SIM900D Hardware Design
Figure6: The minimal VBAT voltage at VBAT drop
3.3.3 Monitoring Power Supply
To monitor the supply voltage, user can use the “AT+CBC” command. It returns charging state, 1-100 percent of
remaining battery capacity and voltage value measured at VBAT pin.
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 command “AT+CBC” is executed.
For detail information about “AT+CBC”, please refer to
related 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 power down as soon as the module detects 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 the 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
(please refer to chapter 3.4.1.2)
;
Note: The “AT” command must be set after the SIM900D is powered on and Unsolicited Result Code “RDY” is
received from the serial port. However if the SIM900D is set to autobauding, the serial port will receive nothing.
The AT commands can be set after the SIM900D is powered 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 powered on. Please refer
to the chapter AT+IPR in related document [1].
3.4.1.1 Turn on SIM900D Using the PWRKEY Pin (Power on)
User can turn on the SIM900D 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.
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