PCF2129AT datasheet(19/68 Pages) NXP

19 / 68 page

PCF2129A_1

Product data sheet

Rev. 01 — 13 January 2010

19 of 68

NXP Semiconductors

PCF2129A

Integrated RTC, TCXO and quartz crystal

8.6 Oscillator stop detection function

The PCF2129A has an on-chip oscillator detection circuit which monitors the status of the

oscillation: whenever the oscillation stops, a reset occurs and the oscillator stop flag OSF

(in register Seconds) is set logic 1.

• Power-on:

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

b. When the oscillator starts running and is stable after power-on, the chip exits from

reset.

c. The flag OSF is still logic 1 and can be cleared (OSF set logic 0) via the interface.

• Power supply failure:

a. When the power supply of the chip (VDD or VBAT) drops below a certain value

(Vlow), typically 1.2 V, the oscillator stops running and a reset occurs.

b. When the power supply returns to normal operation, the oscillator starts running

again, the chip exits from reset.

c. The flag OSF is still logic 1 and can be cleared (OSF set logic 0) via the interface.

(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 using the interface.

Fig 9.

Power failure event due to battery discharge: reset occurs

001aaj409

internal power supply

(1)

Vth(sw)bat

(= 2.5 V)

VSS

VBAT

Vlow

(= 1.2 V)

VDD

VBBS

VDD

VBAT

VSS

(2)

OSF

battery discharge

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