Datasheet SHT25

www.sensirion.com

Version 0.91 – October 2010

9/12

5.5

Soft Reset

This command (see Table 6) is used for rebooting the

sensor system without switching the power off and on

again. Upon reception of this command, the sensor

system reinitializes and starts operation according to the

default settings – with the exception of the heater bit in the

user register (see Sect. 5.6). The soft reset takes less than

15ms.

1

2

3

4

5

6

7

8

9

10 11 12 13 14 15 16 17 18

S 1 0 0 0 0 0 0 0

1 1 1 1 1 1 1 0

P

Soft Reset

Figure 17 Soft Reset – grey blocks are controlled by SHT2x.

5.6

User Register

The content of User Register is described in Table 8.

Please note that reserved bits must not be changed and

default values of respective reserved bits may change

over time without prior notice. Therefore, for any writing to

the User Register, default values of reserved bits must be

read first. Thereafter, the full User Register string is

composed of respective default values of reserved bits

and the remainder of accessible bits optionally with default

or non-default values.

Bit

# Bits

Description / Coding

Default

7, 0

2

Measurement resolution

RH

T

‘00’

12 bit

14 bit

‘01’

8 bit

12 bit

‘10’

10 bit

13 bit

‘11’

11 bit

11 bit

‘00’

6

1

‘0’: VDD > 2.25V

‘1’: VDD < 2.25V

‘0’

3, 4, 5

3

Reserved

2

1

Enable on-chip heater

‘0’

1

1

Disable OTP Reload

‘1’

Table 8 User Register. Cut-off value for End of Battery signal

may vary by ±0.05V. Reserved bits must not be changed. “OTP

reload” = ‘0’ loads default settings after each time a

measurement command is issued.

The end of battery alert is activated when the battery

power falls below 2.25V.

The heater is intended to be used for functionality

diagnosis

–

relative

humidity

drops

upon

rising

temperature. The heater consumes about 5.5mW and

provides a temperature increase of about 0.5 – 1.5°C.

OTP Reload is a safety feature and loads the entire OTP

settings to the register, with the exception of the heater bit,

before every measurement. This feature is disabled per

default and is not recommended for use. Please use Soft

Reset instead – it contains OTP Reload.

User Register is given in Figure 18.

1

2

3

4

5

6

7

8

9

10 11 12 13 14 15 16 17 18

S 1 0 0 0 0 0 0 0

1 1 1 0 0 1 1 1

Read Register

19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36

S 1 0 0 0 0 0 0 1

0 0 0 0 0 0 1 0

Register content

37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54

S 1 0 0 0 0 0 0 0

1 1 1 0 0 1 1 0

Write Register

55 56 57 58 59 60

61 62

63

0 0 0 0 0 0 1 1

P

Register content to be written

Figure 18 Read and write register sequence – grey blocks are

controlled by SHT2x. In this example, the resolution is set to 8bit

/ 12bit.

5.7

CRC Checksum

CRC8 is a well known standard checksum concept. For

implementation please refer to public sources such as

Wikipedia.

5.8

Serial Number

SHT25 provides an electronic identification code. For

instructions on how to read the identification code please

refer to the Application Note “Electronic Identification

Code” – to be downloaded from the web page

6 Conversion of Signal Output

Default resolution is set to 12 bit relative humidity and 14

bit temperature reading. Measured data are transferred in

two byte packages, i.e. in frames of 8 bit length where the

most significant bit (MSB) is transferred first (left aligned).

Each byte is followed by an acknowledge bit. The two

status bits, the last bits of LSB, must be set to ‘0’ before

calculating physical values. In the example of Figure 15

and Figure 16, the transferred 16 bit relative humidity data

is ‘0110’0011’0101’0000’ = 25424.

6.1

Relative Humidity Conversion

humidity RH is obtained by the following formula (result in

%RH), no matter which resolution is chosen: