SIM900D Hardware Design
3.7.3 Wake Up SIM900D from SLEEP Mode (CSCLK=1)
When SIM900D is in SLEEP mode, the following methods can wake up the module.
z
Enable DTR pin to wake up SIM900D.
If DTR pin is pulled down to a low level
，
this signal will wake up SIM900D 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 SIM900D.
z
Receiving a SMS from network to wake up SIM900D.
3.7.4 Sleep Mode II (CSCLK=2)
In this mode, the SIM900D will continuously monitor the main serial port data signal. When there is no data
transferred exceed 5 seconds on the RXD signal and there is no data on air and hardware interrupt (such as GPIO
interrupt or data in serial port), SIM900 will enter SLEEP mode automatically. In this mode, SIM900D can still
receive paging or SMS from network but the serial port is not accessible.
Note: For SIM900, it requests to set AT command “AT+CSCLK=2” to enable the SLEEP mode. The default
value is 0, which can not make the module enter SLEEP mode. For more details please refer to related
document [1].
3.7.5 Wake Up SIM900D from SLEEP Mode (CSCLK=2)
When SIM900D is in SLEEP mode (CSCLK=2), the following methods can wake up the module.
z
User can send data to SIM900D using main serial port, when SIM900D detects the change on the RXD. The
first byte data of user will not be sent via module.
z
Receiving a voice or data call from network to wake up SIM900D.
z
Receiving a SMS from network to wake up SIM900D.
3.8 RTC Backup
The RTC power supply of module can be provided by an external capacitor or a battery (rechargeable or
non-chargeable) through the VRTC.
Note: The VRTC could not be designed to a NC pin in user’s circuit. User should connect the VRTC pin to a
battery or a capacitor.
The following figures show various sample circuits for RTC backup.
SIM900D_Hardware Design_V1.04
2010.10.09
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