PCF2129 datasheet(17/86 Pages) PHILIPS

17 / 86 page

PCF2129

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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 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 8.12.6).

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 I2C-bus communication and battery backup operation, see

Section 9.3 on page 56.

PCF2129 Fiches technique(PDF) 17 Page - NXP Semiconductors