Panasonic KX-FT901CX-B, Service Manual

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8.3.6. SRAM and RTC BACK UP CIRCUIT
1. Function
This unit has a lithium battery (BAT1) which works for the RAM (IC4) 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 (IC4).
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 RAM (IC4) and RTC (IC1).
At this time, the voltage at pin 32 of RAM and pin 14 of RTC (IC1) is +3.3V. When the power is turned OFF, the battery supplies
the power to RAM and RTC through J1, R48, D3. At that time, the voltage at pin 32 of RAM and pin 14 of IC1 are about +2.5V.
When the power is OFF and the +5V and +3.3V voltages decrease, IC3 detects them and LOW is input to pin 114 of IC1. Pin
111 of IC1 outputs the reset signals. Pin 32 of RAM (IC4) and pin 11 of IC1 become low, then RAM and RTC (IC1) go into the
back up mode, when the power consumption is lower.
111
KX-FT901CX-B / KX-FT903CX-B / KX-FT901CX-W / KX-FT903CX-W / KX-FT901CX-S / KX-FT903CX-S