Panasonic KX-FT982LS-B Service Manual page 19

6.3.5. RTC Back up Circuit
1. Function
This unit has a lithium battery (BAT1) which works for the Real Time Clock (RTC, Integrated into ASIC:IC1).
The RTC continues functioning, even when the power switch is OFF, backed up by a lithium battery.
2. Circuit Operation
When the power is turned ON, power is supplied to RTC (IC1).
At this time, the voltage at pin 14 of RTC (IC1) is +3.3V. When the power is turned OFF, the battery supplies the power to RTC
through JS1, R30, D14. At that time, the voltage at pin 14 of IC1 are about +2.5V. When the power is OFF and the +5V and
+3.3V voltages decrease, the LOW is input to pin 114 of IC1. Pin 111 of IC1 outputs the reset signals. Pin 11 of IC1 become
low, then RTC (IC1) go into the back up mode, when the power consumption is lower.
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KX-FT982LS-B / KX-FT984LS-B / KX-FT988LS-B