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 23 of 86
NXP Semiconductors
PCF2127AT
Integrated RTC, TCXO and quartz crystal
8.6.3 Battery low detection function
The PCF2127AT has a battery low detection circuit which monitors the status of the
battery V
BAT
.
When V
BAT
drops below the threshold value V
th(bat)low
(typically 2.5 V), the BLF flag
(register Control_3) is set to indicate that the battery is low and that it must be replaced.
Monitoring of the battery voltage also occurs during battery operation.
An unreliable battery cannot prevent that the supply voltage drops below V
low
(typical
1.2 V) and with that the data integrity gets lost.
When V
BAT
drops below the threshold value V
th(bat)low
, the following sequence occurs (see
Figure 8
):
1. The battery low flag BLF is set logic 1.
2. An interrupt is generated if the control bit BLIE (register Control_3) is enabled
(see Section 8.13.8
).
3. The flag BLF remains logic 1 until the battery is replaced. BLF cannot be cleared by
command. It is cleared automatically by the battery low detection circuit when the
battery is replaced.
Fig 7. Typical driving capability of V
BBS
: (V
BBS
 V
DD
) with respect to the output load
current I
BBS
I
BBS
(mA)
0 8642
001aaj327
−400
−600
−200
0
V
BBS
− V
DD
(mV)
−800
V
DD
= 4.2 V
V
DD
= 3 V
V
DD
= 2 V