PCF2127AT datasheet(26/100 Pages) PHILIPS

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PCF2127

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Product data sheet

Rev. 8 — 19 December 2014

26 of 100

NXP Semiconductors

PCF2127

Accurate RTC with integrated quartz crystal for industrial applications

8.6.3.2

Extra power fail detection when the battery switch-over function is disabled

If the battery switch-over function is disabled and the power fail comparator is enabled,

the power fail output at pin PFO depends only on the result of the comparison between

VPFI and 1.25 V:

• If VPFI > 1.25 V, PFO = HIGH (through the external pull-up resistor)

• If VPFI < 1.25 V, PFO = LOW

8.6.4 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:

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

needs, such as SRAM (see Ref. 3 “AN11266”). For this case, Figure 14 shows the typical

driving capability when VBBS is driven from VDD.

Fig 13. PFO signal behavior when battery switch-over is disabled

Table 26.

Output pin BBS

Battery switch-over function

mode

Conditions

Potential of

Voper(int) and

VBBS

standard

VDD > VBAT OR VDD > Vth(sw)bat

VDD

VDD < VBAT AND VDD < Vth(sw)bat

VBAT

direct switching

VDD > VBAT

VDD

VDD < VBAT

VBAT

disabled

only VDD available,

VBAT must be put to ground

VDD

PCF2127AT Datasheet(PDF) 26 Page - NXP Semiconductors

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