PCF2129A datasheet(19/69 Pages) NXP

19 / 69 page

PCF2129A_2

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Product data sheet

Rev. 02 — 7 May 2010

19 of 69

NXP Semiconductors

PCF2129A

Integrated RTC, TCXO and quartz crystal

The output pin BBS can be used as a supply for external devices with battery backup

needs, such as SRAM (see Ref. 3 “AN10857”). For this case, Figure 7 shows the typical

driving capability when VBBS is driven from VDD.

8.5.3 Battery low detection function

The PCF2129A 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 therewith the data integrity gets lost.

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

Figure 8):

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

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

the battery is replaced.

Fig 8.

Battery low detection behavior with bit BLIE set logic 1 (enabled)

001aaj322

internal power supply (= VBBS)

VBAT

BLF

Vth(bat)low

(= 2.5 V)

VBAT

VDD = VBBS

INT

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