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AC75 Hardware Interface Description
Strictly confidential / Preliminary
3.8
RTC Backup
The internal Real Time Clock of AC75 is supplied from a separate voltage regulator in the
analog controller which is also active when AC75 is in POWER DOWN status. An alarm
function is provided that allows to wake up AC75 to Airplane mode without logging on to the
GSM network.
In addition, you can use the VDDLP pin on the board-to-board connector to backup the RTC
from an external capacitor or a battery (rechargeable or non-chargeable). The capacitor is
charged by the BATT+ line of AC75. If the voltage supply at BATT+ is disconnected the RTC
can be powered by the capacitor. The size of the capacitor determines the duration of
buffering when no voltage is applied to AC75, i.e. the larger the capacitor the longer AC75
will save the date and time.
A serial 1kΩ resistor placed on the board next to VDDLP limits the charge current of an
empty capacitor or battery.
The following figures show various sample configurations. Please refer to Table 26 for the
parameters required.
AC75_HD_V00.202
Baseband
processor
PSU
RTC
Figure 13: RTC supply from capacitor
Baseband
processor
PSU
RTC
Figure 14: RTC supply from rechargeable battery
Baseband
processor
PSU
RTC
Figure 15: RTC supply from non-chargeable battery
Page 50 of 120
BATT+
B2B
1k
VDDLP
+
BATT+
B2B
1k
VDDLP
+
BATT+
B2B
1k
VDDLP
+
+
s
27.04.2006
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