Datasheet STS21

www.sensirion.com

Version 2 – December 2011

8/12

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

„00‟

14 bit

„01‟

12 bit

„10‟

13 bit

„11‟

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. Threshold value for End of Battery

signal may vary by ±0.1V. 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.

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 15.

5.7

CRC Checksum

STS21 provides a CRC-8 checksum for error detection.

and implementation please refer to the application note

“CRC Checksum Calculation for SHT2x”.

5.8

Serial Number

STS21 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

www.sensirion.com/SHT21.

9

This status bit is updated after each measurement

1

2

3

4

5

6

7

8

9

10 11 12 13 14 15 16 17 18

S 1 0 0 1 0 1 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 1 0 1 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 1 0 1 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 15 Read and write register sequence – grey blocks are

controlled by STS21. In this example, the resolution is set to

12bit.

6 Conversion of Signal Output

Default resolution is set to 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 12 and Figure

13, the transferred 16 bit temperature data is

„0110‟0011‟0101‟0000‟ = 25424.

6.1

Temperature Conversion

The temperature T is calculated by inserting temperature

matter which resolution is chosen:

16

T

2

S

175.72

46.85

T

7 Environmental Stability

The SHT2x sensor series were tested based on AEC-

Q100 Rev. G qualification test method where applicable.

Sensor specifications are tested to prevail under the AEC-

Q100 temperature grade 2 test conditions listed in Table