User's Manual Part 1

MC75 Hardware Interface Description
Strictly confidential / Draft
s
MC75_V00.190a Page 38 of 91 15.02.2005
3.5 RTC Backup
The internal Real Time Clock of MC75 is supplied from a separate voltage regulator in the
analog controller which is also active when MC75 is in POWER DOWN status. An alarm
function is provided that allows to wake up MC75 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 MC75. 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 MC75, i.e. the larger the capacitor the longer MC75
will save the date and time.
A serial 1k 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 16 for the
parameters required.
Baseband
processor
RTC
PSU
+
BATT+
1k
B2B
VDDLP
Figure 9: RTC supply from capacitor
RTC
+
BATT+
1k
B2B
VDDLP
Baseband
processor
PSU
Figure 10: RTC supply from rechargeable battery
RTC
+
+
BATT+
1k
VDDLP
B2B
Baseband
processor
PSU
Figure 11: RTC supply from non-chargeable battery