PCF8523TSSLASH1 datasheet(16/75 Pages) NXP

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PCF8523

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Product data sheet

Rev. 4 — 5 July 2012

16 of 74

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

VDD 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:

VDD < VBAT AND VDD <Vth(sw)bat

• Direct switching mode: the power failure condition happens when VDD < VBAT.

Direct switching from VDD to VBAT without requiring VDD to drop below Vth(sw)bat

Vth(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

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 VDD. 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

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