Datasheet SHT21

www.sensirion.com

Version 1.0 – January 2010

7/12

commands from the master (MCU). Current consumption

during start up is 350µA maximum.

5.2

Start / Stop Sequence

Each transmission sequence begins with Start condition

(S) and ends with Stop condition (P) as displayed in Figure

11 and Figure 12.

Figure 11 Transmission Start Condition (S) - a high to low

transition on the SDA line while SCL is high. The Start condition

is a unique state on the bus created by the master, indicating to

the slaves the beginning of a transmission sequence (bus is

considered busy after a Start).

Figure 12 Transmission Stop Condition (P) - a low to high

transition on the SDA line while SCL is high. The Stop condition

is a unique state on the bus created by the master, indicating to

the slaves the end of a transmission sequence (bus is

considered free after a Stop).

5.3

Sending a Command

and an SDA direction bit (Read R: ‘1’, Write W: ‘0’). The

sensor indicates the proper reception of a byte by pulling

SCL clock. After the issue of a measurement command

(‘1110’0011’ for temperature, ‘1110’0101’ for relative

humidity’), the MCU must wait for the measurement to

complete. The basic commands are summarized in Table

6. Hold master or no hold master modes are explained in

next Section.

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

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

communication tasks on a bus while the sensor is

measuring. A communication sequence of the two modes

is displayed in Figure 13 and Figure 14, 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

Command (see Table 6)

19

20

21

22

23

24

25

26

27

S 1 0 0 0 0 0 0 1

Measurement

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 0 1 1

P

Checksum

Figure 13 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

header (1000’0001) as shown in Figure 14. 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.

For both modes, since the maximum resolution of a

measurement is 14 bit, the two last 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.

SDA

SCL

70%

30%

70%

30%

SDA

SCL

70%

30%

70%

30%