PCF2129T datasheet(21/73 Pages) NXP

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PCF2129T

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Product data sheet

Rev. 3 — 12 February 2013

21 of 73

NXP Semiconductors

PCF2129T

Accurate RTC with integrated quartz crystal for industrial

8.6 Oscillator stop detection function

The PCF2129T 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) by command.

• Power supply failure:

a. When the power supply of the chip (VBBS, see Section 8.5.2) 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) by command.

(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.

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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