Panasonic KX-FT21RS - Manual (Page 106)

6.3.6. SRAM and RTC BACK UP CIRCUIT
1. Function
This unit has a lithium battery (BAT1) which works for the RAM (IC3) and Real Time Clock (RTC, Integrated into ASIC:IC1).
The user parameter for auto dial numbers, the transmission ID, the system setup date and so on are stored in the RAM (IC3).
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 RAM (IC3) and RTC (IC1).
At this time, the voltage at pin 28 of RAM is +5V and pin 3 of RTC (IC1) is +3.3V. When the power is turned OFF, the battery
supplies the power to RAM and RTC through J1, R14,D1 or D7. At that time, the voltage at pin 28 of RAM and pin 3 of IC1 are
about +2.5V. When the power is OFF and the +5V and +3.3V voltages decrease, IC4 detects them and LOW is input to pin 17
of IC1. Pin 16 of IC1 outputs the reset signals. Pin 28 of RAM (IC3) and pin 3 of RTC (IC1) become low, then RAM and RTC
(IC1) go into the back up mode, when the power consumption is lower.
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KX-FT21RS