SIM900 Reference Design Guide Notes
2.1.4 Reset design
SIM900 support the reset function, when the MCU find the module is in an abnormal state,
SIM900 can be restarted by pulling the RESET pin to ground for a typical 50uS. Reset is a noise
sensitivity pin, it should be kept away from the high speed signal line (eg. clock) when layout.
This pin is internal pull up to 2.8V through a 100k resistor. So it’s not necessary to add an external
pull up resistor for stable consideration if it is not used. Please pay attention that reset is only used
in emergency situation, such as software break down, module does not response the AT command.
Reset the module continually is not recommended and it may cause some unexpected fault.
SIM900
100K
RESET
2.8V
GPIO
MCU
ARM9
50uS
Figure 5: Reset SIM900 with a MCU
2.1.5 RTC backup design
SIM900 integrates s a RTC backup interface which can be connected to a backup battery or a
capacitor. Please pay attention to the rated voltage of the battery should be 3V.The RTC current
consumption is about 2uA when the VBAT is removed.
2.2. UART interface
2.2.1 UART interface design
SIM900 integrates two UART port, one is Serial Port, and the other is Debug Port. Serial port is
for AT command with the MCU while Debug port is for firmware update and bug trace. It is
suggested to connect Debug port to an external connector for module debug consideration. If
hardware flow control is not used ,DCD、DSR can be left floating. Please refer to the following
figure. DTR can be used to wake up the module from sleep mode and RI can be used to detect a
coming call or SMS. These two should connect to GPIO of the MCU.
NOTE 3:
Please note that the UART level is 2.8V, if the level is not matching, a level shift circuit is needed.
SIM900_AN_Reference Design Guide_V1.01 01.27.2010
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