PCF2127TSLASH2Y datasheet(19/100 Pages) PHILIPS

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PCF2127

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Product data sheet

Rev. 8 — 19 December 2014

19 of 100

NXP Semiconductors

PCF2127

Accurate RTC with integrated quartz crystal for industrial applications

8.6.1 Battery switch-over function

The PCF2127 has a backup battery switch-over circuit which monitors the main power

supply VDD. 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:

VDD < VBAT AND VDD <Vth(sw)bat

Vth(sw)bat is the battery switch threshold voltage. Typical value is 2.5 V. The battery

switch-over in standard mode works only for VDD > 2.5 V

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

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 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 I2C-bus communication and battery backup operation, see

Section 9.3 on page 70.

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