Sram And Rtc Back Up Circuit - Panasonic KX-F900 Service Manual

KX-F900
3-3. SRAM and RTC BACK UP CIRCUIT
(1)Function
This unit has a lithium battery (BATT), which works tor the RAM (IC3) and Real Time Clock IC (RTC,lntegrated into
ASIC:IC1 ). The user parameter of autodial 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, back up by the lithium battery.
(2)Circuit Operation
When the power switch is turned ON, thus supplying the power through the IC9 to the RAM (IC3) and RTC.
At this time, the voltage at pin 28 of the RAM and pin 43 of the RTC are +5 V. When the power switch is turned OFF,
the BATT supplies the power to the RAM and RTC through the J1, R76 and IC9. At the time, the voltage at pin 28 of
the RAM and pin 43 of the RTC are about +2.5 V. When the power switch is OFF and the voltage of +5 V goes down,
the Reset IC. (IC9) outputs the reset signals. Pin 28 of the RAM (IC3) and pin 43 of the RTC become low level, then
the RAM and RTC go into the back up mode, when the power consumption is less.
Circuit Diagram
+5V
C64 1 1
I
I
I
I
IC9
VOLTAGE J_l
DETECTION
VCC O
"
. _1
R76
BATT
5V/BATT 28
ICl
43
----q
RTC
48
--C41
,,. C421
_° _L ', H
x _Tq4 J
47 - i J_7
SRAM
IC403
vcc
C€
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