Datasheet

PCF2127AT All information provided in this document is subject to legal disclaimers. © NXP B.V. 2013. All rights reserved.
Product data sheet Rev. 6 — 11 July 2013 25 of 86
NXP Semiconductors
PCF2127AT
Integrated RTC, TCXO and quartz crystal
(1)
V
th(uvp)
value is usually set to a value that there are several milliseconds before V
DD
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 V
SS
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 V
BAT
, 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 V
DD
, the pin PFO is not driven LOW
anymore and is pulled HIGH through the external pull-up resistance for a certain time
(t
rec
= 15.63 ms to 31.25 ms) and then the power fail comparator is enabled again
For illustration, see Figure 10
and Figure 11.
V
th uvp
R
1
R
2
------
1+


1.25V=
Fig 10. PFO signal behavior when battery switch-over is enabled in standard mode and
V
th(uvp)
>(V
BAT
,V
th(sw)bat
)
001aaj319
internal power supply (= V
BBS
)
comparator
enabled
comparator
disabled
comparator
enabled
V
th(sw)bat
(= 2.5 V)
V
BAT
V
DD
V
BBS
V
BBS
V
th(uvp)
PF0
t
rec
= [15.63 : 31.25] ms
V
DD
(= 0 V)