SHT7x (including SHT71 and SHT75) is Sensirion’s family of relative humidity and temperature sensors with pins. The sensors integrate sensor elements plus signal processing in compact format and provide a fully calibrated digital output. A unique capacitive sensor element is used for measuring relative humidity while temperature is measured by a band-gap sensor. Example of this sensor usage with ATmega8 is given in this article.
The applied CMOSens® technology guarantees excellent reliability and long term stability. Both sensors are seamlessly coupled to a 14bit analog to digital converter and a serial interface circuit. This results in superior signal quality, a fast response time and insensitivity to external disturbances (EMC). Each SHT7x is individually calibrated in a precision humidity chamber. The calibration coefficients are programmed into an OTP memory on the chip. These coefficients are used to internally calibrate the signals from the sensors. The 2-wire serial interface and internal voltage regulation allows for easy and fast system integration. The small size and low power consumption makes SHT7x the ultimate choice for even the most demanding applications. SHT7x is supplied on FR4 with pins which allows for easy integration or replacement. The same sensor is also available as surface mountable packaging (SHT1x) or on flex print (SHTA1).
SHT7x V4 – for which this datasheet applies – features a version 4 Silicon sensor chip. Besides a humidity and a temperature sensor the chip contains an amplifier, A/D converter, OTP memory and a digital interface.
Results that comes from this sensors are very precise and already calibrated so you cannot make mistake. Its measurement characteristics are displayed in tables below.
Power supply voltage can be in range from 2.4 to 5.5 V, but depending on voltage you should choose different constants in result calculation functions.
Sensor component itself has 4 pins.
The supply voltage of SHT7x must be in the range of 2.4 and 5.5V, recommended supply voltage is 3.3V. Decoupling of VDD and GND by a 100nF capacitor is integrated on the backside of the sensor packaging. The serial interface of the SHT7x is optimized for sensor readout and effective power consumption. The sensor cannot be addressed by I2C protocol, however, the sensor can be connected to an I2C bus without interference with other devices connected to the bus. Microcontroller must switch between protocols.
For connecting this sensor with microcontroller you need only two digital pins and one pullup resistor. On picture below you can see example on how to connect it.
Serial clock input (SCK)
SCK is used to synchronize the communication between microcontroller and SHT7x. Since the interface consists of fully static logic there is no minimum SCK frequency.
Serial data (DATA)
The DATA tri-state pin is used to transfer data in and out of the sensor. For sending a command to the sensor, DATA is valid on the rising edge of the serial clock (SCK) and must remain stable while SCK is high. After the falling edge of SCK the DATA value may be changed. For reading data from the sensor, DATA is valid TV after SCK has gone low and remains valid until the next falling edge of SCK. To avoid signal contention the microcontroller must only drive DATA low. An external pull-up resistor (e.g. 10 kS) is required to pull the signal high – it should be noted that pull-up resistors may be included in I/O circuits of microcontrollers.
Communication with Sensor
Start up Sensor
As a first step the sensor is powered up to chosen supply voltage VDD. The slew rate during power up shall not fall below 1V/ms. After power-up the sensor needs 11ms to get to Sleep State. No commands must be sent before that time.
Communication with Sensor is explained in details in SHT71 datasheet that you can download on this link. What you need to pay attention to are next operations:
- Sending a Command
List of supported commands is given in table below
Measurement of RH and T are commands you will use the most often.
- Connection reset sequence
- CRC-8 Checksum calculation
- Status Register
Some of the advanced functions of the SHT7x such as selecting measurement resolution, end-of-battery notice, use of OTP reload or using the heater may be activated by sending a command to the status register.
Conversion of Signal Output
obtaining the full accuracy of the sensor it is recommended to convert the humidity readout (SORH) with the following formula with coefficients given below.
In example on Electronics-Base.com we used 12 bit option since it gives more accurate results. Values higher than 99%RH indicate fully saturated air and must be processed and displayed as 100%RH. Please note that the humidity sensor has no significant voltage dependency.
Temperature compensation of Humidity Signal
For temperatures significantly different from 25°C (~77°F) the humidity signal requires temperature compensation. The temperature correction corresponds roughly to 0.12%RH/°C @ 50%RH. Coefficients for the temperature compensation are given below
The band-gap PTAT (Proportional To Absolute Temperature) temperature sensor is very linear by design. Use the following formula to convert digital readout (SOT) to temperature value, with coefficients given in Table below
SHT7x is not measuring dew point directly, however dew point can be derived from humidity and temperature readings. Since humidity and temperature are both measured on the same monolithic chip, the SHT7x allows superb dew point measurements. For dew point (Td) calculations there are various formulas to be applied, most of them quite complicated. For the temperature range of -40 – 50°C the approximation used in example provided on Electronics-Base.com used with AVR ATmega8 provides good accuracy.
In case you are not sure what is exactly dew point here is short explanation:
The dew point is the temperature to which a given parcel of humid air must be cooled, at constant barometric pressure, for water vapor to condense into liquid water. The condensed water is called dew when it forms on a solid surface. The dew point is a saturation temperature.
That's a good indicator for weather to choose to drive car or motorbike during winter ;) It is better indicator than just temperature because road can get slippery on very different temperature depending on humidity of the air.
As conclusion we can say that SHT71 relatively simple sensor to interface and if example provided on www.Elecftronics-Base.com is used it becomes easy job to do. It will provide your device accurate readings of temperature, humidity and dew point. I can recommend you to use it . If you encounter with any problems try reading more detailed data sheet or contact me via website. Good starting point is definitely example written in Codevision C compiler for ATmega8 to read data from SHT71 and send it to UART.