In agricultural production activities, the collection and transmission of temperature and humidity information is a very important function. In the past, extensive production is now pursuing high-tech and refined production. In agriculture, breeding, breeding and other occasions, the temperature needs to be controlled. In many fields, temperature and pressure data can be collected by sensors. How to effectively manage the temperature of the greenhouse is the focus of current vegetable management. In this paper, wireless communication technology is used for data transmission due to the problem that the spatial position of the sensor in different periods of the crop is not fixed, the wiring is inconvenient, and the reliability is poor. Using a micro-power RF chip (nRF24E1), a point-to-multipoint wireless radio frequency data acquisition system with small size, low cost, stable performance, low power consumption, and reliable data transmission is designed to realize real-time data wireless to multiple sampling points collection. After actual use, the temperature measurement system composed of DSl8820, single-chip AT89S52 and RF chip completely meets the needs of users.
1 Overall system design
What this system wants to achieve is the collection, transmission, display and post-processing of temperature, humidity and pressure. The overall concept of the system is data acquisition module, short-distance wireless communication module, serial communication module, display processing module. The determination of the system plan is mainly focused on the selection of the wireless communication module and the display module.
The hardware system is composed as shown in Figure 1. First, the sensor is used to convert the on-site signal into an electrical signal, which is converted into a digital signal after sampling, quantization, and encoding by an analog-to-digital converter ADC. The transmission chip sends valid data to the receiving end by wireless means. After receiving the valid data, the receiving end sends the data to the mobile phone through the serial port. The mobile phone completes the data display and further processing of the valid data through the control program written in C # language. task.
2 System hardware design
2.1 System hardware composition
Figure 2 shows the hardware composition of the system. DSl8820 is a one-line digital temperature sensor produced by DALLAS, which can directly convert the temperature signal into a serial digital signal for microcomputer processing. It has a 3-pin TO-92 small-volume package; the temperature measurement range is -55 ～ + 125 ℃, and it can be programmed to 9 ～ 12 b A / D conversion accuracy. The temperature measurement resolution can reach O. At 0625 ℃, the measured temperature is serially output using the sign-extended 16b digital method; its working power can be introduced at the remote end or can be generated by parasitic power; multiple DSl8820 can be connected in parallel to 3 or 2 lines, The CPU can communicate with many DS18B20s with only one port line, which occupies less ports of the microprocessor, which can save a lot of leads and logic circuits. The information read from the DSl820 or the information written to the DSl820 requires only one port line (single line interface). The power for reading, writing and temperature conversion comes from the data bus. The bus itself can also supply power to the connected DS1820 without additional power supply. The above characteristics make DSl8B20 very suitable for long-distance multi-point temperature detection system. DSl820 provides nine-bit temperature readings, forming a multi-point temperature detection system without any peripheral hardware. The connection between DSl820 and nRF24E1 is shown in Figure 3.
In this system, after the sensor collects data, it is sent to the lower computer, and the lower computer and the upper computer communicate through the wireless data channel. The wireless data transceiver part is mainly composed of nRF24E1 chip, E2PR-OM, crystal oscillator, sensor signal input and antenna.
2. 2 chip nRF24E1 internal structure
The nRF24E1 chip is the core of the wireless data receiving and sending part. Through the embedded 51 single-chip core, it controls the A / D conversion module, wireless receiving and sending module in the chip. Integrate RF transmission, reception, GMSK modulation, demodulation, enhanced 8051 core, 9-input 12b ADC, 125 channels, UART, SPI, PWM, RTC, WDT into a single chip to achieve data collection, transmission, processing And other functions. The hardware module is shown in Figure 4. The chip is mainly composed of the following parts:
(1) CPU (microprocessor). With enhanced 805l core, ADC, SPI, 1 RF transmitter, 2 RF receivers, 5 wake-up timer interrupt sources, 1 UART and 3 timers.
(2) PWM output. The output of the PWM can be programmable to work in 6, 7, or 8 bits, and the frequency of the PWM signal can be controlled by software.
(3) SPI interface and SPI bus.
(4) RTC wake-up timer, WTD and RC oscillator.
(5) A / D converter. The A / D converter has 9 input channels that can be selected by software.
(6) The wireless transceiver communicates with other modules through an internal parallel port or an internal SPI port and works in the 2.4 to 2.5 GHz frequency band that is open globally.
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