SIM900 Hardware Design
card function will not be accessible.
If SIM900 has been set by “AT+CFUN=4”, the RF function will be closed, the serial port is still active. In this
case all AT commands correlative with RF function will not be accessible.
After SIM900 has been set by “AT+CFUN=0” or “AT+CFUN=4”, it can return to full functionality by
“AT+CFUN=1”.
For detailed information about “AT+CFUN”, please refer to document [1].
3.5.2 Sleep Mode (Slow Clock Mode)
We can control SIM900 module to enter or exit the SLEEP mode in customer applications through DTR signal.
When DTR is in high level and there is no on air and hardware interrupt (such as GPIO interrupt or data on serial
port), SIM900 will enter SLEEP mode automatically. In this mode, SIM900 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=1” to enable the sleep mode; the default value
is 0, that can’t make the module enter sleep mode. For more details please refer to our AT command list.
3.5.3 Wake Up SIM900 from SLEEP Mode
When SIM900 is in SLEEP mode, the following methods can wake up the module.
z Enable DTR pin to wake up SIM900.
If DTR pin is pulled down to a low level，this signal will wake up SIM900 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 SIM900.
z Receiving a SMS from network to wake up SIM900.
3.6 Summary of State Transitions (except SLEEP mode)
The following figure shows how to proceed from one mode to another.
Normal Mode
Normal Mode
Power Down
Mode
Power Down
Mode
AT+CPOWD or Drive
PWRKEY to ground
Drive PWRKEY to ground
Figure 15:
State Transition
SIM900_HD_V1.05 06.23.2010
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