SIM900 Hardware Design
3.5.3 Wake Up SIM900A from SLEEP Mode
When SIM900A is in SLEEP mode, the following methods can wake up the module.
z Enable DTR pin to wake up SIM900A.
If DTR pin is pulled down to a low level,this signal will wake up SIM900A from power saving mode. The
serial port will be active after DTR changed to low level for about 50ms.
z Receiving a voice or data call from network to wake up SIM900A.
z Receiving a SMS from network to wake up SIM900A.
3.6 Summary of State Transitions (except SLEEP mode)
The following figure shows how to proceed from one mode to another.
AT+CPOWD or Drive
PWRKEY to ground
NNoromrmalalMModoede
PPowowererDDowownn
MModoede
Drive PWRKEY to ground
Figure 15: State Transition
3.7 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.
Non-chargeable
Backup Battery
VRTC
SIM900A
10K
RTC
Core
SIM900_HD_V1.01
Figure 16: RTC supply from non-chargeable battery
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26.12.2009