The design of two-way communication test and measurement circuit module

Relying on the perfect Beidou satellite communication positioning system, GPRS communication network and various technical platforms equipped with sensor equipment, it has the ability to measure wind, temperature, humidity, pressure and other meteorological parameters and collect ocean currents, salinity, chemistry and other required oceans The research and development of complete functions such as hydrological parameters, target positioning, emergency alarm reporting, information transmission, statistical reports, etc. has a broader application prospect. This paper presents a design scheme of two-way communication circuit system. It has the characteristics of low power consumption, miniaturization, rich interfaces, and high reliability. It can provide multiple functional services such as ship positioning, maritime communication, and distress calls. It can improve safety and prevention capabilities, and is therefore of great significance to relevant industry authorities.
ARM9 processor S3C2440A
The S3C2440A based on the ARM920T core is used as the system central processing unit, equipped with 128M SDRAM, 256M NAND FLASH and 16M NOR FLASH to cope with the memory consumed by the embedded Linux operating system. S3C2440A integrates MMC/SD card read-write controller, LCD and touch screen interface, 3 UART serial ports, 1 master control and 1 slave USB interface, 1 IDE interface can be connected to a large-capacity hard disk, real-time clock, AC' 97 audio interfaces, up to 130 general-purpose IO ports and many other hardware resources. S3C2440A's internal integrated 3-channel serial ports can be connected to the Beidou satellite communication positioning terminal, GPRS module and wind sensor through an external simple RS232 level conversion chip. In addition, the ARM9 processor S3C2440A can be connected to a 16-bit ADC through the internal integrated two-way SPI interface to connect sensors such as temperature, humidity, salinity, and pressure. The general IO port of S3C2440A is used as a switch input detection and output control interface, and can be expanded according to user needs. At the same time, S3C2440A can also meet the needs of scalable applications such as connecting Ethernet controllers, USB peripherals, electronic compasses, ocean current meters, and underwater acoustic transmission.
Beidou satellite communication module circuit design
The design of the Beidou satellite communication module circuit can be expanded from three levels of signal reception, signal processing and functional interface display. Correspondingly, the Beidou satellite communication terminal with high cost performance is adopted, and the serial communication circuit and LCD liquid crystal display circuit are designed. The Beidou satellite communication module can choose the UM220 module, which supports the Beidou second-generation (BD2) and GPS dual-system navigation timing. It has the advantages of small size (only 40×30×3.7mm), low power consumption (only 350mW), and high integration. , The module's three-dimensional positioning accuracy is 3m, the speed accuracy is 0.1m/s, and the data update rate can reach 1Hz. UM220 is also equipped with satellite display and control software CDT (Control&Display Tool), which provides a simple graphical user interface, which can easily control the satellite receiver and set the functions to obtain the required information. Beidou satellite communication module circuit mainly includes UM220 interface, antenna, backup power supply, reset and serial communication circuit. The circuit schematic diagram is shown in Figure 4. The serial port 0 pins TXD0 and RXD0 of ARM9 processor S3C2440A are connected with UM220's RXD3 and TXD3 to form a serial communication circuit. The pin GPE0 of S3C2440A is connected to the PPS pin of UM220, which can receive the PPS signal with adjustable pulse width and polarity output by UM220; pin GPE1 of S3C2440A is programmable.


Figure 4 UM220 circuit schematic
The event signal with adjustable output pulse width and polarity is provided to the EVENT pin of UM220. Electrolytic capacitor CT1 and ceramic capacitor C1 are connected in parallel between the UM220 power supply pins VCC and GND to filter out high-frequency and low-frequency clutter signals, so that the UM220 power supply is stable and pure, and the peak-to-peak voltage does not exceed 50mV. UM220 has a time service function. In order to maintain the system clock, the VBAT pin of UM220 is connected to a 3V lithium battery via a current limiting resistor R1 and voltage drop diodes D2 and D3. Among them, the voltage drop of diode D3 is 0.3V, and the voltage drop of D2 is 0.7V to ensure that the VBAT pin of UM220 is normally powered by the main power supply 3.3V, and when the main power supply fails, it is powered by the backup power supply 3V lithium battery. The GNSS_ANT pin of UM220 needs to be wired 50Ω on the PCB circuit board to match the antenna impedance, and then connect to the Beidou satellite cellular antenna. In addition, the resistors R2, R3, the capacitor C2 and the diode D1 form a stable low-level reset circuit, and the low-level retention time is longer than 2ms.
GPRS module circuit design
The measuring instrument mainly relies on the satellite communication network for data communication and positioning. In order to reduce the power consumption and operating cost of the measuring instrument system, the GPRS/CDMA network can be used for remote communication. The peripheral application circuit design of GPRS module includes module startup circuit, data communication circuit, voice communication circuit and SIM card application circuit. The GPRS module can choose Siemens MC55. MC55 is a new generation launched by Siemens.


Figure 5 MC55 circuit schematic
The wireless communication GPRS module can quickly and reliably realize the data, voice transmission, short message service and fax in the system plan. The module has a compact structure, light weight, built-in TCP/IP protocol, and is controlled by AT commands to make the application easy Access the network. The GPRS module circuit mainly includes MC55 interface, SIM card circuit, starting power supply and serial communication circuit connected with S3C2440A. The circuit schematic diagram is shown in Figure 5. The RXD1 and TXD1 pins of S3C2440A serial port 1 are connected with the TXD0 and RXD0 pins of MC55 to form a serial communication circuit. The CCGND, CCIN, CCREST, CCIO, CCVCC and CCCLK pins of MC55 form the SIM card interface, and are connected with filter capacitors C4 and C5. The power supply VBATT of MC55 is obtained by stepping down 5V through diode D2. The GPE3 of S3C2440A drives and controls the IGT pin of MC55 to enter the normal operating mode; GPE2 of S3C2440A can detect the pulse signal output by the RING pin of MC55 to determine whether the system sleeps and enters a low-power state. In order to generate a synchronization signal when there is GPRS data information transmission, it can be realized by configuring the SYNC pin of MC55 to control the light-emitting LED status indicator. When data is sent, the SYNC pin outputs a high level to make the base of the transistor T1 conductive, and the red light-emitting diode LED1 is lit.
16-bit ADC circuit design
S3C2440A can cascade multiple high-precision and fast 16-bit ADC chips through its own SPI interface to realize the acquisition and data conversion of various sensor analog signals such as temperature, humidity, salinity, and pressure. The 16-bit ADC chip selects ADI’s AD7798. The AD7798 has a low-power, low-noise, and complete analog front end suitable for high-precision measurement applications. It has a built-in low-noise 16-bit Σ-Δ ADC with 3 differential analog inputs and Integrated on-chip low-noise instrumentation amplifier, which can directly measure input small signals. Figure 6 shows that the S3C2440A is connected to an AD7798 through the SPI interface for measurement


Figure 6 AD7798 circuit schematic
Schematic diagram of the circuit principle of temperature, pressure and salinity data. S3C2440A's SPICLK1, SPIMSI1, SPIMISO1, and GPE4 pins form the SPI interface to connect the serial clock (SCLK), data input (DIN), data output (DOUT) and chip select of AD7798, respectively. Taking the third analog channel of AD7798 to measure temperature as an example, the thermistor and three precision resistors R14, R15, R16 form an unbalanced bridge, and the output differential small signal is a dual path formed by R18, C13, R17, C14 and C12 The RCπ type first-order low-pass filter circuit is connected to the AIN3+ and AIN3- pins of the AD7798.