Datasheet

PCF2129 All information provided in this document is subject to legal disclaimers. © NXP Semiconductors N.V. 2014. All rights reserved.
Product data sheet Rev. 7 — 19 December 2014 20 of 86
NXP Semiconductors
PCF2129
Accurate RTC with integrated quartz crystal for industrial applications
8.5.1.4 Battery switch-over architecture
The architecture of the battery switch-over circuit is shown in Figure 8
.
V
oper(int)
is at V
DD
or V
BAT
potential.
Remark: It has to be assured that there are decoupling capacitors on the pins V
DD
, V
BAT
,
and BBS.
8.5.2 Battery low detection function
The PCF2129 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. (For further information about V
low
see
Section 8.6
.)
When V
BAT
drops below the threshold value V
th(bat)low
, the following sequence occurs (see
Figure 9
):
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 8.12.7
).
3. The flag BLF remains logic 1 until the battery is replaced. BLF cannot be cleared by
command. It is automatically cleared by the battery low detection circuit when the
battery is replaced or when the voltage rises again above the threshold value. This
could happen if a super capacitor is used as a backup source and the main power is
applied again.
Fig 8. Battery switch-over circuit, simplified block diagram
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