PCF2131 datasheet(21/85 Pages) NXP

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NXP Semiconductors

PCF2131

Nano-power highly accurate RTC with integrated quartz crystal

When VBAT drops below the threshold value Vth(bat)low (typical 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 the supply voltage from dropping below Vlow (typical 1.2 V) and with that

the data integrity gets lost. (For further information about Vlow see Section 7.6.)

When VBAT drops below the threshold value Vth(bat)low, the following sequence occurs (see Figure 7):

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 7.13.7).

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.

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internal operating voltage (Voper(int))

VBAT

BLF

Vth(bat)low

(= 2.5 V)

VBAT

VDD = Voper(int)

INT

Figure 7. Battery low detection behavior with bit BLIE set logic 1 (enabled)

7.5.3 Battery backup supply

The VBBS voltage on the output pin BBS is at the same potential as the internal operating voltage Voper(int),

depending on the selected battery switch-over function mode:

Battery switch-over function mode

Conditions

Potential of

Voper(int) and VBBS

VDD > VBAT OR VDD > Vth(sw)bat

VDD

standard

VDD < VBAT AND VDD < Vth(sw)bat

VBAT

VDD > VBAT

VDD

direct switching

VDD < VBAT

VBAT

disabled

only VDD available,

VBAT must be put to ground

VDD

Table 23. Output pin BBS

PCF2131

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Product data sheet

Rev. 2.0 — 5 October 2023

21 / 85

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