Service manual

2-4
2.4.1 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.
2.4.2 Circuit Description
The RTC module circuit, shown in Figure 2-4, is powered by the MODB/VSTBY pin and PI6/PI7 from
the crystal oscillator circuit. A clock frequency of 38.4kHz from a crystal oscillator provides the
reference signal which is divided down to 1Hz 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 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 3.3V regulator charges the Lithium battery.
Figure 2-4: RTC Circuit
2.4.1 MODB/VSTBY Supply
The supply to the MODB/VSTBY pin varies depending on the conditions listed in Table 2-4.
Table 2-4 MODB/VSTBY Supply Modes
Condition Circuit Operation
Radio On Vddd supply voltage via CR411
Radio Off • Vddd turned off
• Q416 gate pulled low by R462
• Q416 switched on
• U410 supplies 3.2V to MODB_VSTBY
Primary battery removed • Vddd turned off
• Q416 gate pulled low by R462
• Q416 switched on
• Lithium battery provides 3.2V to MODB_VSTBY
TP405
TEST_POINT
BOOT_CTRL
R463
LI_ION
R462
R419
3
4
2
15
CR411
1
2
3
Q416
C434
U410
3.3V
3
2
1
VIN
VOUT
VSS
UNSWB+
R460
C435
R461
Vddd
HC11FL0
MODA
MODB
R420
R426
FL401
C436
C437
PI6
PI7
OUT
IN
GND
38.4kHz