Panasonic KX-FP81BX Service Manual page 134

KX-FP81BX / KX-FP81CX / KX-FP82CX
6.3.8. SRAM AND RTC BACKUP CIRCUIT
1. Function
This unit has a lithium battery (BAT501) which works for the 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 BAT501 supplies power to the SRAM through J501,
R541 and D504. 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 BAT501 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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