SHTW2 datasheet(6/14 Pages) ETC2

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Version 2 – August 2016

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5 Operation and Communication

All commands and memory locations of the SHTW2 are

mapped to a 16-bit address space which can be accessed

via the I2C protocol.

SHTW2

Bin.

Dec.

Hex.

I2C address

111’0000

112

0x70

Table 8 SHTW2 I2C device address.

5.1 Power-Up and Communication Start

Upon VDD reaching the power-up voltage level VPOR, the

SHTW2 enters idle state after a duration of tPU. In idle

state, the SHTW2 is ready to receive commands from the

master (microcontroller).

Each transmission sequence begins with START condition

(S) and ends with an (optional) 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.

Please note that in case VDD is set to 0 V (GND), e.g. in

case of a power off of the SHTW2, the SCL and SDA pads

are also pulled to GND. Consequently, the I2C bus is

blocked while VDD of the SHTW2 is set to 0 V.

5.2 Measurement Commands

The SHTW2 provides the possibility to define the sensor

behavior during measurement as well as the transmission

sequence of measurement results. These characteristics

are defined by the appropriate measurement command

(see Table 9). Each measurement command triggers both

a temperature and a humidity measurement.

Clock Stretching

Enabled

Clock Stretching

Disabled

Read T

First

Read H

First

Read T

First

Read H

First

0x7CA2

0x5C24

0x7866

0x58E0

Table 9 Measurement commands.

5.3 Starting a Measurement

A measurement communication sequence consists of a

START condition followed by the I2C header with the 7-bit

I2C device address and a write bit (write W: ‘0’). The

sensor indicates the proper reception of a byte by pulling

the SDA pin low (ACK bit) after the falling edge of the 8th

SCL clock. Then the sensor is ready to receive a 16-bit

measurement

command.

Again,

the

SHTW2

acknowledges the proper reception of each byte with ACK

condition. A complete measurement cycle is presented in

Figure 7.

With the acknowledgement of the measurement

command, the SHTW2 starts measuring humidity and

temperature.

5.4 Sensor Behavior during Measurement and

Clock Stretching

In general, the sensor does not respond to any I2C activity

during measurement, i.e. I2C read and write headers are

not acknowledged (NACK). However, when clock

stretching has been enabled by using a corresponding

measurement command, the sensor responds to a read

header with an ACK and subsequently pulls down the SCL

line until the measurement is complete. As soon as the

measurement is complete, the sensor starts sending the

measurement results.

For best possible repeatability of humidity and temperature

measurements, it is recommended to avoid any

communication on the I2C bus while the SHTW2 is

measuring. For more information, see application note

“SHTC1/SHTW1 Optimization of Repeatibility”.

During measurement, the sensor has a current

consumption according to Table 3.

5.5 Readout of Measurement Results

After a measurement command has been issued and the

sensor has completed the measurement, the master can

read the measurement results by sending a START

condition followed by an I2C read header. The sensor will

acknowledge the reception of the read header and send

two bytes of data followed by one byte CRC checksum

and another two bytes of data followed by one byte CRC

checksum. Each byte must be acknowledged by the

microcontroller with an ACK condition for the sensor to

continue sending data. If the SHTW2 does not receive an

ACK from the master after any byte of data, it will not

continue sending data.

Whether the sensor sends out humidity or temperature

data first depends on the measurement command that

was sent to the sensor to initiate the measurement (see

Table 9).

The I2C master can abort the read transfer with a NACK

condition after any data byte if it is not interested in

subsequent data, e.g. the CRC byte or the second

measurement result, in order to save time.

In case the user needs humidity and temperature data but

does not want to process CRC data, it is recommended to

read the first two bytes of data with the CRC byte (without

processing the CRC data) and abort the read transfer after

reading the second two data bytes with a NACK. This

procedure is more time efficient than starting two different

measurements and aborting the read transfer after the first

two data bytes each time.

5.6 Soft Reset

The SHTW2 provides a soft reset mechanism that forces

the system into a well-defined state without removing the

power supply. If the system is in idle state (i.e. if no

measurement is in progress) the soft reset command can

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