Datasheet SHT21
1
2
3
4
5
6
7
8
9
10 11 12 13 14 15 16 17 18
5.5 Soft Reset
S 1 0 0 0 0 0 0 0
1 1 1 1 0 1 0 1
Command (see Table 6)
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. The soft reset takes less than 15 ms.
I2C address + write
19 20 21 22 23 24 25 26 27
Measurement
S 1 0 0 0 0 0 0 1
I2C address + read
1
2
3
4
5
6
7
8
9
10 11 12 13 14 15 16 17 18
1 1 1 1 1 1 1 0
Soft Reset
measuring
S 1 0 0 0 0 0 0 0
P
19 20 21 22 23 24 25 26 27
I2C address + write
Measurement
S 1 0 0 0 0 0 0 1
I2C address + read
Figure 15 Soft Reset – grey blocks are controlled by SHT2x.
continue measuring
28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45
5.6 User Register
0 1 1 0 0 0 1 1
Data (MSB)
0 1 0 1 0 0 1 0
The content of user register is described in Table 8.
Please note that reserved bits must not be changed.
Data (LSB) Stat.
46 47 48 49 50 51 52 53 54
Bit
# Bits
2
Description / Coding
Default
‘00’
0 1 1 0 0 0 1 1
P
7, 0
Measurement resolution
Checksum
RH
T
‘00’
‘01’
‘10’
‘11’
12 bit
8 bit
10 bit
11 bit
14 bit
12 bit
13 bit
11 bit
Figure 14 No Hold master communication sequence – grey
blocks are controlled by SHT2x. 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.
6
1
Status: End of battery16
‘0’: VDD > 2.25 V
‘0’
‘1’: VDD < 2.25 V
In the examples given in Figure 13 and Figure 14 the
sensor output is SRH = ‘0110’0011’0101’0000’. For the
calculation of physical values Status Bits must be set to ‘0’
– see Chapter 6.
3, 4, 5
3
1
1
Reserved
2
1
Enable on-chip heater
Disable OTP reload
‘0’
‘1’
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.
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.
Resolution RH typ RH max T typ
T max
85
Units
ms
14 bit
13 bit
12 Bit
11 bit
10 bit
8 bit
66
33
17
9
The heater is intended to be used for functionality
43
ms
diagnosis
– relative humidity drops upon rising
22
12
7
29
15
9
22
ms
temperature. The heater consumes about 5.5mW and
provides a temperature increase of about 0.5 – 1.5°C.
11
ms
ms
OTP Reload is a safety feature and loads the entire OTP
settings to the register before every measurement. This
feature is disabled per default and is not recommended for
use. Please use Soft Reset instead – it contains OTP
Reload.
3
4
ms
Table 7 Measurement times for RH and 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.
An example for I2C communication reading and writing the
User Register is given in Figure 16. It is important that first
the content of the register is read in order getting the
default values. Hereafter, the resolution is changed by
configuring register. Please note that the reserved user-
register bits must not be changed.
Please note: I2C communication allows for repeated Start
conditions (S) without closing prior sequence with Stop
condition (P) – compare Figures 13, 14 and 16. Still, any
sequence with adjacent Start condition may alternatively
be closed with a Stop condition.
16 This status bit is updated after each measurement
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Version 1.0 – January 2010
8/12
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