Specifications

TC35 / TC37 Hardware Interface Description
TC35_37_HD_02_V03.10 - Released Page 38 of 92 21.12.2001
3.4 RTC backup
The internal Real Time Clock of TC35/TC37 is supplied from a dedicated voltage regulator
which is part of the power supply ASIC and remains active while the module is powered
down. An alarm function is included that allows to wake up the GSM engine without logging
on to the GSM network.
In addition, you can use the VDDLP pin on the ZIF connector (pin no. 30) to backup the RTC
from an external capacitor, a battery or an accumulator. The capacitor is charged by the
VBATT+ line of TC35/TC37. If the voltage supply at VBATT+ 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 module, i.e. the greater capacitor the longer TC35/TC37
will save the date and time. For example, a capacitor C=100µF typically buffers the date and
time for 30 seconds, i.e. you have 30 seconds to change the battery or accumulator without
losing the information.
If you need to adjust the date and time use the AT+CCLK command. To set the alarm time
enter AT+CALA. For further instructions please refer to Chapter 3.3.1.4 and to the AT
Command Set.
When designing the GSM application it is recommended to add a serial resistor to the
VDDLP line in order to limit the input current of an empty capacitor.
The following figures show various sample configurations. The voltage applied at VDDLP
can be in the range from 2 to 5.5V. Please refer to Table 18 for the parameters required.
Baseband
processor
RTC
PSU
ZIF
+
VBatt+
VDDLP
R
Figure 11: RTC supply from capacitor
Baseband
processor
RTC
PSU
ZIF
+
VBatt+
VDDLP
R
Figure 12: RTC supply from rechargeable battery (accumulator)