Datasheet SHT20 (Sensirion) - 10

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5.8 I2C Communication Recommendation 5.9 Serial Number
Uncontrolled operation in the No-Hold Master Mode (see SHT20 provides an electronic identification code. For section 5.4) can very rarely result in false offsets of the instructions on how to read the identification code please temperature or relative humidity measurement values14. refer to the Application Note “Electronic Identification These offsets can occur, if a rising edge on the SCL falls Code” – to be downloaded from the web page within 15 µs to 18.5 µs after the sensor has sent an www.sensirion.com/products/catalog/SHT20/ ACK bit, which confirms the correct reception of a measurement command. In this time window the sensor’s calibration data stored in the OTP memory, might be
6 Conversion of Signal Output
corrupted during transmission to the working memory. Once the offsets manifest in the measurement data, it will Default resolution is set to 12 bit relative humidity and remain until the next soft or hard reset. These offsets can 14 bit temperature reading. Measured data are transferred be completely avoided using one of the following four in two byte packages, i.e. in frames of 8 bit length where workarounds: the most significant bit (MSB) is transferred first (left aligned). Each byte is followed by an acknowledge bit. The 1.
Recommended:
Avoiding of rising edges in clock line (SCL) during critical time window. To this end the stop two status bits, the last bits of LSB, must be set to ‘0’ condition from the master should be sent >18.5 µs before calculating physical values. In the example of after the ACK bit is send from the slave. This prevents Figure 15 and Figure 16, the transferred 16 bit relative the offset to occur under all circumstances. humidity data is ‘0110’0011’0101’0000’ = 25424.
6.1 Relative Humidity Conversion
With the relative humidity signal output SRH the relative humidity RH is obtained by the following formula (result in %RH), no matter which resolution is chosen: S RH RH = − 6 +125 
Figure 19
Stop condition after critical time window 16 2 In the example given in Figure 15 and Figure 16 the 2. Sending the stop condition before the critical time window, i.e. <15 µs and other communications relative humidity results to be 42.5 %RH. after 18.5 µs. The physical value RH given above corresponds to the relative humidity above liquid water according to World Meteorological Organization (WMO). For relative humidity above ice RHi the values need to be transformed from relative humidity above water RHw at temperature t. The equation is given in the following, compare also Application Note “Introduction to Humidity”:
Figure 20
Stop condition before critical time window  β t   β t  RH = RH exp exp i  w w   i  3. Operating only in the Hold-Master mode (see section  λ + t w   λ + t i  5.4). Units are %RH for relative humidity and °C for 4. Performing an OTP reload each time a measurement temperature. The corresponding coefficients are defined command is issued. This entails copying the content as follows: β of the OTP into the working memory, which results in w = 17.62, λw = 243.12°C, βi = 22.46, λ an increased measurement time of around 2.5 ms. i = 272.62°C. This can be achieved by changing the relevant bit in
6.2 Temperature Conversion
the user register. As part of the OTP reload, all values The temperature T is calculated by inserting temperature in the user register are reinitialized to their default values (except the heater bit). Therefore, the bit signal output ST into the following formula (result in °C), no matter which resolution is chosen: (BIT 1) in the user register that controls the OTP must be changed prior to every measurement (see section ST T = − 4 6.85 +175.72  5.6). 16 2 14 If the sensor is operated using Sensibus commands the of set wil not occur www.sensirion.com Version 5 – October 2022 10/14