Figure 3-3. Rtc Circuit - Motorola HT Series Detailed Service Manual

Keypad: Controller Board
3.1.2 Real Time Clock
Radios with displays support a real time clock (RTC) module for purposes of message time stamping
and time keeping. The RTC module resides in the microcontroller. The clock uses a back-up lithium-
ion battery for operating power when the primary battery is removed.
3.1.3 Circuit Description
The RTC module circuit, shown in
the crystal oscillator circuit. A clock frequency of 38.4 kHz from a crystal oscillator provides the
reference signal which is divided down to 1 Hz in the processor.
As the RTC module is powered separately from the processor Vdd, the RTC is kept active through
the MODB/VSTBY pin which provides the lithium-ion battery back-up power when the radio is
switched off.
A MOSFET transistor (Q416) switches in the battery supply when Vdd is removed. Q416 also
provides isolation from BOOT_CTRL function. The 3.3 V regulator charges the lithium-ion battery.
HC11FL0
MODA
MODB
PI6
R420
PI7
3.1.4 MODB/VSTBY Supply
The supply to the MODB/VSTBY pin varies depending on the conditions listed in
Radio On
Radio Off
Primary battery removed
Figure
FL401 C436
OUT
38.4kHz
GND
IN
R426
C437

Figure 3-3. RTC Circuit

Table 3-2. MODB/VSTBY Supply Modes
Condition
Vddd supply voltage via CR411
• Vddd turned off
• Q416 gate pulled low by R462
• Q416 switched on
• U410 supplies 3.2V to MODB_VSTBY
• Vddd turned off
• Q416 gate pulled low by R462
• Q416 switched on
• Lithium-ion battery provides 3.2V to MODB_VSTBY
3-3, is powered by the MODB/VSTBY pin and PI6/PI7 from
Vddd
R461 C435
CR411
1
Q416
2
3 2
1
R462
R463
BOOT_CTRL
TP405
TEST_POINT
Circuit Operation
UNSWB+
U410
3.3V
2
3
VIN VOUT
R460
5
4
R419
3
LI_ION
Table
3-3
C434
3-2.