MC75i Hardware Interface Description
3.8 RTC Backup
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MC75i_HD_v01.100a
Page 50 of 122
2009-08-13
Confidential / Released
3.8
RTC Backup
The internal Real Time Clock of MC75i is supplied from a separate voltage regulator in the an-
alog controller which is also active when MC75i is in POWER DOWN status. An alarm function
is provided that allows to wake up MC75i to Airplane mode without logging on to the GSM net-
work.
In addition, you can use the VDDLP line to backup the RTC from an external capacitor or a
battery (rechargeable or non-chargeable). The capacitor is charged by the BATT+ line of
MC75i. If the voltage supply at BATT+ is disconnected the RTC can be powered by the capac-
itor. The size of the capacitor determines the duration of buffering when no voltage is applied
to MC75i, i.e. the larger the capacitor the longer MC75i 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
rameters required.
Figure 13:
RTC supply from capacitor
Figure 14:
RTC supply from rechargeable battery
Figure 15:
RTC supply from non-chargeable battery
