SHT21 SENSIRION, SHT21 Datasheet - Page 9

• Manufacturer Part Number: SHT21
• Description: Humidity/Temperture Sensor
• Manufacturer: SENSIRION

Specifications of SHT21

• Operating Temperature Range: -40°C To +125°C
• No. Of Pins: 6
• Supply Voltage: 3V
• Humidity Sensor Type: Capacitive
• Sensor Case Style: DFN
• Humidity Range: 0% To 100%
• Response Time: 8s
• Accuracy: ± 2%
• Supply Current: 300µA

Datasheet SHT21
5.5
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.
Figure 17 Soft Reset – grey blocks are controlled by SHT2x.
5.6
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.
Table 8 User Register. Cut-off value for End of Battery signal
may vary by ±0.05V. 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.
The heater is intended to be used for functionality
diagnosis – relative humidity drops upon rising
temperature. The heater consumes about 5.5mW and
provides a temperature increase of about 0.5 – 1.5°C.
OTP Reload is a safety feature and loads the entire OTP
settings to the register, with the exception of the heater bit,
15
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S 1 0 0 0 0 0 0 0
Bit
7, 0
6
3, 4, 5
2
1
This status bit is updated after each measurement
1
2
I
Soft Reset
User Register
2
C address + write
3
# Bits
2
1
3
1
1
4
5
6
7
Description / Coding
Measurement resolution
Status: End of battery
‘0’: VDD > 2.25V
‘1’: VDD < 2.25V
Reserved
Enable on-chip heater
Disable OTP Reload
‘00’
‘01’
‘10’
‘11’
8
9
10 11 12 13 14 15 16 17 18
1 1 1 1 1 1 1 0
RH
12 bit
8 bit
10 bit
11 bit
Soft Reset
T
14 bit
12 bit
13 bit
11 bit
15
Default
‘00’
‘0’
‘0’
‘1’
Version 1.1 – May 2010
P
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
User Register is given in Figure 18.
Figure 18 Read and write register sequence – grey blocks are
controlled by SHT2x. In this example, the resolution is set to 8bit
/ 12bit.
5.7
CRC8 is a well known standard checksum concept. For
implementation please refer to public sources such as
Wikipedia.
5.8
SHT21 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.
6 Conversion of Signal Output
Default resolution is set to 12 bit relative humidity and 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 15
and Figure 16, the transferred 16 bit relative humidity data
is ‘0110’0011’0101’0000’ = 25424.
6.1
With the relative humidity signal output S
humidity RH is obtained by the following formula (result in
%RH), no matter which resolution is chosen:
S 1 0 0 0 0 0 0 0
S 1 0 0 0 0 0 0 1
S 1 0 0 0 0 0 0 0
55 56 57 58 59 60 61 62 63
0 0 0 0 0 0 1 1
19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36
37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54
1
Register content to be written
2
I
I
I
CRC Checksum
Serial Number
Relative Humidity Conversion
2
2
2
C address + write
C address + read
C address + write
3
4
5
2
C communication reading and writing the
6
7
8
9
P
10 11 12 13 14 15 16 17 18
1 1 1 0 0 1 1 0
1 1 1 0 0 1 1 1
0 0 0 0 0 0 1 0
Register content
Read Register
Write Register
RH
the relative
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