PCF2127ATSLASH1 datasheet(21/80 Pages) NXP

21 / 80 page

PCF2127A_2

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Product data sheet

Rev. 02 — 7 May 2010

21 of 80

NXP Semiconductors

PCF2127A

Integrated RTC, TCXO and quartz crystal

8.6.3 Battery low detection function

The PCF2127A 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 8.13.8).

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 7.

Typical driving capability of VBBS: (VBBS − VDD) with respect to the output load

current IBBS

IBBS (mA)

001aaj327

−400

−600

−200

VBBS − VDD

(mV)

−800

VDD = 4.2 V

VDD = 3 V

VDD = 2 V

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