PCF2123 datasheet(21/86 Pages) NXP

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NXP Semiconductors

PCA2131

Nano-power highly accurate RTC with integrated quartz crystal for automotive applications

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:

PCA2131

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Product data sheet

Rev. 1.0 — 26 July 2021

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