therefore, its own power, since Vdd comes
from SW B+ via U708) by setting B+ CNTL=0. In sum-
mary, we see that turning the ON/OFF switch ON
always supplies power to the radio circuitry, but the
radio can only power down when the switch is OFF and
the microprocessor has set B+ CNTL=0.
1. Low-Battery/ Detect Circuit (Controller Board)
The low-battery detect circuit is used to warn the
user that the radio's battery needs recharging.
The implementation of this function on open archi-
tecture radios takes advantage of the micro-
processor's on-chip 8-bit, 8-channel A/D converter
(pins PE0-PE7 of U705). The mechanically
switched 7.5V (MECH SWB+) is divided down to a
nominal 3.92 V by resistors R725 and R726 and
fed to port PE1 of U705. This voltage is converted
by the A/D to digital format. The microprocessor
compares this voltage to a preset low-battery trip
threshold, which corresponds to a battery voltage
~ =
of
7.0V in standby mode or
mode. If the measured digitized voltage is lower
than either low battery threshold, the low battery
alert tone or flashing icon is generated to warn the
user that only about 20 minutes of usable battery
capacity remains.
2. Power To/From External Accessories
The switched 7.5V also powers external acces-
sories used with the radio. The voltage is picked up
from the controller board and passed to the front
cover/display flex via connector jack J701 pin 16
(OPT B+/BOOT SEL). The front cover/display flex
then applies the voltage to pin 4 of the universal
connector, where it is picked up by external acces-
sories. Resistor R714, with a 1W power rating,
provides current limiting to the external circuit to
prevent internal damage should the external con-
nector short.
~ =
6.20V in transmit
11
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Troubleshooting
