As the demand for wireless data services is increasing on the market, operators have to develop their own data and services in the field, and strive to stay ahead in the fierce competition. Traditional GSM networks can only support 9.6 kb / s data transmission rate, which can not meet the user demand for high-speed wireless data services. GPRS (General packet radio service) is architected for traditional GSM network a standardized packet switched data service, he can provide up to 115 kb / s rate of the packet data services, including so that pictures, voice and video, multimedia services in a wireless network transmission becomes a reality.
GPRS is considered to be the second generation mobile communication system an important step towards the third generation mobile communication evolution. Relative to the original GSM dial-up circuit-switched data transmission mode, GPRS is a packet switching technology, with “high speed” and “always on” advantages. GPRS allows users to send and receive data at end packet transfer mode, without the need to use the circuit-switched mode network resources, thereby providing an efficient, low-cost wireless packet data service. With the rapid development of wireless data transmission technology, GPRS wireless data services have become the best destination.
2.1 USR GM3 GPRS module
This study module is optional GPRS module Siemens GM3, this wireless module is the smallest on the market today tri-band module. GM3-end via the 50-pin connector is a 0.5 mm wide pin is connected to the chip application platform. Pin 50 is used to control each logic unit, the transmission data and audio signals, and supplies power to provide queue. The concept of a multi-purpose audio offers a variety of audio interfaces, interfaces are available on each end connector: a Digital Audio Interface (DAI) and two analog audio jacks. Use AT commands can be easily switched back and forth, and select the command different audio modes.
The most important is the state of GM3 at 0 V (for low data bits and the ON state) and 2.65 V (for high-data bits and the OFF state). The chip is designed for use as a data communications equipment (DCE), and based on the traditional DCE-DTE connection, he and serial communication end user applications.
2.2 GPRS data transmission module hardware design
The study design is the application of GPRS data transmission module with the microcontroller, so the hardware is mainly involved in microcontroller and GPRS modules, where the choices GPRS module is USR-GM3, while the microcontroller is a selection of 51 series.
GPRS data communication between the microcontroller module and the main port TXD0 between TXD, and RXD between RXD0 and data transfer through to complete. Wherein TXD0 port GPRS module for receiving data coming from the microcontroller, while the TXD port on the microcontroller is used to transfer data to GPRS module. RXD0 port GPRS module is used to send data to the microcontroller, the microcontroller RXD port is used to receive data transmission from the GPRS module.
Since the power supply module has five pins, and the voltage is 3.3 ~ 4.8 V, so there will be five pins together, directly connected to the external power supply. Pin IGT GPRS module is mainly used in the ignition reset, where GPRS module role is to do a reset conversion. On GM3 baseband processor has an integrated SIM interface, he was directly wired to the host interface (end connector) for connection to an external SIM card holder. Here then SIM card has six pins CCCLK, CCVCC, CCIO, CCRST, CCIN and CCGND respectively connected to the MC55 on the first to sixth pins. The distance between the pin module connector and SIM card holder should not exceed 20 cm, in order to achieve the best results, in the SIM holder isolated laying a layer of copper mesh, is connected to the copper layer and SIM card pin CCGND . And two capacitors CCVCC CCGND to leave between the pin as close as possible, and try to trace impedance is low, in order to meet regulatory requirements. SCM and keyboard can be connected to send data to the microcontroller via the keyboard. The single-chip crystal oscillator for 11.059 2 MHz, the data transfer rate to 9 600 b / s. MCU reset key RESET is high reset.