Datasheet SHT21

www.sensirion.com / D1

Version 5 – October 2022

9/14

Resolution RH typ RH max

T typ

T max

Units

14 bit

66

85

ms

13 bit

33

43

ms

12 bit

22

29

17

22

ms

11 bit

12

15

9

11

ms

10 bit

7

9

ms

8 bit

3

4

ms

Table 7 Measurement times for RH and T measurements at

different resolutions. Typical values are recommended for

calculating energy consumption while maximum values shall be

applied for calculating waiting times in communication.

Please note: I2C communication allows for repeated Start

conditions (S) without closing prior sequence with Stop

condition (P) – compare Figures 15, 16 and 18. Still, any

sequence with adjacent Start condition may alternatively

be closed with a Stop condition.

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. 5.4). The soft reset takes less than

15 ms.

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

I2C address + write

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.

The end of battery alert is activated when the battery

power falls below 2.25 V.

The heater is intended to be used for functionality

diagnosis – relative humidity drops upon rising

temperature. The heater consumes about 5.5 mW 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, except for 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.

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

Status: End of battery13

‘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 ±0.1V. Reserved bits must not be changed. “OTP

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

measurement command is issued.

An example for I2C communication reading and writing the

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

I2C address + write

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

I2C address + read

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

I2C address + write

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

SHT21 provides a CRC-8 checksum for error detection.

The polynomial used is x8 + x5 + x4 +1. For more details

and implementation please refer to the application note

“CRC Checksum Calculation for SHT2x”.

13

This status bit is updated after each measurement