Real Time Clock; Circuit Description; Figure 2-4:Rtc Circuit; Modb/Vstby Supply - Motorola Professional Radio 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.
HC11FL0
MODA
MODB
PI6
R420
PI7
2.4.1

MODB/VSTBY Supply

The supply to the MODB/VSTBY pin varies depending on the conditions listed in Table 2-4.
Radio On
Radio Off
Primary battery removed
FL401 C436
OUT
38.4kHz
GND
IN
R426
C437
Figure 2-4: RTC Circuit
Table 2-4 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 battery provides 3.2V to MODB_VSTBY
Vddd
R461 C435
R460
CR411
1 5
Q416
4
3 2 2 3
1
R462
R463
BOOT_CTRL
TP405
TEST_POINT
Circuit Operation
UNSWB+
U410
3.3V
2 3
VIN VOUT
C434
R419
LI_ION