Sensirion offers multiple SHT series of digital sensors for measuring both relative humidity and temperature. The temperature is measured using a band-gap sensor, whereas the humidity sensor is capacitive; which means the presence of moisture in air changes the dielectric constant of the material in between the two plates of a parallel-plate capacitor, and hence varies the capacitance. The required signal conditioning, analog-to-digital conversion, and digital interface circuitries are all integrated onto the sensor chip. The various SHT series sensors have different levels of accuracy for humidity and temperature measurements, as described below.
SHT1x are available in surface mount type whereas SHT7x are supplied with four pins which allows easy connection. The SHT11 and SHT75 sensors both provide fully calibrated digital outputs that can be read through a two-wire (SDA for data and SCK for clock) serial interface which looks like I2C but actually it is not compatible with I2C. An external pull-up resistor is required to pull the signal high on the SDA line. However, the SCK line could be driven without any pull-up resistor. The signaling detail of the serial bus is described in the datasheet, which we will implement for PIC18F2550 microcontroller using mikroC pro for PIC compiler. The operating range of both the sensors is 0 to 100% for relative humidity, and -40.0 to 123.8 °C for temperature. The sensor consumes 3 mW power during measurement, and 5 ?W, while in sleep mode.
The SHT11 module that I have got is from mikroElektronika. The sensor (SMD) is soldered on a tiny proto board with all the four pins accessible through a standard 0.1 inch spacing male header. The board comes with pull-up resistors connected to both SDA and SCK lines. One concern in this type of arrangement is the heat dissipated by the pull-up resistors could affect the measurements if the resistors and the sensor are close in the board. We will discuss about this issue later too. The SHT75 module from Sensirion, however, does not include any pull-up resistor for SDA line and therefore must be included externally.
Brief description of SHT1x/7x sensors
Please read the datasheets for SHT1x and SHT7x for detail information about these sensors. I am only providing a brief summary here.
SHT11 and SHT75 are functionally same with SHT75 being more accurate (±1.8% vs ±3%) in measuring relative humidity. Both the sensors can operate from 2.4-5.5 V supply voltage, however the datasheet recommends to use 3.3V for highest accuracy. The default measurement resolution is 14-bit for temperature and 12-bit for relative humidity, which can be reduced to 12- and 8-bit respectively by changing the bit settings of the Status Register (discussed later) inside the sensor chip. We will be using the default resolution settings for measurements.
SCK is the clock line that is used to synchronize the communication between the micrcontroller and the sensor. It is an input only pin on the sensor’s side and therefore the microcontroller should be responsible to generate the clock signal. DATA or SDA is a bidirectional data transfer pin for sending data in and out of the sensor. The sensor should receive a conversion command from the microcontroller in order to start measuring temperature or relative humidity. The measurement commands for relative humidity and temperature are 00000101 (05H) and 0000011 (03H), respectively [The first three most-significant bits are actually the address bits, which are always zero for SHT1x and SHT7x sensors, and the remaining 5 bits are the command bits]. Prior to sending a command, a Transmission Start sequence must be issued by the microcontroller which consists of a lowering of the DATA line while SCK is high, followed by a low pulse on SCK and raising the DATA line high again, while the SCK is already high.
After receiving a command from microcontroller, the sensor issues an acknowledge pulse by pulling the DATA line low for one clock cycle. It takes place after the falling edge of the 8th clock (corresponding to the 8th bit of command sent) on the SCK line, and the DATA line is pulled low until the end of the 9th clock on the SCK line.
After issuing a measurement command (‘00000101’ for relative humidity, ‘00000011’ for temperature) the microcontroller has to wait for the measurement to complete, which takes a maximum of 20/80/320 ms for a 8-/12-/14-bit measurement. During this time, the microcontroller can stop generating clocks on SCK line, and release DATA line. Upon the completion of measurement, the sensor generates a Data Ready signal by pulling the DATA line low. Upon receiving the Data Ready signal, the microcontroller can restart the clock on SCK line to readout the measurement data. The measurement data is kept stored into the memory of the sensor until readout.