Design of Wireless Temperature and Humidity Measurement Circuit with Intelligent Radio Frequency Chip nRF9E5

NRF9E5 is a smart RF chip operating at 433/868/915MHz, which integrates 8051 microcontroller, 4-channel 10-bit A/D conversion and multi-channel RF transceiver.

This paper introduces the circuit design and programming implementation of wireless temperature and humidity measurement using this RF chip, temperature sensor LM71, humidity sensor HS1101. The design is simple, reliable, flexible and convenient.

NRF9E5 is a smart RF chip with 433/868/915MHz operating frequency. The chip is powered by 1.9V~3.6V single power supply and 32-foot QFN package (5) × 5mm), transmission power 10dBm, reception sensitivity -100dBm, current only 2.5 at low power consumption μ A, particularly suitable for battery power, for wireless keyboards, wireless phones, wireless headphones, industrial wireless sensors, remote controls, wireless alarms, etc.

Compared with wired sensor measurement, wireless sensor measurement has the advantages of high reliability, easy installation, simple structure, easy reuse and easy system maintenance.

This design takes full advantage of the low power consumption of each device. Using 3.6V lithium battery in the wireless temperature and humidity sensor can not only prolong the life of the battery, but also make the wireless data transmission more convenient and flexible.

Introduction to nRF9E5 function

Figure 1 shows the internal structure of nRF9E5. NRF9E5 is a 10-bit ADC with embedded nRF905 wireless transceiver chip, integrated enhanced 8051 microprocessor and 4 channels. The sampling rate is 80ksps. It contains 1.22V voltage reference, power management, PWM output, UART asynchronous serial port, SPI communication interface, logic interface circuit, watchdog sufficiency circuit, multi-channel programmable wake-up, CRC inspection and multi-point communication control. High frequency inductance and filter are all built-in, with few peripheral elements. The chip has good consistency, stability and is not subject to interference.

NRF9E5 has no complex communication protocol. It is completely transparent to users and can communicate freely with the same products. The built-in CRC error correction hardware circuit and protocol eliminates software error correction programming and error correction operation of micro-controllers by software developers, and reduces the difficulty of wireless application development.

The EEPROM 25320 shown below in Fig. 1 is an external program memory for nRF9E5 (U2 in Fig. 2) with a capacity of 4K bytes (4096). × 8b), for storing applications running on nRF9E5. When powered on, nRF9E5 imports applications into on-chip RAM through its internal solidified boot loader through the SPI interface.

Design of Wireless Temperature and Humidity Sensor Measurement Circuit

A wireless temperature and humidity sensor is composed of RF chip nRF9E5, 14-bit digital temperature sensor LM71 and humidity sensor HS1101.

LM71 is a low-power, 14-bit (1-bit) product produced by National Semiconductor Corporation. ΔΣ ADC temperature sensor, with a temperature measurement range of -40 ~150 C, a temperature resolution of 0.03125 C, a temperature measurement range of -40 ~150 C, +1.5 C, and a power voltage of 2.65V~5.5V, has a three-wire serial interface SPI/MICROWIRE compatible interface, and is easy to connect with microprocessor.

Therefore, in Figure 2, the serial clock SC of U3 (LM71), the serial input and output SI/O, the selected CS of U1 (nRF9E5) are connected to the P02, P01 and P00 of U1 (nRF9E5), respectively. The RF circuits of LM71, nRF9E5 and single-ended 50 antenna are shown in Figure 2, which can work at 818/915MHz. The output of ambient temperature and LM71 temperature sensor is shown in Table 1.

The HS1101 humidity sensor is designed with solid polymer structure, which has fast response time, high reliability and long-term stability, and does not require full interchangeability of calibration. The HS1101 humidity sensor is equivalent to a capacitor C in the circuit_XIn the range of relative humidity 0%-100%RH, the capacity of the capacitor changes from 160pF to 200pF, the error is not more than (+) 2%RH, the response time is less than 5s, and the temperature coefficient is 0.04pF/C.

As shown in Figure 3, the humidity-sensitive capacitance C_XPlaced in the 555 oscillator circuit, the change of capacitance is converted to a voltage frequency signal which is inversely proportional to it, and the frequency signal can be collected directly by the micro controller. The high-level time of the two transient output frequency-varying square-wave signals of the oscillating circuit (the 3-foot output of U4 in Figure 3) is

Output low level time is

So the period of the output square wave signal is

That is

EEPROM Programming

The main program of nRF9E5 is stored in the external serial EEPROM. When the chip starts to power on, the internal solidified factory bootstrapper imports the main program of the external EEPROM into the RAM of nRF9E5 through SPI interface (4K bytes of space, which is not occupied by the main program as in-chip RAM), and then executes jump instructions to start the main program running. The main program needs to write to EEPROM according to the rules shown in Table 2, where SPEED (bit3) is the reading and writing speed of EEPROM, with 0 being 1 MHz and 1 being 0.5 MHz. XO_ FREQ (bit2,1,0) is the frequency of the external crystal vibration of nRF9E5, 000=4MHz, 001=8MHz, 010=12MHz, 011=16MHz, 100=20MHz.

The built-in 8051 compatible microcontroller of nRF9E5, together with its own unique control registers, can be programmed with MCS51 series of simulation software and written into EEPROM to debug.

Write radio frequency communication, data collection and data conversion programs using WAVE6000 (Weifu) simulation software, compile the data file "XXXXXX.HEX" suffixed with HEX, which is in text format: the first character of each line is fixed to ":", indicating the beginning of a line; The 2nd and 3rd characters are the number of instruction data M (hexadecimal) in the line, M is 20H, which means the line has M double-byte data, 2M characters, the 4th, 5th, 6th and 7th characters are the address of the line, the 8th and 9th characters are "00" or "01", the rest are 2M instruction data characters in sequence, and the 2nd is the verification character bit. The HEX file ends with ":00000001FF".

Delphi 6.0 is used to open the data file in the Windows 2000 environment, read out each row of data in turn, remove the first 9 characters of each line and the two characters of each line, the rest is the user's main program file, add the program header in Table 2 before the main program file, which constitutes the program format of the external EEPROM of nRF9E5. As shown in Figure 4, it can be written to EEPROM chip 25320 by RS232 communication and AT89C2052.