Datasheet SHT25

www.sensirion.com

Version 0.91 – October 2010

8/12

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 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

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 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

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.

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

Command (see Table 6)

19 20 21 22 23 24 25 26 27

Measurement

S 1 0 0 0 0 0 0 1

measuring

19 20 21 22 23 24 25 26 27

Measurement

S 1 0 0 0 0 0 0 1

continue measuring

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 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

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.

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.

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.