SIM900D Hardware Design
Normal Mode
Normal Mode
Power Down
Mode
Power Down
Mode
AT+CPOWD or
PWRKEY
Charge-only
Mode
Charge-only
Mode
PWRKEY
Insert charger
Figure12:
State Transition
3.8 RTC Backup
The RTC (Real Time Clock) power supply of module can be provided by an external capacitor or a battery
(rechargeable or non-chargeable) through the VRTC.
Note: If the RTC function is enabled, a battery or capacitor should be connected with the VRTC pin.
Otherwise the VRTC pin can be keep open.
The following figures show various sample circuits for RTC backup.
RTC
Core
10K
Module
VRTC
Non-chargeable
Backup Battery
Figure13: RTC supply from non-chargeable battery
SIM900D_HD_V1.02
09.04.2010
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