Datasheet SHT21

www.sensirion.com / D1

Version 5 – October 2022

8/14

Command

Comment

Code

Trigger T measurement

hold master

1110’0011

Trigger RH measurement

hold master

1110’0101

Trigger T measurement

no hold master 1111’0011

Trigger RH measurement

no hold master 1111’0101

Write user register

1110’0110

Read user register

1110’0111

Soft reset

1111’1110

Table 6 Basic command set, RH stands for relative humidity,

and T stands for temperature

Hold master or no hold master modes are explained in

next Section.

5.4

Hold / No Hold Master Mode

There are two different operation modes to communicate

with the sensor: Hold Master mode or No Hold Master

mode. In the first case the SCL line is blocked (controlled

by sensor) during measurement process while in the latter

case the SCL line remains open for other communication

while the sensor is processing the measurement. No hold

master mode allows for processing other I2C

communication tasks on a bus while the sensor is

measuring. A communication sequence of the two modes

is displayed in Figure 15 and Figure 16, respectively.

In the hold master mode, the SHT2x pulls down the SCL

line while measuring to force the master into a wait state.

By releasing the SCL line the sensor indicates that internal

processing is terminated and that transmission may be

continued.

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 0 1

I2C address + write

Command (see Table 6)

19 20 21 22 23 24 25 26 27

S 1 0 0 0 0 0 0 1

Measurement

I2C address + read

Hold during measurement

28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45

0 1 1 0 0 0 1 1

0 1 0 1 0 0 1 0

Data (MSB)

Data (LSB) Stat.

46 47 48 49 50 51 52 53 54

0 1 1 0 0 1 0 0

P

Checksum

Figure 15 Hold master communication sequence – grey blocks

are controlled by SHT2x. Bit 45 may be changed to NACK

followed by Stop condition (P) to omit checksum transmission.

In no hold master mode, the MCU has to poll for the

termination of the internal processing of the sensor. This is

done by sending a Start condition followed by the I2C

header (1000’0001) as shown in Figure 16. If the internal

processing is finished, the sensor acknowledges the poll of

the MCU and data can be read by the MCU. If the

measurement processing is not finished the sensor

answers no ACK bit and the Start condition must be

issued once more.

When using the no hold master mode it is recommended

to include a wait period of 18.5 µs after the reception of

the sensor’s ACK bit (bit 18 in Figure 16) and before the

Stop condition (see section 5.8).

For both modes, since the maximum resolution of a

measurement is 14 bit, the two last least significant bits

(LSBs, bits 43 and 44) are used for transmitting status

information. Bit 1 of the two LSBs indicates the

measurement type (‘0’: temperature, ‘1’ humidity). Bit 0 is

currently not assigned.

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 0 1 0 1

wait P

I2C address + write

Command (see Table 6)

20µs

19 20 21 22 23 24 25 26 27

Measurement

S 1 0 0 0 0 0 0 1

P

measuring

I2C address + read

19 20 21 22 23 24 25 26 27

Measurement

S 1 0 0 0 0 0 0 1

continue measuring

I2C address + read

28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45

0 1 1 0 0 0 1 1

0 1 0 1 0 0 1 0

Data (MSB)

Data (LSB) Stat.

46 47 48 49 50 51 52 53 54

0 1 1 0 0 1 0 0

P

Checksum

Figure 16 No Hold master communication sequence – grey

blocks are controlled by SHT2x. If measurement is not

completed upon “read” command, sensor does not provide ACK

on bit 27 (more of these iterations are possible). If bit 45 is

changed to NACK followed by Stop condition (P) checksum

transmission is omitted.

In the examples given in Figure 15 and Figure 16 the

sensor output is SRH = ‘0110’0011’0101’0000’. For the

calculation of physical values Status Bits must be set to ‘0’

– see Chapter 6.

The maximum duration for measurements depends on the

type of measurement and resolution chosen – values are

displayed in Table 7. Maximum values shall be chosen for

the communication planning of the MCU.