Panasonic KX-FM89BX, Service Manual, Page: 175 / 276

SRAM (IC504) and Real Time Clock IC (RTC: inside IC501).
The user parameters for autodial numbers, the system setup data
and others are stored in the SRAM (IC504).
The RTC continues to work, backed up by a lithium battery even
when the power switch is OFF.
2. SRAM (IC504) Backup Circuit Operation
When the power switch is turned ON, power is supplied through
pin 12 of IC501 to the SRAM (IC504). At this time, the voltage at
pin 28 of the SRAM is 5V. When the power switch is turned OFF,
the BATT supplies power to the SRAM through J501, R543 and
D508. The voltage at pin 28 of the SRAM is about +2.5V. When the
power switch is OFF and the voltage of +5V decreases, the
voltage detect IC (IC507) outputs "Low " level and the IC501
outputs the reset signal. Pin 28 of the SRAM becomes roughly the
same voltage as the battery voltage. At this point, pin 20 (CS) of
IC504 becomes high level, causing the SRAM to go into the
backup mode, in which the power consumption is lower.
3. RTC Inside (IC501) Backup Circuit Operation
When the power switch is turned ON, power is supplied through
DA501 to the RTC (inside IC501). At this time, the voltage at pin 7
of the IC501 is +3.3V. When the power switch is turned OFF, the
BATT supplies power to RTC through DA501. Thevoltage at pin 7
of IC501 is about +2.5V. When the power switch is OFF and the
voltage of +3.3V decreases, pin 7 of RTC (IC501) becomes roughly
the same voltage as the battery voltage. RTC goes into the backup
mode, in which the power consumption is lower.
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