SHT35 datasheet(9/20 Pages) ETC2

9 / 20 page

Datasheet SHT3x-DIS

www.sensirion.com

August 2016 - Version 3

9/20

Please note that the I2C address is represented through

the 7 MSBs of the I2C read or write header. The LSB

switches between read or write header. The wiring for

the default address is shown in Table 7 and Figure 11.

The ADDR pin mustnot be left floating. Please note that

only the 7 MSBs of the I2C Read/Write header constitute

the I2C Address.

SHT3x-DIS I2C Address in Hex.

representation

Condition

I2C address A

0x44 (default)

ADDR (pin 2)

connected to VSS

I2C address B

0x45

ADDR (pin 2)

connected to VDD

Table 7 I2C device addresses.

3.5

ALERT Pin

The alertpin may be used to connectto the interrupt pin

of a microcontroller. The output of the pin depends on

the value of the RH/T reading relative to programmable

limits. Its function is explained in a separate application

note. If not used, this pin must be left floating. The pin

switches high, when alert conditions are met. The

maximum driving loads are listed in Table 3. Be aware

that self-heating mightoccur,depending on the amount

of currentthatflows. Self-heating can be prevented ifthe

Alert Pin is only used to switch a transistor.

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. If not used it is

recommended to leave the pin floating.

4 Operation and Communication

The

SHT3x-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 specification12.

All SHT3x-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 alreadyinclude a 3 bitCRC 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 SHT3x-DIS only accepts data if it

http://www.nxp.com/documents/user_manual/UM10204.pd f

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 3. 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. The stop

condition is optional. Whenever the sensor is powered

up, but not performing a measurement or

communicating, it automatically enters idle state for

energysaving. This idle state cannotbe controlled bythe

user.

4.2

Starting a Measurement

A measurementcommunication 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 measurementcycleis

depicted in Table 8.

With the acknowledgement of the measurement

command, the SHT3x-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 4.4.

In single shot mode different measurement commands

can be selected. The 16 bit commands are shown in

Table 8. 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 ofthe

sensor. This is explained in section 2.

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