www.sensirion.com / D1

Version 1

– December 2022

11/26

3.6 nRESET Pin

The nReset pin may be used to generate a reset of the sensor. A minimum pulse duration of 1 µs

is required to reliably trigger a reset of the sensor. Its function is explained in more detail in section

4.9. If not used it is recommended to leave the pin floating or to connect it to VDD with a series

resistor of R

≥ 2 kΩ. However, the nRESET pin is internally connected to VDD with a pull up resistor

of R = 50

kΩ (typ.).

4 Sensor Operation

The SHT33-DIS supports I2C fast mode (and frequencies up to 1000 kHz). Clock stretching can

be enabled and disabled through the appropriate user command. For detailed information on the

I2C protocol, refer to NXP I2C-bus specification14 [2]. After sending a command to the sensor a

minimal waiting time of 1 ms is needed before another command can be received by the sensor.

All SHT33-DIS commands and data are mapped to a 16-bit address space. Additionally, data and

commands are protected with a CRC checksum. This increases communication reliability. The 16

bits commands to the sensor already include a 3 bit CRC checksum. Data sent from and received

by the sensor is always succeeded by an 8 bit CRC.In write direction it is mandatory to transmit

the checksum, since the SHT33-DIS only accepts data if it is followed by the correct checksum. In

read direction it is left to the master to read and process the checksum.

4.1 Power-Up and Communication Start

The sensor starts powering-up after reaching the power-up threshold voltage VPOR specified in

Table 4. After reaching this threshold voltage the sensor needs the time tPU to enter idle state. Once

the idle state is entered it is ready to receive commands from the master (microcontroller). Each

transmission sequence begins with a START condition (S) and ends with a STOP condition (P) as

described in the I2C-bus specification. Whenever the sensor is powered up, but not performing a

measurement or communicating, it automatically enters idle state for energy saving. This idle state

cannot be controlled by the user.

4.2 Starting a Measurement

A measurement communication sequence consists of a START condition, the I2C write header (7-

bit I2C device address plus 0 as the write bit) and a 16-bit measurement command. The proper

reception of each byte is indicated by the sensor. It pulls the SDA pin low (ACK bit) after the falling

edge of the 8th SCL clock to indicate the reception. A complete measurement cycle is depicted in

Table 10. With the acknowledgement of the measurement command, the SHT33-DIS starts

measuring humidity and temperature.

4.3 Measurement Commands for Single Shot Data Acquisition Mode

In this mode one issued measurement command triggers the acquisition of one data pair. Each

data pair consists of one 16 bit temperature and one 16 bit humidity value (in this order). During

transmission each data value is always followed by a CRC checksum, see section 4.13. In single

shot mode different measurement commands can be selected. The 16 bit commands are shown in

Table 10. They differ with respect to repeatability (low, medium and high) and clock stretching

(enabled or disabled). The repeatability setting influences the measurement duration and thus the

overall energy consumption of the sensor. This is explained in section 2.

14

http://www.nxp.com/documents/user_manual/UM10204.pdf