Datasheet
Datasheet SHT21
www.sensirion.com Version 1.1 – May 2010 9/12
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
0
0
0
0
0
ACK
1
1
1
1
1
1
1
0
ACK
P
I
2
C address + write
Soft Reset
Figure 17 Soft Reset – grey blocks are controlled by SHT2x.
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
RH T
‘00’ 12 bit 14 bit
‘01’ 8 bit 12 bit
‘10’ 10 bit 13 bit
‘11’ 11 bit 11 bit
‘00’
6 1 Status: End of battery
15
‘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. 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
This status bit is updated after each measurement
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 18.
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
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
0
0
0
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
0
0
0
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 18 Read and write register sequence – grey blocks are
controlled by SHT2x. In this example, the resolution is set to 8bit
/ 12bit.
5.7 CRC Checksum
CRC8 is a well known standard checksum concept. For
implementation please refer to public sources such as
Wikipedia.
5.8 Serial Number
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 Relative Humidity Conversion
With the relative humidity signal output S
RH
the relative
humidity RH is obtained by the following formula (result in
%RH), no matter which resolution is chosen: