PCF2131 datasheet(22/85 Pages) NXP

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NXP Semiconductors

PCF2131

Nano-power highly accurate RTC with integrated quartz crystal

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

(see [1]).

7.6 Oscillator stop detection function

The PCF2131 has an on-chip oscillator detection circuit which indicates the status of the oscillation by

monitoring the supply of oscillator: whenever the supply is out of the expected range, a reset occurs and the

oscillator stop flag OSF (in register Seconds) is set logic 1.

• Power-on:

1. The oscillator is not running, the chip is in reset (OSF is logic 1).

2. When the oscillator starts running and supply is OK after power-on, the chip exits from reset.

3. The flag OSF is still logic 1 and can be cleared (OSF set logic 0) by command.

• Power supply failure:

1. When the power supply of the chip drops below a certain value (Vlow), typically 1.2 V, the oscillator supply

also fails and a reset occurs.

2. When the power supply returns to normal operation, the oscillator supply is OK again, the chip exits from

reset.

3. The flag OSF is still logic 1 and can be cleared (OSF set logic 0) by command.

4. When OSF flag is cleared an OTP refresh should be performed (see Section 7.3.2).

001aaj409

(1)

Vth(sw)bat

(= 2.5 V)

VSS

VBAT

Vlow

(= 1.2 V)

VDD

Voper(int)

VDD

VBAT

VSS

(2)

OSF

battery discharge

VDD

1. Theoretical state of the signals since there is no power.

2. The oscillator stop flag (OSF), set logic 1, indicates that the oscillation has stopped and a reset has occurred since the

flag was last cleared (OSF set logic 0). In this case, the integrity of the clock information is not guaranteed. The OSF

flag is cleared by command.

Figure 8. Power failure event due to battery discharge: reset occurs

7.7 Power-On Reset function

The PCF2131 has a Power-On Reset (POR) and a Power-On Reset Override (PORO) function implemented.

7.7.1 Power-On Reset (POR)

The POR is active whenever the oscillator is stopped. The oscillator is considered to be stopped during the time

between power-on and stable crystal resonance (see Figure 9). This time may be in the range of 200 ms to 2 s

PCF2131

All information provided in this document is subject to legal disclaimers.

Product data sheet

Rev. 2.0 — 5 October 2023

22 / 85

PCF2131 Datasheet(PDF) 22 Page - NXP Semiconductors

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