Datasheet
Datasheet STS21
www.sensirion.com Version 2 – December 2011 8/12
5.6 User Register
The content of User Register is described in Table 8.
Please note that reserved bits must not be changed and
default values of respective reserved bits may change
over time without prior notice. Therefore, for any writing to
the User Register, default values of reserved bits must be
read first. Thereafter, the full User Register string is
composed of respective default values of reserved bits
and the remainder of accessible bits optionally with default
or non-default values.
Bit
# Bits
Description / Coding
Default
7, 0
2
Measurement resolution
„00‟
14 bit
„01‟
12 bit
„10‟
13 bit
„11‟
11 bit
„00‟
6
1
Status: End of battery
9
„0‟: VDD > 2.25V
„1‟: VDD < 2.25V
„0‟
3, 4, 5
3
Reserved
2
1
Enable on-chip heater
„0‟
1
1
Disable OTP Reload
„1‟
Table 8 User Register. Threshold value for End of Battery
signal may vary by ±0.1V. Reserved bits must not be changed.
“OTP reload” = „0‟ loads default settings after each time a
measurement command is issued.
The end of battery alert is activated when the battery
power falls below 2.25V.
OTP Reload is a safety feature and loads the entire OTP
settings to the register, with the exception of the heater bit,
before every measurement. This feature is disabled per
default and is not recommended for use. Please use Soft
Reset instead – it contains OTP Reload.
An example for I
2
C communication reading and writing the
User Register is given in Figure 15.
5.7 CRC Checksum
STS21 provides a CRC-8 checksum for error detection.
The polynomial used is x
8
+ x
5
+ x
4
+1. For more details
and implementation please refer to the application note
“CRC Checksum Calculation for SHT2x”.
5.8 Serial Number
STS21 provides an electronic identification code. For
instructions on how to read the identification code please
refer to the Application Note “Electronic Identification
Code” – to be downloaded from the web page
www.sensirion.com/SHT21.
9
This status bit is updated after each measurement
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
1
1
1
ACK
I
2
C address + write
Read Register
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
S
1
0
0
1
0
1
0
1
ACK
0
0
0
0
0
0
1
0
NACK
I
2
C address + read
Register content
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
S
1
0
0
1
0
1
0
0
ACK
1
1
1
0
0
1
1
0
ACK
I
2
C address + write
Write Register
55
56
57
58
59
60
61
62
63
0
0
0
0
0
0
1
1
ACK
P
Register content to be written
Figure 15 Read and write register sequence – grey blocks are
controlled by STS21. In this example, the resolution is set to
12bit.
6 Conversion of Signal Output
Default resolution is set to 14 bit temperature reading.
Measured data are transferred in two byte packages, i.e.
in frames of 8 bit length where the most significant bit
(MSB) is transferred first (left aligned). Each byte is
followed by an acknowledge bit. The two status bits, the
last bits of LSB, must be set to „0‟ before calculating
physical values. In the example of Figure 12 and Figure
13, the transferred 16 bit temperature data is
„0110‟0011‟0101‟0000‟ = 25424.
6.1 Temperature Conversion
The temperature T is calculated by inserting temperature
signal output S
T
into the following formula (result in °C), no
matter which resolution is chosen:
16
T
2
S
175.7246.85 T
7 Environmental Stability
The SHT2x sensor series were tested based on AEC-
Q100 Rev. G qualification test method where applicable.
Sensor specifications are tested to prevail under the AEC-
Q100 temperature grade 2 test conditions listed in Table
9
10
.
10
Temperature range is -40 to 105°C (AEC-Q100 temperature grade 2).