PCF2127ATSLASH1 datasheet(23/80 Pages) NXP

23 / 80 page

PCF2127A_2

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Product data sheet

Rev. 02 — 7 May 2010

23 of 80

NXP Semiconductors

PCF2127A

Integrated RTC, TCXO and quartz crystal

(1)

Vth(uvp) value is usually set to a value that there are several milliseconds before VDD falls

below the minimum operating voltage of the system, in order to allow the microcontroller

to perform early backup operations.

If the extra power fail detection function is not used, pin PFI must be connected to VSS and

pin PFO must be left open circuit.

8.6.4.1

Extra power fail detection when the battery switch over function is enabled

• When the power switches to the backup battery supply VBAT, the power fail

comparator is switched off and the power fail output at pin PFO goes (or remains)

LOW

• When the power switches back to the main VDD, the pin PFO is not driven LOW

anymore and is pulled HIGH through the external pull-up resistance for a certain time

(trec = 15.63 ms to 31.25 ms) and then the power fail comparator is enabled again

For illustration see Figure 10 and Figure 11.

th uvp

()

------

⎝⎠

⎛⎞

1.25V

Fig 10. PFO signal behavior when battery switch-over is enabled in standard mode and

Vth(uvp) >(VBAT,Vth(sw)bat)

001aaj319

internal power supply (= VBBS)

comparator

enabled

comparator

disabled

comparator

enabled

Vth(sw)bat

(= 2.5 V)

VBAT

VDD

VBBS

Vth(uvp)

PF0

trec = [15.63 : 31.25] ms

VDD (= 0 V)

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