Panasonic KX-FLM652CX Service Manual page 167

7.3.2. RTC BACKUP CIRCUIT
1. Function
This unit has a lithium battery (BAT600) which works for the Real Time Clock IC (RTC: inside IC604).
The RTC continues to work, backed up by a lithium battery even when the power switch is OFF.
2. RTC Inside (IC604) Backup Circuit Operation
When the power switch is turned ON, power is supplied to the RTC (inside IC604). At this time, the voltage at pin 128 of the
IC604 is +3.3V. When the power switch is turned OFF, the BAT600 supplies power to RTC through DA600.
When the power switch is OFF and the voltage of +3.3V decreases, pin 128 of RTC (IC604) 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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KX-FLM652CX