SIM900D Hardware Design
3.3.3 Monitoring Power Supply
To monitor the supply voltage, user can use the “AT+CBC” command which include three parameters: charging
state, voltage percentage and voltage value (in mV). It returns charge state
，
the battery voltage 1-100 percent of
capacity and actual value measured at VBAT and GND.
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 SIM900D while it is beyond the safety limits of voltage and temperature stated
in Chapter 3.4.2. SIM900D 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 SIM900D
SIM900D 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 VCHG Pin: starts Charge-only mode (see chapter 3.4.1.2);
Note: The AT command must be set after the SIM900D is power on and Unsolicited Result Code “RDY” is
received from the serial port. However if the SIM900D is set autobauding, the serial port will receive nothing.
The AT commands can be set after the SIM900D is power on. 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 SIM900D is power on. Please refer to the
chapter AT+IPR in document [1].
3.4.1.1 Turn on SIM900D Using the PWRKEY Pin (Power on)
The SIM900D can be turned on by driving the PWRKEY to a low level voltage for a short time, then release.
This pin has pulled up to 3V in the module. The simple circuit illustrates as the following figures.
SIM900D_HD_V1.02
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