PCF2127ATSLASH1 datasheet(18/80 Pages) NXP

18 / 80 page

PCF2127A_2

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Product data sheet

Rev. 02 — 7 May 2010

18 of 80

NXP Semiconductors

PCF2127A

Integrated RTC, TCXO and quartz crystal

8.6.1 Battery switch-over function

The PCF2127A has a backup battery switch-over circuit which monitors the main power

supply VDD and automatically switches 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.

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 8.13.7).

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 8.12.4).

4. The battery switch flag BF is cleared via the interface; 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

8.6.1.1

Standard mode

If VDD > VBAT OR VDD >Vth(sw)bat the internal power supply is VDD.

If VDD < VBAT AND VDD <Vth(sw)bat the internal power supply is VBAT.

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