Datasheet

PCF8523 All information provided in this document is subject to legal disclaimers. © NXP B.V. 2013. All rights reserved.
Product data sheet Rev. 6 — 17 September 2013 16 of 78
NXP Semiconductors
PCF8523
Real-Time Clock (RTC) and calendar
8.5.2 Battery switch-over function
The PCF8523 has a backup battery switch-over circuit. It monitors the main power supply
V
DD
and switches automatically to the backup battery when a power failure condition is
detected.
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
• 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
V
th(sw)bat
is the battery switch threshold voltage. Typical value is 2.5 V.
Generation of interrupts from the battery switch-over is controlled via the BSIE bit (see
register Control_2). If BSIE is enabled, the INT1
follows the status of bit BLF (register
Control_3). Clearing BLF immediately clears INT1
.
When a power failure condition occurs and the power supply switches to the battery, the
following sequence occurs:
1. The battery switch flag BSF (register Control_3) is set logic 1
2. An interrupt is generated if the control bit BSIE (register Control_3) is enabled
The battery switch flag BSF can be cleared by using the interface after the power supply
has switched to V
DD
. 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