Datasheet STS21

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Version 2 – December 2011

7/12

In the hold master mode, the STS21 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 1 0 1 0 0

1 1 1 0 0 0 1 1

Command (see Table 6)

19 20 21 22 23 24 25 26 27

S 1 0 0 1 0 1 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 0 0

Data (MSB)

Data (LSB) Stat.

46 47 48 49 50 51 52 53 54

0 0 0 0 0 1 1 0

P

Checksum

Figure 12 Hold master communication sequence – grey blocks

are controlled by STS21. 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 (1001‟0101) as shown in Figure 13. 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 plus header

byte must be issued once more.

For both modes, since the maximum resolution of a

measurement is 14 bit, the two 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). Bit 0 is currently not

assigned.

In the examples given in Figure 12 and Figure 13 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.

conditions (S) without closing prior sequence with Stop

condition (P) – compare Figure 12, Figure 13 and Figure

18. Still, any sequence with adjacent Start condition may

alternatively be closed with a Stop condition.

1

2

3

4

5

6

7

8

9

10 11 12 13 14 15 16 17 18

S 1 0 0 1 0 1 0 0

1 1 1 1 0 0 1 1

Command (see Table 6)

19 20 21 22 23 24 25 26 27

Measurement

S 1 0 0 1 0 1 0 1

measuring

19 20 21 22 23 24 25 26 27

Measurement

S 1 0 0 1 0 1 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 0 0

Data (MSB)

Data (LSB) Stat.

46 47 48 49 50 51 52 53 54

0 0 0 0 0 1 1 0

P

Checksum

Figure 13 No Hold master communication sequence – grey

blocks are controlled by STS21. 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.

Resolution

T typ

T max

Units

14 bit

66

85

ms

13 bit

33

43

ms

12 Bit

17

22

ms

11 bit

9

11

ms

Table 7 Measurement times for 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.

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. 5.6). The soft reset takes less than

15ms.

1

2

3

4

5

6

7

8

9

10 11 12 13 14 15 16 17 18

S 1 0 0 1 0 1 0 0

1 1 1 1 1 1 1 0

P

Soft Reset

Figure 14 Soft Reset – grey blocks are controlled by STS21.