15 Battery backup switchover function
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ETM50E-07 Seiko Epson Corporation 43
3) SMPTSEL1, SMPTSEL0 bit
V
DD
voltage detection: V
DET
1 (reset), V
DET
2 (voltage down):
Battery backup detection (V
DET
2) is done constantly and SW1 is opened periodically for V
DD
measurement.
During SW1:ON, external voltage to V
DD
cannot be measured precisely because RTC internal voltage is
leaked to V
DD
external voltage supply circuit even external circuit is down. So periodical SW1 = OFF
makes disconnection between RTC internal voltage and external voltage supply, thus V
DD
external voltage
is measured precisely. V
DD
external voltage drop moves to backup mode.
See Figure 30 V
DD
detection (V
DET
2) timing.
V
BAT
voltage detection: V
DET
3 (V
BAT
full charge), V
DET
4(V
BAT
over discharge)
In case of normal mode, V
BAT
voltage is precisely and periodically detected with condition (SW1 OFF,
SW2 OFF). The detection period is determined by combination of SMPTSEL1, SMPTSEL0.
See Figure 31 for VDET3, V
DET
4 voltage detection timing)
(In case of CHGEN = 0, There is no charge operation SW2 = OFF)
Table 36 Voltage detection timing
* In normal mode, when operating the RTC on VDD-supply, SW1 has to be opened (Off) to perform the V
DD
voltage
detection for below mentioned times
Table 37 VDET3, VDET4 intermittent detection period
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