Design of GSM Module TC35 and Its Peripheral Circuit

　
1. Introduction

With the development of communication industry, the application field of mobile communication is expanding, and the design of mobile terminal is also getting more and more attention. This paper describes in detail the GSM module TC35 of Siemens Company, the level conversion chip MAX3238 of TI Company, and the hardware circuit of mobile terminal. This design can complete functions such as SMS sending and receiving, voice transmission, data transmission with PC, etc. It has been applied in GPS vehicle monitoring system based on GSM SMS. 2. TC35 module

TC35 is a new generation wireless communication GSM module introduced by Siemens Company, which can quickly, safely and reliably implement data, voice transmission, Short Message Service and fax in the system scheme. The module operates at a voltage of 3.3-5.5V and can operate at 900MHz and 1800MHz with power consumption of 2 W (900M) and 1 W (1800M), respectively. The module has an AT command set interface that supports text and PDU mode SMS, group 3 second-class faxes, and 2.4k, 4.8k, 9.6K non-transparent modes. In addition, the module also has the phonebook function, multi-party calls, roaming detection function, and the common working modes are power saving mode, IDLE, TALK and so on. The unique 40 pin ZIF connector enables bidirectional transmission of power connection, command, data, voice signal, and control signal. The SIM card holder and antenna can be connected separately through the ZIF connector and the 50 antenna connector.
TC35 module mainly consists of GSM baseband processor, GSM radio frequency module, power supply module (ASIC), flash memory, ZIF connector and antenna interface. As TC35, the baseband processor mainly processes voice and data signals in GSM terminals, and covers all analog and digital functions in cellular RF devices. FR, HR, and EFR speech channel encoding can be supported without requiring additional hardware circuitry.

Figure 1 TC35 functional block diagram
3. Peripheral Application Circuits

The normal operation of TC35 module requires the corresponding peripheral circuit to cooperate with it. TC35 has 40 pins, which are connected with power circuit, startup and shutdown circuit, data communication circuit, voice communication circuit, SIM card circuit, indicator circuit and so on through ZIF connector. As shown in Figure 2,

Figure 2 Peripheral circuit connection of TC35
3.1 Power supply and startup circuit

The power circuit is divided into two parts: the rechargeable battery and the regulated power module: the rechargeable battery mainly provides 3.6V operating voltage for the whole system, and at the same time generates the high level required by MAX3238; TI's three-terminal power module UA7806 converts the external +12V DC power supply to +6V, connects to 11 and 12 pins of ZIF connector, and provides +6V, 500mA charging voltage for TC35 in charging mode.
The starting circuit consists of an open-drain triode and an on-power reset circuit. After the module is powered on for 10ms (the battery voltage must be greater than 3V), in order for it to work properly, a low level signal with an additional time of at least 100ms at 15 feet (/IGT) must be added, and the signal decrease along the time is less than 1ms. After startup, the signal of 15 feet should be kept at a high level. As shown in the figure below, the signal generated for the startup circuit shows a delay of 10ms and a low level of 100ms.

Startup signal downward collected in Figure 3

3.2 Data Communication Circuit

The data communication circuit mainly completes the functions of sending and receiving short messages, communicating with PC, software flow control and so on. The data interface of TC35 is serial asynchronous transceiver, conforms to ITU-T RS-232 interface circuit standard, and works at CMOS level (2.65V). The data interface is configured with 8-bit data bits, 1-bit stop bits, and no check bits, and can operate at a baud rate of 300 bps to 115 kbps, with a supported automatic baud rate of 4.8 Kbps to 115 kbps (except 14.4 Kbps and 28.8 kbps). The TC35 module also supports RTS0/CTS0 hardware handshake and XON/XOFF software flow control.
The data communication circuit uses TI's MAX3238 chip as its chip to realize level conversion and serial communication functions.
The power supply voltage of TI's MAX3238 chip is 3-5.5V, which meets TIA/EIA-232-F and ITU V.28 standards. It has a unique (+) 15KV electrostatic protection for human body, compatible with 5V logical input, and contains 3 receiving and 5 sending serial communication interfaces. The data transfer rate can reach 250 kbps. The chip is characterized by flexible power management without data input in the serial port, that is, when FORCEON (13 feet) is low level and/FORCEOFF (14 feet) is high level, Auto-Powerdown Plus function is effective. In normal operation mode, if the chip does not detect a valid signal at the receiving and sending pins within about 30 seconds of event, it will automatically enter Powerdown mode, which consumes 1uA of power. If both FORCEON and/or FORCEOFF pins are high, then the Auto-Powerdown Plus function is invalid. When the Auto-Powerdown Plus function is valid, the chip is automatically activated to normal operation if signal input is detected for receiving or sending pins. If the input voltage of any receiving channel is higher than 2.7V or less than -2.7V, or if the time between -0.3V and 0.3V is less than 30uS, the/INVALID pin is high (data valid). If the input voltage of all receiving channels is between -0.3V and 0.3V for more than 30uS, the/INVALID pin is low (data invalid).
Table 1 Output control truth table


The above characteristics of the chip meet the connection requirements of three receiving and five sending circuits of TC35 as mobile terminal. When the MAX3238 is connected to the corresponding pin of the ZIF connector, it is important to note that the sending and receiving pins are connected correctly. MAX3238 also needs to be connected with four 0.1uF capacitors to complete the level conversion function. TC35 module outputs RxD0, CTS0, DSR0, DCD0, RING0 signals through the pins of RS-232 interface, and the input signals are TxD0, RTS0, DTR0.
Because the interface circuit of TC35 uses all pins of 9-pin serial port, TC35 can obtain DTR0, DSR0, DCD0 and RING0 control signals. The signal RING0 is used to indicate to cellular devices that an Unsolicited Result Code (URC) is received. Through AT command, different operation modes of TC35 can be set.
3.3 Voice Communication Circuit

Because the GSM baseband processor of TC35 integrates audio filter, ADC, DAC, speech synthesis and so on, the peripheral circuit connection of module speech interface is relatively simple. TC35 has two voice interfaces, each with analog microphone input and analog headphone output. For different peripherals, there are six voice modes in the module, which can be selected by instruction AT^SNFS. The default configuration of the Votronic HH-SI-30.3/V1.1/0 handheld microphone is Voice mode 1 (default), 4, 5 with mode 1 parameter fixed. The second voice interface is headset and microphone settings, with voice mode 2, 3, 6.
To prevent the introduction of high frequency interference from the microphone and headphone wires, the normal operation of TC35 is affected. When designing the circuit, inductance is connected to the sockets of the microphone, headphone and handset. In addition, considering the electrostatic protection, all voice signal input terminals are coupled with GND through capacitance.
3.4 SIM card circuit

The baseband processor integrates an SIM interface compatible with the ISO 7816-3 IC Card standard. To fit the external SIM interface, the interface connects to the primary interface (ZIF connector). On the basis of reserving 5 pins for SIM card in GSM11.11, TC35 reserved 6 pins for SIM card interface on ZIF connector. The added CCINN pins are used to detect whether there is a SIM card inserted in the SIM card holder. When a SIM card is inserted, the pin is set to a high level, so that the system can enter normal operation. However, at present, SIM cards provided by mobile operators do not have CCIN pins, so pin CCIN is connected to CCCC in the design of the circuit.
In the design for the SIM card cabling, a noticeable problem was found: if the fourth leg of the SIM card CCGND is connected directly to the GND of the printed circuit board without any signal isolation protection, the volume of the call will be small. Considering the factors such as EMC and static protection in the design, a copper isolation net is laid on the top layer of the printed circuit board under the SIM bracket, which is connected with the pin of the SIM card's CCGND, and the GND of the CCGND and the circuit board is coupled by two capacitors and inductors in parallel. This constitutes a separation area for the SIM card, shielding the interference of other signal lines to the SIM card. When you make another voice call, the voice is clear.
4. The closing remark is a mobile device composed of Siemens'GSM module TC35 and its peripheral circuit, which can well complete SMS transmission, data and voice communication. The mobile terminal, which combines Rockwell's Jupiter GPS OEM board and is composed of a single-chip computer controlled mobile unit, can well complete the work of using GSM SMS to transmit vehicle positioning information.
Reference


[1].MAX3238 Datasheethttp://www.dzsc.com/datasheet/MAX3238_1074312.html.
[2].EFRdatasheethttp://www.dzsc.com/datasheet/EFR_2339529.html.
[3].30uS Datasheethttp://www.dzsc.com/datasheet/30uS_2513333.html.
[4]. Siemens TC35/TC37 Hardware Interface Description Vision 04.00, 2002.5.29
[5]. Cellular Engine TC35 Audio Interface Vision 02.00, 2001.11.21
[6]. GSM11.11 Vision 6.2.0, 1999.5


Source:Xiang Xueqin