Issue 151 February 2003
GPS-GSM Mobile Navigator

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GSM TRANSMITS DATA

A committee of telecom vendors and manufacturers in Europe—the European Telecommunications Standards Institute (ETSI)—designed GSM as a digital wireless communications system. Commercial service began in mid­1991, and by 1993 there were 36 GSM networks in 22 countries, with 25 additional countries looking to participate. Furthermore, the standard spread quickly beyond Europe to South Africa, Australia, and many Middle and Far Eastern countries. By the beginning of 1994, there were 1.3 million subscribers worldwide. Today, GSM is also the most widely used communications standard in China, and covers almost all of the country. So, we didn’t need to set up a communications base station for our system; this, of course, significantly reduced the total cost of the project.

The most basic teleservice supported by GSM is telephony. Group 3 fax, an analog method described in ITU­T recommendation T.30, is also supported by the use of an appropriate fax adapter. SMS is one of the unique features of GSM compared to older analog systems. For point­to­point SMS, a message can be sent to another subscriber to the service, and an acknowledgment of receipt is sent to the sender. SMS also can be used in Cell Broadcast mode to send messages such as traffic or news updates. Messages can be stored on the SIM card for later retrieval.

SMS is effective because it can transmit short messages within 3 to 5 s via the GSM network and doesn’t occupy a telephony channel. Moreover, the cost savings makes it a worthwhile choice (i.e., in China, each message sent costs $ 0.01 and receiving messages is free). With SMS transmitting, gathering position data is easy and convenient.

As with GPS modules, stand-alone GSM OEM modules are available. We used the FALCOM A2D from Wave-com for this project. The FALCOM A2D is a dual-band embedded GSM module (GSM900/DCS1800). It features the following services: telephony, SMS, data, and fax.

The GSM module has one TTL-level serial data interface. We use AT commands to control and program the FALCOM A2D. The data and control commands are exchanged between the microcontroller and GSM module through the serial interface.

There are many groups of AT commands, including: Call Control, Data Card Control, Phone Control, Computer Data Card Control, Reporting Operation, Network Communication Parameter, Miscellaneous, and Short Message Service. We use some of the SMS commands to communicate with the control center. The main AT commands for using SMS are listed in Table 3. You can download the GSM 07.07 and GSM 07.05 protocols for more details about the AT commands that are used in GSM communications. [1, 2]

Let’s review an example of how to make a GSM module send and read a sample SMS in Text mode. First, initialize the GSM module with AT commands AT+CSCA and AT+CMGF. Using the former sets the SMS center number to be used with outgoing SMS messages. Remember, the number will be saved on the SIM card just like in normal mobile phones. There are two different modes—Text mode and Protocol Data Unit (PDU) mode—for handling short messages. The system default is PDU mode; however, Text mode is easier to understand. So, use the AT+CMGF=1 command to set the module to the GSM 07.05 standard SMS Text mode.

The AT+CMGS command is used to send a short message. The format of this command is:

AT+CMGS=<da><CR>Message Texts<CTRL-Z>

Here, <da> is a subscriber’s mobile phone number that you want to send the short message to. The GSM module can receive incoming short messages and save them on the SIM card automatically. You can use the AT+CMGR command to read an incoming short message from the SIM card storage, and then use the AT+CMGD command to delete it when you’re finished.

If you want to read an SMS message, then send a AT+CMGR=x command to tell the GSM module which short message you want to read. Next, check the serial port to receive the message from the GSM module. Rs232_r is a subroutine used to receive data from the UART. Listing 1 demonstrates sending and reading a short message in a BASCOM-AVR program. In this code segment, chr(34) converts the ASCII value 34 to the right quote character ("). It also converts chr(13) to <CR> and chr(26) to <CTRL-Z>. As you can see, "My SMS Message" is the message you want to send.

CIRCUIT DESCRIPTION

The difficult part of designing this project was learning both the NMEA GPS message and GSM AT command protocols. The easy part was designing the hardware circuit (see Figure 2). You may download a table of the pin assignments from the Circuit Cellar ftp site. As you can see from the schematic, there are three jack ports. JP1 (20 pins) is used for the GPS module, JP2 (40 pins) is for the GSM module, and JP3 is used for communication with the PC.

(Click here to enlarge)

Figure 2—Jack port JP1 is the 20-pin GPS socket header. Jack port JP2 is the 40-pin GSM socket header. U2 is the dual four-channel multiplexer controlled by PA2 through PA3. All of the data traffic runs at 9600 bps.

The AT90S8515 (U1) is the core of the circuit. This low-power CMOS 8-bit microcontroller is based on the AVR-enhanced RISC architecture. By executing powerful instructions in a single clock cycle, the AT90S8515 achieves throughputs approaching 1 MIPS per megahertz, allowing you to optimize power consumption versus processing speed. The AT90S8515 features 8 KB of in-system programmable flash memory, 512 bytes of EEPROM, 512 bytes of SRAM, and 32 general-purpose I/O lines. Flexible timer/counters with compare modes, internal and external interrupts, a programmable serial UART, an SPI serial port, and two software-selectable power-saving modes are also available.

The high speed of the AT90S8515 makes it possible to complete multiple tasks between the GPS and GSM modules, although it has only one UART serial port. With the programmable flash memory, you have high reliability and can update your system. The EEPROM makes it possible to store system parameters such as the SMS center number, control center number, and predetermined time intervals.

Other components on the board are the four-channel multiplexer, a large capacity data memory, and the user interface. The latter consists of a 2 × 16 LCD, a buzzer, and three push buttons.