Datasheet
Datasheet STS21
www.sensirion.com 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
ACK
1
1
1
0
0
0
1
1
ACK
I
2
C address + write
Command (see Table 6)
19
20
21
22
23
24
25
26
27
S
1
0
0
1
0
1
0
1
ACK
Measurement
I
2
C 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
ACK
0
1
0
1
0
0
0
0
ACK
Data (MSB)
Data (LSB)
Stat.
46
47
48
49
50
51
52
53
54
0
0
0
0
0
1
1
0
NACK
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
done by sending a Start condition followed by the I
2
C
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
sensor output is S
T
= „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.
Please note: I
2
C communication allows for repeated Start
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
ACK
1
1
1
1
0
0
1
1
ACK
I
2
C address + write
Command (see Table 6)
19
20
21
22
23
24
25
26
27
Measurement
S
1
0
0
1
0
1
0
1
NACK
measuring
I
2
C address + read
19
20
21
22
23
24
25
26
27
Measurement
S
1
0
0
1
0
1
0
1
ACK
continue measuring
I
2
C 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
ACK
0
1
0
1
0
0
0
0
ACK
Data (MSB)
Data (LSB)
Stat.
46
47
48
49
50
51
52
53
54
0
0
0
0
0
1
1
0
NACK
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
ACK
1
1
1
1
1
1
1
0
ACK
P
I
2
C address + write
Soft Reset
Figure 14 Soft Reset – grey blocks are controlled by STS21.