PCF2129 datasheet(20/86 Pages) PHILIPS

20 / 86 page

PCF2129

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Product data sheet

Rev. 7 — 19 December 2014

20 of 86

NXP Semiconductors

PCF2129

Accurate RTC with integrated quartz crystal for industrial applications

8.5.1.4

Battery switch-over architecture

The architecture of the battery switch-over circuit is shown in Figure 8.

Voper(int) is at VDD or VBAT potential.

Remark: It has to be assured that there are decoupling capacitors on the pins VDD, VBAT,

and BBS.

8.5.2 Battery low detection function

The PCF2129 has a battery low detection circuit which monitors the status of the battery

VBAT.

When VBAT drops below the threshold value Vth(bat)low (typically 2.5 V), the BLF flag

(register Control_3) is set to indicate that the battery is low and that it must be replaced.

Monitoring of the battery voltage also occurs during battery operation.

An unreliable battery cannot prevent that the supply voltage drops below Vlow (typical

1.2 V) and with that the data integrity gets lost. (For further information about Vlow see

Section 8.6.)

When VBAT drops below the threshold value Vth(bat)low, the following sequence occurs (see

Figure 9):

1. The battery low flag BLF is set logic 1.

2. An interrupt is generated if the control bit BLIE (register Control_3) is enabled

(see Section 8.12.7).

3. The flag BLF remains logic 1 until the battery is replaced. BLF cannot be cleared by

command. It is automatically cleared by the battery low detection circuit when the

battery is replaced or when the voltage rises again above the threshold value. This

could happen if a super capacitor is used as a backup source and the main power is

applied again.

Fig 8.

Battery switch-over circuit, simplified block diagram

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