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N.B.: The RH conversion formula (1) allows values to be reported which are outside of the range of

0 %RH … 100 %RH. Relative humidity values which are smaller than 0 %RH and larger than 100 %RH are non-

physical; however, these “uncropped” values might be found beneficial in some cases (e.g. when the

distribution of the sensors at the measurement boundaries are of interest). For all who do not want to

engage in evaluation of these non-physical values, cropping of the RH signal to the range of 0 %RH …

100 %RH is advised.

N.B. 2: From a computational perspective, in formulae ( 1 ), ( 2 ) and ( 3 ) the division by 216 − 1 can be

simplified to a division by only 216. The introduced accuracy deviations are <0.002 %RH and <0.003 °C,

respectively.

4.7 Serial Number

Each sensor has a unique serial number, that is assigned by Sensirion during production. It is stored in the

one-time-programmable memory and cannot be manipulated after production. The serial number is

accessible via I2C command 0x89 and is transmitted as two 16-bit words, each followed by an 8-bit CRC.

4.8 Reset & Abort

A reset of the sensor can be achieved in three ways:

1. Soft reset: send the reset command described in Table 8.

2. I2C general call reset: all devices on I2C bus are reset by sending the command 0x06 to the I2C

address 0x00.

3. Power down (incl. pulling SCL and SDA low).

Any command that triggers an action at the sensor can be aborted via I2C general call reset or soft reset.

4.9 Heater Operation

The sensor incorporates an integrated on-package heater which can be switched on by the set of commands

given in Table 8. Three heating powers and two heating durations are selectable which are given in Table 9.

After reception of a heater-on command, the sensor executes the following procedure:

1. The heater is enabled, and the timer starts its count-down.

2. On timer expiration a temperature and humidity measurement with the highest repeatability is

started, the heater remains enabled.

3. After the measurement is finished the heater is turned off.

4. Temperature and humidity values are now available for readout.

The maximum on-time of the heater commands is one second in order to prevent overheating of the sensor

by unintended usage of the heater. Thus, there is no dedicated command to turn off the heater. For extended

heating periods it is required to send periodic heater-on commands, keeping in mind that the heater is

designed for a maximal duty cycle of less than 10%. To obtain a fast increase in temperature the idle time

between consecutive heating pulses shall be kept minimal.

Parameter

Selectable Values

Heater Power (typical for VDD=3.3V)

0 (=off), 20, 110, 200 mW

Heater-on Duration (tHeat)

0.1, 1 s

Maximal duty cycle

10%

Table 9. SHT4x heater specifications.