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SHT15 in embedded system applications

In Electronic Infomation Category: S | on April 12,2011

Abstract: To improve radon monitor temperature and SI4425bDY-T1-E3 datasheet and humidity measurement accuracy and SI4425bDY-T1-E3 price and real-time, made SHT15 digital temperature and SI4425bDY-T1-E3 suppliers and humidity sensors in measuring radon in the applications. First introduced the digital temperature and humidity sensors SHT15 the basic features, working principle, internal structure, measurement resolution and measurement range, the operation command and the startup sequence, and gives the SHT15 temperature and humidity sensor in the embedded system, the use of SHT15 S3C2440A microcontroller formed with the radon monitor temperature and humidity control system, and gave details of hardware design, software design and temperature, humidity, dew point is calculated. Experimental results show that the traditional analog temperature and humidity sensors, based on measuring radon SHT15 the temperature and humidity control system with simple circuit structure, real time and high accuracy, good stability.

Measurement of radon and its daughters is a cover intelligent instruments, nuclear radiation detection, nuclear technology application, the environment * prices, industrial measurement, industrial automation, computer communications and software and other problems in the field. However, the concentration of radon and its daughters greatly influenced by temperature and humidity, the temperature and humidity measurements for measuring radon is very important. Traditional analog temperature and humidity sensors are generally required to involve signal conditioning circuit and require a complex calibration and calibration process, so accuracy is not guaranteed. SHT15 produced by a single-chip sensor is a digital signal containing a calibrated temperature and humidity combined output of sensors, the patent application process for industrial micro-processing technology COMS (CMOSens), to ensure that products with high reliability and excellent long-term stability. Sensor includes a capacitive polymer humidity measuring element and a gap-type temperature measurement devices, and with a 14-bit A / D converter and serial interface circuit on the same chip to achieve seamless connectivity. Therefore, the product has excellent quality, fast response, anti-interference ability, cost-effective advantages. System uses a micro-controller S3C2440A, S3C2440A ARM920T core by providing a complete set of common system peripherals, there are 130 multi-purpose input / output ports, so the direct use in the design S3C2440A general I / O port to analog communication timing control SHT15, the circuit design simple.

1 SHT15 Introduction

1.1 SHT15 Features

SHT15 Sensirion Swiss company introduced a digital temperature and humidity sensor chip. The chips are widely used in HVAC, automotive, consumer electronics, automatic control and other fields. Its main features are as follows:

1) the temperature and humidity sensor, signal conversion, A / D converter, and I2C bus interface and other functions into one chip;

2) to provide two-wire digital serial interface SCK and DATA, and to support the transmission CRC checksum;

3) precision programmable regulator, built-in A / D converter;

4) to provide temperature compensation and humidity dew point measurement and high-quality computing capabilities;

5) As a result of CMOSensTM technology, immersion in water were measured.

1.2 SHT15 performance parameters

SHT15 performance parameters are as follows:

1) Humidity measuring range: 0 ~ 100% RH; 2) Temperature Range: -40 ~ +123.8 ; 3) Humidity measuring accuracy: 2.0% RH; 4) temperature measurement accuracy: 0.3 ; 5 ) Response time: 8 s (tau63%); 6) can be completely submerged.

As temperature and humidity sensor which has high accuracy, and can measure and so immersed in the water, and radon monitor designed to require a measurement of soil, air and water radon concentration of the feature, so use this type of temperature and humidity sensors to fully meet requirements.

1.3 SHT15 internal structure and working principle

SHT15 digital temperature and humidity sensor is an 8-pin SMD (LCC) surface mount package, shown in Figure 1. One pin 1 ground, pin 4 connected power supply, working voltage is 2.4 ~ 5.5 VDC, the sensor in order to achieve the highest accuracy, supply voltage of 3.3 V is appropriate. Pin 2 is data line, clock line pin 3, pin 5 to pin 8 is empty.

Figure 1 SHT15 pin diagram

SHT15 digital temperature and humidity sensor the temperature sensor, humidity sensor, signal conversion, A / D conversion and heaters and other functions into one chip, its internal structure shown in Figure 2.

Figure 2 SHT15 internal structure

The chip includes a capacitive polymer humidity sensor and a bandgap material with temperature-sensitive components. These two sensitive components, respectively, humidity and temperature into electrical signals, the signal first enters the amplifier to amplify weak signals, and then enter a 14-bit A / D converter, and finally after a two-wire serial digital interface output digital signals. SHT15 in the factory, will be in humidity or temperature environment, calibration, calibration coefficients stored in the calibration register, in the measurement process, the calibration coefficients are automatically calibrate the signals from the sensors. In addition, SHT15 also incorporates an internal heating elements, heating elements can be connected to SHT15 after about 5 temperature rise, while consumption will increase. This function is mainly to compare the heating temperature and humidity values ??before and after. Two sensor elements can be integrated authentication performance. In high humidity (> 95% RH) environment, the sensor can prevent condensation heat sensors, while reducing response time and improve accuracy. SHT15 heating temperature, relative humidity lower than that before heating, measurements will be slightly different.

2 Hardware Design

Microprocessor by wire serial digital interface for communication with the SHT15, because of its communication protocol of the I2C bus protocol with GM is not compatible, so S3C2440A directly in the design of a common I / O port to simulate the communication timing control SHT15 . S3C2440A 130 general-purpose I / O ports, is divided into 9 groups (GPAGPJ), which GPG including 16 I / O port. S3C2440A 289-FBGA package with pins, GPG9 and GPG10 the corresponding pin functions is shown in Table 1.

Table 1 GPG9 and GPG10 corresponding pin function chart

S3C2440A the GPG9 design using analog clock signal, GPG10 to simulate the data signal (data lines require an external pull-up resistor), the hardware connection diagram shown in Figure 3.

SHT15 Figure 3 microcontroller and the hardware connection diagram

3 Software Design

The program began, the controller S3C2440A need a set of "Transmission Start" sequence to indicate the initialization data. It includes: When the clock is high, SCK is low DATA flip, followed by SCK goes low to start, shown in Figure 4, followed by the clock at the SCK high is high when the flip DATA. SCK next goes low, and later goes high, SCK clock is high, DATA once again turn to high.

Figure 4 start transmission sequence

Start command issued by the controller, then the command to send a follow-up 8 yards. Follow-up order includes three address bits (currently only supports "000" ) and 5 command bits. Code corresponding to the appropriate command set shown in Table 2.

Table 2 SHT15 command set

SHT15 received after the above address and command codes in the first 8 clock falling edge, the DATA pull down low as a slave of the ACK; clock falling edge in the first 9 months after the slave releases DATA (recovery high) bus; release the bus after the machine started from the current humidity measurements, measurements after the DATA bus again pulled low; host detects DATA bus is pulled low, the humidity measurement over that given SCK clock signal; from the machine at the 8th clock falling edge, high byte first output data; at the 9th clock falling edge, the host will be low as the ACK signal DATA bus. Then release the bus DATA; in the next eight falling edge SCK cycle, the low byte of data sent from the machine; the next falling edge of SCK, the host again as a DATA bus low data received ACK signal; the last eight from the SCK falling edge CRC issued machine calibration data, not the host response (NACK), said end of the measurement.

The design of microprocessor Samsung S3C2440A, through the I / O registers programmed to simulate the communication process. The processors I / O ports can be set as input, output, and other high impedance state. In the software process to achieve through the Functions I / O port state changes.

Above function can be achieved in a variety of SCK and DATA bus input and output. In SHT15 also need to delay the communication function, designed with function void DelaylIn8 (U32 tt) to achieve the soft delay. This communication protocol can be completed SHT15 simulation.

4 temperature, humidity and dew point calculations

4.1 Relative Humidity

SHT15 I2C bus can be directly output digital humidity, relative humidity coefficient of the output characteristic curve shown in Figure 5.

Figure 5 is converted to relative humidity from SORH

Figure 5 shows, SHT15 was a certain output characteristics of the nonlinear, in order to compensate for humidity sensor to obtain accurate non-linear data, using equation (1) to correct the output value.

Where C1, C2, C3 values ??shown in Table 3, SHT15 relative humidity output value (SORH) is 12 bit.

Table 3, the linear temperature compensation coefficient

4.2 Temperature and relative humidity calculation of temperature compensation

The temperature and humidity sensor has good linearity, available in (2) the digital output is converted to temperature. Temperature conversion factors shown in Table 4.

Table 4, the temperature conversion factor

Actual measurement using equation (3) the amendment to the temperature coefficient. Temperature compensation coefficient shown in Table 5.

Table 5 Temperature compensation coefficient

4.3 dew point calculation

Dew point is a special temperature, humidity of the air to maintain a certain minimum temperature to be achieved. When the air temperature below the dew point, air can not hold too much water, the water will become a fog, dew or frost. Dew point relative humidity according to the current calculated value and the temperature, the specific formula is as follows:

Where, T is the current temperature value, SORH relative humidity values, Dp is the dew point.

5 results

By measuring radon concentration of indoor radon measurements, simultaneous measurement of temperature and humidity, in order to ensure the accuracy of measurements in different places in the atmosphere has been measured, and test and measurement of 20 times, an average calculation, the measurement results shown in Table 6.

Table 6 radon concentration and temperature and humidity measurements

6 Conclusion

This paper SHT15 digital temperature and humidity sensors in embedded systems applications, and details the radon monitor temperature and humidity control module of the design process. As SHT15 integrated 14-bit A / D converter, digital output, with high precision, small size, immunity to interference and so on. As radon monitor temperature and humidity control system to meet system design requirements. The design for the application of other monitoring devices to provide references.

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