SIM900D Hardware Design
start 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: 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
Null modem port
z
Only contain Data lines TXD and RXD
z
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
Debug port supports the communication rates 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 need 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.
MODULE(DCE)
DBG_TXD
DBG_RXD
GND
PWRKEY
TXD
RXD
GND
PWRKEY
Serial port
I/O Connector
Figure18: Connection of software upgrade and software debug
The serial port and the debug port don’t support the RS_232 level and it only supports the CMOS level. It should
add the level converter IC between the DCE and DTE, if you connect it to the computer. Please refer to the
following figure.
SIM900D_HD_V1.02
09.04.2010
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