Specifications

XT55 Hardware Interface Description
Confidential / Preliminary
s
mo b i l e
XT55_hd_v00.02 Page 53 of 116 17.03.2004
3.8 RTC backup for GSM/GPRS part of XT55
The internal Real Time Clock of the XT55 GSM/GPRS part is supplied from a separate
voltage regulator in the power supply ASIC which is also active when the GSM/GPRS part of
the XT55 is in POWER DOWN status. An alarm function is provided that allows to wake up
XT55 without logging on to the GSM network.
In addition, you can use the GSM_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 GSM_BATT+ line of XT55. If the voltage supply at GSM_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 the GSM/GPRS part of
the XT55, i.e. the greater capacitor the longer the GSM/GPRS part of the XT55 will save the
date and time.
The following figures show various sample configurations. The voltage applied at
GSM_VDDLP can be in the range from 2 to 5.5V. Please refer to Table 27 for the
parameters required.
Baseband
processor
RTC
PSU
+
GSM_BATT+
1k
B2B
GSM_VDDLP
Figure 14: RTC supply from capacitor
RTC
PSU
+
GSM_BATT+
1k
B2B
GSM_VDDLP
Baseband
processor
Figure 15: RTC supply from rechargeable battery
RTC
PSU
+
+
GSM_BATT+
1k
GSM_VDDLP
B2B
Baseband
processor
Figure 16: RTC supply from non-chargeable battery