Design of wireless data transmission system based on ARM

With the rapid development of social economy in the 21st century, people's demand for mobile computing and broadband wireless communication that can provide information services anytime and anywhere is becoming more and more urgent. In industrial production, it is often necessary to collect a large amount of field data, such as temperature, weight, air pressure, etc., and transmit these data to the host for processing. The host transmits the control signal to the field execution module for various operations based on the processing results. It can be seen that whether the data is from the acquisition device to the processing terminal, or from the monitoring and control instructions from the processing terminal to the acquisition device, it must go through the important link of the transmission process. However, when the wired network is not smooth or it is inconvenient to set up lines due to the limitations of on-site environmental factors, the use of wireless communication technology for data collection and transmission is more practical, efficient and fast.

1 Working principle of wireless data transmission system
As shown in Figure 1, the entire system first uses a sensor to convert the field signal into an electrical signal, which is converted into a digital signal after sampling, quantization and encoding by the analog/digital converter ADC, and sent to the microcontroller for preliminary processing, and then uses the nRF905 wireless data transmission chip to send the valid data to the receiving end by wireless means. After receiving the valid data, the receiving end sends the data to the output device through the serial port and further processes the valid data.

1.1 ARM Microcontroller LPC1766
The LPC1766 chip uses a high-performance ARM CortexTM-M3 32-bit RISC core with an operating frequency of 100 MHz. It has built-in high-speed memory (up to 512 k bytes of flash memory and 64 k bytes of SRAM), rich enhanced I/O ports and peripherals connected to two APB buses. The board includes 8-channel 12-bit ADC and 10-bit DAC, 4 general-purpose 16-bit timers, motor control PWM interface and multiple standard and advanced communication interfaces: up to 3 I2C, SPI, 2 I2S, 1 SDIO, 4 USART, a USB Host/Device/OTG interface and two CAN, Ethernet MIC interface, Quadrature Encoderinterface. The LPC1766 chip operates in a temperature range of -40 to +105°C and a supply voltage of 2.0 to 3.6 V. Its series of power-saving modes highlight its low power consumption. The rich peripheral configuration makes the LPC1700 microcontroller suitable for a variety of application fields: motor drive and application, control of medical and handheld devices, automotive electronics and other fields.
1.2 nRF905 wireless data transmission module
nRF905 is a single-chip RF transceiver launched by Nordic VLSI. The operating voltage is 1.9~3.6 V. It works in three ISM bands of 433 MHz, 868 MHz and 915 MHz. The channel switching time is less than 650μs and the maximum data rate is 100 kbit/s. nRF905 can automatically complete the processing of headers and CRC (cyclic redundancy check), and Manchester encoding/decoding can be automatically completed by the on-chip hardware. It uses the SPI interface to communicate with the microcontroller. The configuration is very convenient and its power consumption is very low. The nRF905 working mode mainly includes active mode and power saving mode. In active mode, data can enter from the processor at a low speed and then be transmitted at a high speed; at the same time, the data frame header can be automatically generated and the CRC checksum can be calculated when sending data. It has a carrier detection function when receiving data, performs address detection on the received code, and calculates CRC to ensure the accuracy of the data.

2 Hardware design of wireless data transmission system
The hardware of the wireless data transmission system is based on the LPC1766 microcontroller and its peripheral components. It sends and receives data through the serial port and controls the wireless transmission module to finally meet the system requirements.
2.1 Design of microcontroller module
This design uses the AMR microcontroller chip LPC1766, which has 100 pins and more than 70 input and output ports. Other ports are used to connect to power, ground and resistors; there are also watchdog clocks, real-time clocks, 10-bit 8-channel digital-to-analog conversion, etc. Among them, LPC1766 and nRF905 communicate through the SIP protocol.
2.2 Design of wireless transmission module
Wireless data transmission is mainly achieved by controlling the wireless RF chip nRF905 by the single-chip microcomputer LPC1766. The transmission process is as follows: First, LPC1766 sets various configuration registers of nRF905 (such as communication frequency, local address, target address, data length, etc.) through the SPI interface, and puts the data to be sent into the transmission storage area; then LPC1766 sets TRX_CE and TX_EN high to start nRF905 transmission, nRF905 starts the RF circuit, completes data packaging, and transmits data at a rate of 100 kbps using GFSK; if AUTO_RETRAN is set high, nRF905 repeatedly sends data until 'FRX-CE is set low; when the transmission is completed, if TRX_CE is set low, nRF905 automatically enters the standby state. [page]

The receiving process is as follows: first, by setting TRX_CE and TX_EN to low, select the receiving state; after 650μs, nRF905 starts to detect the signal in the air; when nRF905 detects the carrier on the receiving frequency, the carrier detection (CD) pin is set high; when a valid address is received, the address match (AM) pin is set high; when a valid data packet is received, that is, the CRC detection is correct, nRF905 opens the data packet, removes the header and tail, and sets the receive data or send data ready (DR) pin high; MCU sets the TRX_CE pin low to enter the standby state, and takes out the data through the SPI interface; after the data is taken out, nRF905 sets the DR and AM pins low again.

3 Software Design of Wireless Data Transmission System
This design only realizes point-to-point communication. To achieve this step, it is mainly necessary to configure LPC1766 and NRF905 appropriately, write the NRF905 transceiver program, and configure the LPC1766 pin function. At the sending end, in order to save power as much as possible, NRF905 is set to STANDBY static mode at the beginning, and SPI_CSN is set to invalid. This can achieve energy saving effect on the one hand, and can monitor whether there is data to be sent at any time. When there is data to be sent, it is changed to the sending mode and SPI_CSN is set to valid to start chip selection. The program flow charts of the sending and receiving ends are shown in Figures 3 and 4.

4 Conclusion
After the system was successfully debugged, multiple data transmission experiments were conducted. This system has passed the verification of actual data communication experiments, with stable operation and reliable communication, and can achieve wireless communication of about 50 meters. This system can be applied to some actual working environments of point-to-point and point-to-multipoint communication.