Datasheet

PCF2129 All information provided in this document is subject to legal disclaimers. © NXP Semiconductors N.V. 2014. All rights reserved.
Product data sheet Rev. 7 — 19 December 2014 17 of 86
NXP Semiconductors
PCF2129
Accurate RTC with integrated quartz crystal for industrial applications
8.5.1 Battery switch-over function
The PCF2129 has a backup battery switch-over circuit which monitors the main power
supply V
DD
. When a power failure condition is detected, it automatically switches to the
backup battery.
One of two operation modes can be selected:
Standard mode — the power failure condition happens when:
V
DD
< V
BAT
AND V
DD
<V
th(sw)bat
V
th(sw)bat
is the battery switch threshold voltage. Typical value is 2.5 V. The battery
switch-over in standard mode works only for V
DD
> 2.5 V
Direct switching mode — the power failure condition happens when V
DD
< V
BAT
. Direct
switching from V
DD
to V
BAT
without requiring V
DD
to drop below V
th(sw)bat
When a power failure condition occurs and the power supply switches to the battery, the
following sequence occurs:
1. The battery switch flag BF (register Control_3) is set logic 1.
2. An interrupt is generated if the control bit BIE (register Control_3) is enabled
(see Section 8.12.6
).
3. If the control bit BTSE (register Control_3) is logic 1, the timestamp registers store the
time and date when the battery switch occurred (see Section 8.11.4
).
4. The battery switch flag BF is cleared by command; it must be cleared to clear the
interrupt.
The interface is disabled in battery backup operation:
• Interface inputs are not recognized, preventing extraneous data being written to the
device
• Interface outputs are high-impedance
For further information about I
2
C-bus communication and battery backup operation, see
Section 9.3 on page 56
.