SIM900 Hardware Design
3.3.2 Minimizing Power Losses
When designing the power supply for your application please pay specific attention to power losses. Ensure that
the input voltage VBAT never drops below 3.4V even in a transmit burst where current consumption can rise to
typical peaks of 2A. If the power voltage drops below 3.4V, the module may be switched off. The PCB traces
from the VBAT pins to the power source must be wide enough to decrease voltage drops in the transmitting burst
mode.
3.3.3 Monitoring Power Supply
To monitor the supply voltage, you can use the “AT+CBC” command which include a parameter: voltage value
(in mV).
The voltage is continuously measured at intervals depending on the operating mode. The displayed voltage (in
mV) is averaged over the last measuring period before the AT+CBC command is executed.
For details please refer to document [1]
3.4 Power Up and Power Down Scenarios
In general, be sure not to turn on SIM900A while it is beyond the safety limits of voltage and temperature stated
in Chapter 3.4.2. SIM900A would immediately switch off after having started and detected these inappropriate
conditions. In extreme cases this can cause permanent damage to the module.
3.4.1 Turn on SIM900A
SIM900A can be turned on by two ways, which are described in following chapters:
z Via PWRKEY pin: starts normal operating mode (please refer to chapter 3.4.1.1);
z Via PWRKEY pin and PWRKEY_OUT pin:starts normal operating mode
Note: The AT command must be set after the SIM900A is power on and Unsolicited Result Code “RDY” is
received from the serial port. However if the SIM900A is set autobauding, the serial port will receive nothing.
The AT commands can be set after the SIM900A is power on. You can use AT+IPR=x to set a fixed baud rate
and save the configuration to non-volatile flash memory. After the configuration is saved as fixed baud rate,
the Code “RDY” should be received from the serial port all the time that the SIM900A is power on. Please
refer to the chapter AT+IPR in document [1].
3.4.1.1 Turn on SIM900A Using the PWRKEY Pin (Power on)
You can turn on the SIM900A by driving the PWRKEY to a low level voltage with a limiting current resistor (1K
is recommended) in series for a short time and then release. This pin has pulled up to VDD_EXT in the module.
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26.12.2009