Hardware Design V1.01(091226)
Table Of Contents
- Contents
- Version history
- 1 Introduction
- 2 SIM900 Overview
- 3 Application Interface
- 3.1 SIM900 Pin Description
- 3.2 Operating Modes
- 3.3 Power Supply
- 3.4 Power Up and Power Down Scenarios
- 3.5 Power Saving
- 3.6 Summary of State Transitions (except SLEEP mode)
- 3.7 RTC Backup
- 3.8 Serial Interfaces
- 3.9 Audio Interfaces
- 3.10 SIM Card Interface
- 3.11 LCD Display Interface
- 3.12 Keypad Interface
- 3.13 ADC
- 3.14 Behaviors of the RI
- 3.15 Network Status Indication
- 3.16 General Purpose Input Output (GPIO)
- 3.17 External Reset
- 3.18 PWM
- 3.19 I2C Bus
- 4 Antenna Interface
- 5 Electrical, Reliability and Radio Characteristics
- 6 Mechanics
SIM900 Hardware Design
3.8.1 Function of serial port & debug port supporting
Serial port
z Modem device.
z Contains data lines TXD and RXD, State lines RTS and CTS, Control lines DTR, DCD, DSR and RI.
z Serial port can be used for CSD FAX, GPRS service and send AT command of controlling module. Also
serial port can be used for multiplexing function. SIM900 supports only basic mode of multiplexing so far.
z Serial port supports the communication rates as following:
1200,2400, 4800, 9600, 19200, 38400, 57600, 115200 Default as 115200bps.
z Autobauding supports baud rates as following:
1200, 2400, 4800, 9600, 19200, 38400, 57600 and 115200bps.
Autobauding allows the GSM engine to automatically detect the baud rate configured in the host application. The
serial port of the GSM engine supports autobauding for the following baud rates: 1200, 2400, 4800, 9600,
19200, 38400, 57600, 115200bps. Factory setting is autobauding enabled. This gives you the flexibility to put the
GSM engine into operation no matter what baud rate your 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, it is recommended to wait 2 to 3 seconds before sending
the first AT character. You must first send “A” to synchronize the band rate. 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 you
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: You can use AT+IPR=x;&W 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 SIM900 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
3.8.2 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 SIM900. The PWRKEY pin is recommended to connect to the IO connector. The user also can
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