Datasheet SHT21

www.sensirion.com

Version 1.0 – January 2010

9/12

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 16 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

For implementing CRC8 checksum please refer to

5.8

Serial Number

SHT21 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.

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 13

and Figure 14, 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:

16

RH

2

S

125

6

RH









In the example given in Figure 13 and Figure 14 the

relative humidity results to be 42.5%RH.

The physical value RH given above corresponds to the

relative humidity above liquid water according to World

Meteorological Organization (WMO). For relative humidity

equation is given in the following, compare also

Application Note “Introduction to Humidity:













































t

λ

t

β

t

λ

t

β

RH

RH

i

i

w

w

w

i

exp

exp

Units are %RH for relative humidity and °C for

temperature. The corresponding coefficients are defined

272.62°C.

6.2

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 according to AEC-

Q100

Rev.

F

qualification

test

method.

Sensor

specifications are tested to prevail under the AEC-Q100

Sensor performance under other test conditions cannot be

guaranteed and is not part of the sensor specifications.

Especially, no guarantee can be given for sensor

performance in the field or for customer’s specific

application.

Environment Standard

HTSL

125°C, 1000 hours

Within

specifications

TC

-50°C - 125°C, 1000 cycles

Acc. JESD22-A104-C

Within

specifications

UHST

130°C / 85%RH, 96h

Within

specifications

THU

85°C / 85%RH, 1000h

Within

specifications

ESD immunity MIL STD 883E, method 3015

(Human Body Model at ±2kV)

Qualified

Latch-up

force current of ±100mA with

Qualified

Table 9: Qualification tests: HTSL = High Temperature Storage

Lifetime, TC = Temperature Cycles, UHST = Unbiased Highly

accelerated Stress Test, THU = Temperature Humidity

Unbiased.

temperature grade 2.