SIM900D Hardware Design
38400, 57600bps. Factory setting is autobauding enabled. This gives you the flexibility to put the GSM engine
into operation no matter what baud rate user host application is configured to. To take advantage of autobauding
mode, specific attention should be paid to the following requirements:
Synchronization between DTE and DCE
:
When DCE powers on with the autobauding enabled, user must first send “AT” to synchronize the baud rate. It is
recommended to wait 2 to 3 seconds before sending “AT” character. After receiving the “OK” response, DTE and
DCE are correctly synchronized. The more information please refer to the AT command “AT+IPR”.
Restrictions on autobauding operation
z
The serial port has to be operated at 8 data bits, no parity and 1 stop bit (factory setting).
z
The Unsolicited Result Codes like "RDY", "+CFUN: 1" and "+CPIN: READY” are not indicated when user
starts up the ME while autobauding is enabled. This is due to the fact that the new baud rate is not detected
unless DTE and DCE are correctly synchronized as described above.
Note: User 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 Unsolicited Result Codes like "RDY" should
be received from the serial port all the time that the SIM900D is power on.
Debug port
z
Only contain Data lines TXD and RXD
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Debug Port used for debugging and upgrading firmware. It cannot be used for CSD call, FAX call. And the
Debug port can not use multiplexing function. It does not support autobauding function.
z
The communication rate of debug port is 115200bps
3.9.2 Software Upgrade and Software Debug
The DBG_TXD, DBG_RXD and GND must be connected to the IO connector when user needs to upgrade
software and debug software, the DBG_TXD, DBG_RXD should be used for software upgrade and for software
debugging. The TXD and RXD also should be connected to the IO connector, if user wants to send AT command
or data stream to SIM900D. The PWRKEY pin is recommended to connect to the IO connector. The user also can
add a switch between the PWRKEY and the GND. The PWRKEY should be connected to the GND when
SIM900D is upgrading software. Please refer to the following figure.
SIM900D_Hardware Design_V1.04
2010.10.09
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