Datasheet SHT21
Humidity and Temperature Sensor IC Fully calibrated
 Digital output, I2C interface
 Low power consumption
 Excellent long-term stability
 DFN type package – reflow solderable
Product Summary
The SHT21 humidity and temperature sensor of Sensirion
has become an industry standard in terms of form factor
and intelligence: Embedded in a reflow solderable Dual
Flat No leads (DFN) package of 3 x 3mm foot print and
1.1mm height it provides calibrated, linearized sensor
signals in digital, I2C format.
The SHT2x sensors contain a capacitive type humidity
sensor, a band gap temperature sensor and specialized
analog and digital integrated circuit – all on a single
CMOSens® chip. This yields in an unmatched sensor
performance in terms of accuracy and stability as well as
minimal power consumption.

Dimensions

Every sensor is individually calibrated and tested. Lot
identification is printed on the sensor and an electronic
identification code is stored on the chip – which can beread out by command. Furthermore, the resolution of
SHT2x can be changed by command (8/12bit up to
12/14bit for RH/T) and a checksum helps to improve
communication reliability.
With this set of features and the proven reliability andlong-term stability, the SHT2x sensors offer an
outstanding performance-to-price ratio. For testing SHT2x
two evaluation kits EK-H4 and EK-H5 are available.

Sensor Chip
3.0

SHT21 features a generation 4C CMOSens® chip.
Besides the capacitive relative humidity sensor and theband gap temperature sensor, the chip contains anamplifier, A/D converter, OTP memory and a digitalprocessing unit.

2.4 max

3.0

SHT21
D0AC4

0.3 typ

1.4 max

0.2

0.3

Bottom
View
NC

VDD

SCL

0.75

0.4

Material Contents

1.1

2.2

While the sensor itself is made of Silicon the sensors’
housing consists of a plated Cu lead-frame and green
epoxy-based mold compound. The device is fully RoHS
and WEEE compliant, e.g. free of Pb, Cd and Hg.

0.4

1.5

2.4

1.0

Additional Information and Evaluation Kits
1.0

NC

VSS

SDA

Figure 1: Drawing of SHT21 sensor package, dimensions are
given in mm (1mm = 0.039inch), tolerances are ±0.1mm. The
die pad (center pad) is internally connected to VSS. The NC
pads must be left floating. VSS = GND, SDA = DATA.
Numbering of E/O pads starts at lower right corner (indicated bynotch in die pad) and goes clockwise (compare Table 2).
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Additional information such as Application Notes isavailable from the web page www.sensirion.com/sht21.
For more information please contact Sensirion via
info@sensirion.com.
For SHT2x two Evaluation Kits are available: EK-H4, afour-channel device with Viewer Software, that also serves
for data-logging, and a simple EK-H5 directly connecting
one sensor via USB port to a computer.

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Sensor Performance
Relative Humidity1234
Parameter

Temperature567

Condition
12 bit
8 bit

Value
0.04
0.7

Units
%RH
%RH

typ

2
see Figure 2%RH

Repeatability

0.1

%RH

Repeatability

Hysteresis

1
<0.1

%RH

Resolution 1
Accuracy tolerance 2

max

Condition
14 bit12 bit

Value
0.01
0.04

Units
°C
°C

typ

°C

max

0.3
see Figure 3°C

Operating Range

extended 4

0.1-40 to 125%RH

Response Time 7

s

8

s

 63%
Typ.

5 to 30

Long Term Drift 8< 0.02°C/yr

0 to 100< 0.25%RH%RH/yrNonlinearity
 63%
extended 4
Typ.

Response time 3
Operating Range
Long Term Drift 5%RH

DRH (%RH)± 10
maximum accuracy±8

typical accuracy

Parameter
Resolution 1
Accuracy tolerance 2°C

DT (°C)± 2.0
maximum accuracy
typical accuracy

± 1.5

±6

± 1.0
±4

± 0.5

±2

± 0.0

±0
0

10

20

30

40

50

60 70 80 90 100
Relative Humidity (%RH)

Figure 2 Typical and maximal tolerance at 25°C for relative
humidity. For extensive information see Users Guide, Sect. 1.2.

Electrical Specification
Parameter
Condition
Supply Voltage, VDD
Supply Current, IDD 6

min
2.1

average 8bit

typ
3.0
0.15
300
0.5
0.9
3.2

VDD = 3.0 V

5.5mW, DT = + 0.5-1.5°C

sleep modemeasuring200

sleep mode

Power Dissipation 6
HeaterCommunication

measuring

0.6

max Units
3.6
V
0.4
µA
330 µA
1.2 µW
1.0 mW
µW

-40

-20

0

20

40

60

80
100 120
Temperature (°C)

Figure 3 Typical and maximal tolerance for temperature sensor
in °C.

Packaging Information
Sensor Type
SHT21

Packaging
Tape & Reel
Tape & Reel
Tape & Reel

Quantity
400
1500
5000

Order Number
1-100707-01
1-100645-01
1-100694-01

This datasheet is subject to change and may be amended
without prior notice

digital 2-wire interface, I2C protocol

Table 1 Electrical specification. For absolute maximum values
see Section 4.1 of Users Guide.

1 Default measurement resolution is 14bit (temperature) / 12bit (humidity). It can

5 Typical value for operation in normal RH/T operating range. Max. value is < 0.5

be reduced to 12/8bit, 11/11bit or 13/10bit by command to user register.
2 Accuracies are tested at Outgoing Quality Control at 25°C and 3.0V. Values
exclude hysteresis and long term drift and are applicable to non-condensing
environments only.
3 Time for achieving 63% of a step function, valid at 25°C and 1m/s airflow.
4 Normal operating range: 0-80%RH, beyond this limit sensor may read areversible offset with slow kinetics (+3%RH after 60h at humidity >80%RH). For
more details please see Section 1.1 of the Users Guide.

%RH/y. Value may be higher in environments with vaporized solvents, outgassing tapes, adhesives, packaging materials, etc. For more details pleaserefer to Handling Instructions.
6 Min and max values of Supply Current and Power Dissipation are based on
fixed VDD = 3.0V and T<60°C. The average value is based on one 8bit
measurement per second.
7 Response time depends on heat conductivity of sensor substrate.
8Max. value is < 0.04°C/y.

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Users Guide SHT21
For details on how Sensirion is specifying and testing
accuracy performance please consult Application Note
“Statement on Sensor Specification”.

Relative Humidity (%)

1.1 Operating Range
The sensor works stable within recommended Normal
Range – see Figure 4. Long term exposure to conditions
outside Normal Range, especially at humidity >80%RH,
may temporarily offset the RH signal (+3%RH after 60h).
After return into the Normal Range it will slowly returntowards calibration state by itself. Prolonged exposure to
extreme conditions may accelerate ageing.

1.3 Electrical Specification
Current consumption as given in Table 1 is dependent on
temperature and supply voltage VDD. For estimations onenergy consumption of the sensor Figures 6 and 7 may beconsulted. Please note that values given in these Figures
are of typical nature and the variance is considerable.
Supply Current IDD (μA)

1 Extended Specification

100

8
7
6
5
4
3
2
1
0

80

0

60

Normal
Range

40

20

40

60

80

100

120

Temperature (°C)

Max.
Range

Figure 6 Typical dependency of supply current (sleep mode)
versus temperature at VDD = 3.0V. Please note that thevariance of these data can be above ±25% of displayed value.

20

-40

-20

0

20

40

60

80
100 120
Temperature (°C)

Figure 4 Operating Conditions

1.2 RH accuracy at various temperatures
Typical RH accuracy at 25°C is defined in Figure 2. For
other temperatures, typical accuracy has been evaluatedto be as displayed in Figure 5.

Supply Current IDD (nA)

0
20
18
16
14
12
10
8
6

Relative Humidity [%RH]

2.1

100
90
80
70
60
50
40
30
20
10
0

±3.5 ±3 ±3 ±3 ±3 ±3 ±3.5 ±4 ±4±3.5 ±3 ±2.5 ±2.5 ±2.5 ±2.5 ±3 ±3.5 ±4±3 ±2.5 ±2 ±2 ±2 ±2.5 ±2.5 ±3 ±3.5
±3 ±2.5 ±2 ±2 ±2 ±2 ±2.5 ±2.5 ±3±2.5 ±2 ±2 ±2 ±2 ±2 ±2 ±2.5 ±2.5
±2.5 ±2 ±2 ±2 ±2 ±2 ±2 ±2 ±2.5
±2.5 ±2 ±2 ±2 ±2 ±2 ±2 ±2 ±2±2.5 ±2 ±2 ±2 ±2 ±2 ±2 ±2 ±2±2.5 ±2.5 ±2 ±2 ±2 ±2 ±2 ±2 ±2±3 ±3 ±2.5 ±2.5 ±2.5 ±2.5 ±2.5 ±2.5 ±2.5
±3.5 ±3.5 ±3 ±3 ±3 ±3 ±3 ±3 ±3
0

10

20

30 40 50 60
Temperature [°C]

70

2.3

2.5

2.7

2.9

3.1

3.3

3.5

Supply Voltage (VDD)

Figure 7 Typical dependency of supply current (sleep mode)
versus supply voltage at 25°C. Please note that deviations may
be up to ±50% of displayed value. Values at 60°C scale with afactor of about 15 (compare Table 1).

80

Figure 5 Typical accuracy of relative humidity measurements
given in %RH for temperatures 0 – 80°C.

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Datasheet SHT21

2.1 Soldering Instructions
The DFN’s die pad (centre pad) and perimeter I/O pads
are fabricated from a planar copper lead-frame by overmolding leaving the die pad and I/O pads exposed formechanical and electrical connection. Both the I/O pads
and die pad should be soldered to the PCB. In order toprevent oxidation and optimize soldering, the bottom sideof the sensor pads is plated with Ni/Pd/Au.
On the PCB the I/O lands9 should be 0.2mm longer thanthe package I/O pads. Inward corners may be rounded tomatch the I/O pad shape. The I/O land width should matchthe DFN-package I/O-pads width 1:1 and the land for thedie pad should match 1:1 with the DFN package – see
Figure 8.

0.3

0.7

0.2

1.5

2.4

1.0

Due to the low mounted height of the DFN, “no clean”
type 3 solder paste11 is recommended as well as Nitrogenpurge during reflow.
TP

tP

TL

tL

TS (max)

preheating

critical zone

Time

Figure 9 Soldering profile according to JEDEC standard. TP <=260°C and tP < 30sec for Pb-free assembly. TL < 220°C and tL <150sec. Ramp-up/down speeds shall be < 5°C/sec.

It is important to note that the diced edge or side faces of
the I/O pads may oxidise over time, therefore a solder fillet
may or may not form. Hence there is no guarantee forsolder joint fillet heights of any kind.
For soldering SHT2x, standard reflow soldering ovens may
be used. The sensor is qualified to withstand solderingprofile according to IPC/JEDEC J-STD-020 with peaktemperatures at 260°C during up to 30sec for Pb-free
assembly in IR/Convection reflow ovens (see Figure 9).
For manual soldering contact time must be limited to 5
seconds at up to 350°C.

0.4

0.4

0.2

The solder mask10 design for the land pattern preferably isof type Non-Solder Mask Defined (NSMD) with solder
mask openings larger than metal pads. For NSMD pads,
the solder mask opening should be about 120μm to150μm larger than the pad size, providing a 60μm to 75μm
design clearance between the copper pad and solder
mask. Rounded portions of package pads should have amatching rounded solder mask-opening shape to minimize
the risk of solder bridging. For the actual pad dimensions,each pad on the PCB should have its own solder mask
opening with a web of solder mask between adjacentpads.

centered on the thermal land area. It can also be split in
two openings.

Temperature

2 Application Information

1.0

Figure 8 Recommended metal land pattern for SHT2x. Values
in mm. Die pad (centre pad) may be left floating or be connected
to ground, NC pads shall be left floating. The outer dotted line
represents the outer dimension of the DFN package.

For solder paste printing a laser-cut, stainless steel stencil
with electro-polished trapezoidal walls and with 0.125mm
stencil thickness is recommended. For the I/O pads the
stencil apertures should be 0.1mm longer than PCB pads
and positioned with 0.1mm offset away from the centre of
the package. The die pad aperture should cover about 70– 90% of the pad area – say up to 1.4mm x 2.3mm

Immediately after the exposure to high temperatures thesensor may temporarily read a negative humidity offset(typ. -1 to -2 %RH after reflow soldering). This offset
slowly disappears again by itself when the sensor is
exposed to ambient conditions (typ. within 1-3 days). If RH
testing is performed immediately after reflow soldering,
this offset should be considered when defining the test
limits.
In no case, neither after manual nor reflow soldering, aboard wash shall be applied. Therefore, and as mentioned
above, it is strongly recommended to use “no-clean” solder
paste. In case of applications with exposure of the sensor
to corrosive gases or condensed water (i.e. environmentswith high relative humidity) the soldering pads shall be
sealed (e.g. conformal coating) to prevent loose contacts
or short cuts.
2.2 Storage Conditions and Handling Instructions
Moisture Sensitivity Level (MSL) is 1, according to
IPC/JEDEC J-STD-020. At the same time, it is

9 The land pattern is understood to be the metal layer on the PCB, onto which

the DFN pads are soldered to.
10 The solder mask is understood to be the insulating layer on top of the PCB
covering the connecting lines.

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11 Solder types are related to the solder particle size in the paste: Type 3 covers

the size range of 25 – 45 µm (powder type 42).

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Datasheet SHT21

recommended to further process the sensors within 1 year
after date of delivery.
It is of great importance to understand that a humidity
sensor is not a normal electronic component and needs tobe handled with care. Chemical vapors at highconcentration in combination with long exposure times
may offset the sensor reading.
For this reason it is recommended to store the sensors inoriginal packaging including the sealed ESD bag atfollowing conditions: Temperature shall be in the range of
10°C – 50°C and humidity at 20 – 60%RH (sensors thatare not stored in ESD bags). For sensors that have been
removed from the original packaging we recommend to
store them in ESD bags made of metal-in PE-HD12.
In manufacturing and transport the sensors shall be
prevented of high concentration of chemical solvents andlong exposure times. Out-gassing of glues, adhesive tapes
and stickers or out-gassing packaging material such as
bubble foils, foams, etc. shall be avoided. Manufacturing
area shall be well ventilated.
For more detailed information please consult the
document “Handling Instructions” or contact Sensirion.
2.3 Temperature Effects
Relative humidity reading strongly depends on
temperature. Therefore, it is essential to keep humidity
sensors at the same temperature as the air of which therelative humidity is to be measured. In case of testing or
qualification the reference sensor and test sensor mustshow equal temperature to allow for comparing humidity
readings.
If the sensor shares a PCB with electronic componentsthat produce heat it should be mounted in a way thatprevents heat transfer or keeps it as low as possible.
Measures to reduce heat transfer can be ventilation,
reduction of copper layers between the sensor and therest of the PCB or milling a slit into the PCB around thesensor – see Figure 10.
Furthermore, there are self-heating effects in case themeasurement frequency is too high. To keep self-heating
below 0.1°C, SHT2x should not be active for more than
10% of the time – e.g. maximum two measurements per
second at 12bit accuracy shall be made.

Figure 10 Top view of example of mounted SHT2x with slits
milled into PCB to minimize heat transfer.

2.4 Light
The SHT2x is not light sensitive. Prolonged direct
exposure to sunshine or strong UV radiation may age thesensor.
2.5 Materials Used for Sealing / Mounting
Many materials absorb humidity and will act as a buffer
increasing response times and hysteresis. Materials in thevicinity of the sensor must therefore be carefully chosen.
Recommended materials are: Any metals, LCP, POM(Delrin), PTFE (Teflon), PEEK, PP, PB, PPS, PSU, PVDF,
PVF.
For sealing and gluing (use sparingly): Use high filled
epoxy for electronic packaging (e.g. glob top, underfill),and Silicone. Out-gassing of these materials may also
contaminate the sensor (see Section 2.2). Therefore try to
add the sensor as a last manufacturing step to theassembly, store the assembly well ventilated after
manufacturing or bake at >50°C for 24h to outgas
contaminants before packing.
2.6 Wiring Considerations and Signal Integrity
Carrying the SCL and SDA signal parallel and in close
proximity (e.g. in wires) for more than 10cm may result incross talk and loss of communication. This may beresolved by routing VDD and/or VSS between the two
SDA signals and/or using shielded cables. Furthermore,slowing down SCL frequency will possibly improve signal
integrity. Power supply pins (VDD, VSS) must be
decoupled with a 100nF capacitor – see next Section.

12 For example, 3M antistatic bag, product “1910” with zipper.

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Datasheet SHT21

of MCUs. See Table 4 and Table 5 for detailed I/Ocharacteristic of the sensor.

3 Interface Specifications
Pin Name
Comment
1 SDA Serial Data, bidirectional 42
VSS Ground
5
5 VDD Supply Voltage
6
SCL Serial Clock, bidirectional 6
3,4 NC Not Connected

4 Electrical Characteristics
3
2
1

Table 2 SHT2x pin assignment, NC remain floating (top view)

3.1 Power Pins (VDD, VSS)
The supply voltage of SHT2x must be in the range of 2.1 –3.6V, recommended supply voltage is 3.0V. Power supply
pins Supply Voltage (VDD) and Ground (VSS) must be
decoupled with a 100nF capacitor, that shall be placed as
close to the sensor as possible – see Figure 11.
3.2 Serial clock (SCL)
SCL is used to synchronize the communication between
microcontroller (MCU) and the sensor. Since the interface
consists of fully static logic there is no minimum SCL
frequency.
3.3 Serial SDA (SDA)The SDA pin is used to transfer data in and out of thesensor. For sending a command to the sensor, SDA isvalid on the rising edge of SCL and must remain stable
while SCL is high. After the falling edge of SCL the SDA
value may be changed. For safe communication SDA shallbe valid tSU and tHD before the rising and after the fallingedge of SCL, respectively – see Figure 12. For reading
data from the sensor, SDA is valid tVD after SCL has gone
low and remains valid until the next falling edge of SCL.
VDDMCU (master)
SCL INRPRP

SCL OUTSDA INSDA

SHT2x(slave)C = 100nF

SCL

SDA OUT
GND

Figure 11 Typical application circuit, including pull-up resistors
RP and decoupling of VDD and VSS by a capacitor.

To avoid signal contention the micro-controller unit (MCU)
must only drive SDA and SCL low. External pull-up
resistors (e.g. 10kΩ), are required to pull the signal high.
For the choice of resistor size please take bus capacity
requirements into account (compare Table 5). It should be
noted that pull-up resistors may be included in I/O circuits
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4.1 Absolute Maximum Ratings
The electrical characteristics of SHT2x are defined inTable 1. The absolute maximum ratings as given in Table
3 are stress ratings only and give additional information.
Functional operation of the device at these conditions is
not implied. Exposure to absolute maximum rating
conditions for extended periods may affect the sensor
reliability (e.g. hot carrier degradation, oxide breakdown).
Parameter
VDD to VSS
Digital I/O Pins (SDA, SCL)
to VSS
Input Current on any Pin

min
-0.3

max
5

Units
V

-0.3

VDD + 0.3

V

-100

100

mA

Table 3 Electrical absolute maximum ratings

ESD immunity is qualified according to JEDEC JESD22A114 method (Human Body Model at 4kV), JEDECJESD22-A115 method (Machine Model 200V) and ESDA
ESD-STM5.3.1-1999 and AEC-Q100-011 (Charged
Device Model, 750V corner pins, 500V other pins). Latchup immunity is provided at a force current of 100mA with
Tamb = 125°C according to JEDEC JESD78. For exposure
beyond named limits the sensor needs additional
protection circuit.
4.2 Input / Output Characteristics
The electrical characteristics such as power consumption,
low and high level input and output voltages depend on
the supply voltage. For proper communication with thesensor it is essential to make sure that signal design isstrictly within the limits given in Table 4 & 5 and Figure 12.
Parameter

Conditions

min

typ

max

Units

Output Low
Voltage, VOL
Output Sink
Current, IOL
Input Low
Voltage, VIL
Input High
Voltage, VIH

VDD = 3.0 V,-4 mA < IOL < 0mA

0

-

0.4

V

-

-

-4

mA

0

-

30%
VDD

V

70%
VDD

-

VDD

V

Input Current

VDD = 3.6 V,
VIN = 0 V to 3.6 V

-

-

±1

uA

Table 4 DC characteristics of digital input/output pads. VDD =2.1V to 3.6V, T = -40°C to 125°C, unless otherwise noted.

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Datasheet SHT21

5.1

1/fSCL
tSCLH

tR

tSCLL

70%

SCL

30%
tSU

SDA valid writetHD

DATA IN

70%

SDA

30%
SDA valid read

tR

tF

tVD
DATA OUT

70%

SDA

30%

Figure 12 Timing Diagram for Digital Input/Output Pads,
abbreviations are explained in Table 5. SDA directions are seenfrom the sensor. Bold SDA line is controlled by the sensor, plain
SDA line is controlled by the micro-controller. Note that SDA
valid read time is triggered by falling edge of anterior toggle.
Parameter
SCL frequency, fSCLSCL High Time, tSCLH
SCL Low Time, tSCLLSDA Set-Up Time, tSU
SDA Hold Time, tHD
SDA Valid Time, tVD
SCL/SDA Fall Time, tF
SCL/SDA Rise Time, tR
Capacitive Load on Bus Line, CB

min
0
0.6
1.3
100
0
0
0
0
0

typ
-

max
0.4
900
400
100
300
400

Units
MHz
µs
µs
ns
ns
ns
ns
ns
pF

Table 5 Timing specifications of digital input/output pads for I2C
fast mode. Entities are displayed in Figure 12. VDD = 2.1V to
3.6V, T = -40°C to 125°C, unless otherwise noted. For further
information
regardingtiming,pleaserefer
tohttp://www.standardics.nxp.com/support/i2c/.

5 Communication with Sensor
SHT21 communicates with I2C protocol. For information on
I2C beyond the information in the following Sections
please refer to the following website:
http://www.standardics.nxp.com/support/i2c/.
Please note that all sensors are set to the same I2C
address, as defined in Section 5.3.
Furthermore, please note, that Sensirion provides an
exemplary sample code on its home page – compare
www.sensirion.com/sht21.
Please note that in case VDD is set to 0 V (GND), e.g. incase of a power off of the SHT2x, the SCL and SDA pads
are also pulled to GND. Consequently, the I2C bus isblocked while VDD of the SHT2x is set to 0 V.

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Start Up Sensor

tF

As a first step, the sensor is powered up to the chosen
supply voltage VDD (between 2.1V and 3.6V). After
power-up, the sensor needs at most 15ms, while SCL ishigh, for reaching idle state, i.e. to be ready accepting
commands from the master (MCU). Current consumption
during start up is 350µA maximum. Whenever the sensor
is powered up, but not performing a measurement orcommunicating, it is automatically in idle state (sleepmode).
5.2 Start / Stop Sequence
Each transmission sequence begins with Start condition(S) and ends with Stop condition (P) as displayed in Figure
13 and Figure 14.
SCL

SDA

70%
30%

70%
30%

Figure 13 Transmission Start condition (S) - a high to lowtransition on the SDA line while SCL is high. The Start condition
is a unique state on the bus created by the master, indicating to
the slaves the beginning of a transmission sequence (bus isconsidered busy after a Start).

SCL

SDA

70%
30%

70%
30%

Figure 14 Transmission Stop condition (P) - a low to hightransition on the SDA line while SCL is high. The Stop condition
is a unique state on the bus created by the master, indicating to
the slaves the end of a transmission sequence (bus isconsidered free after a Stop).

5.3 Sending a Command
After sending the Start condition, the subsequent I2C
header consists of the 7-bit I2C device address ‘1000’000’
and an SDA direction bit (Read R: ‘1’, Write W: ‘0’). The
sensor indicates the proper reception of a byte by pulling
the SDA pin low (ACK bit) after the falling edge of the 8th
SCL clock. After the issue of a measurement command(‘1110’0011’ for temperature, ‘1110’0101’ for relative
humidity’), the MCU must wait for the measurement tocomplete. The basic commands are summarized in Table
6.

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Datasheet SHT21

In the hold master mode, the SHT2x 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 becontinued.
3

4

5

6

7

8

S 1 0 0 0 0 0 0 0

9

1 1 1 0 0 1 0 1

I2C address + write

ACK

Measurement
Hold during measurement

ACK

ACK

0 1 0 1 0 0 1 0

Data (MSB)

Data (LSB) Stat.

NACK

P

Checksum

Figure 15 Hold master communication sequence – grey blocks
are controlled by SHT2x. 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 thetermination of the internal processing of the sensor. This iswww.sensirion.com

7

8

9

10 11 12 13 14 15 16 17 18

1 1 1 1 0 1 0 1

I2C address + write

ACK

6

Command (see Table 6)

wait P
20µs

19 20 21 22 23 24 25 26 27

Measurement

S 1 0 0 0 0 0 0 1

measuringP

I2C address + read
19 20 21 22 23 24 25 26 27

Measurement

S 1 0 0 0 0 0 0 1

continue measuring

I2C address + read28 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

0 1 0 1 0 0 1 0
Data (LSB) Stat.

46 47 48 49 50 51 52 53 54

P

Figure 16 No Hold master communication sequence – grey
blocks are controlled by SHT2x. If measurement is notcompleted upon “read” command, sensor does not provide ACK
on bit 27 (more of these iterations are possible). If bit 45 ischanged to NACK followed by Stop condition (P) checksumtransmission is omitted.

In the examples given in Figure 15 and Figure 16 thesensor output is SRH = ‘0110’0011’0101’0000’. For the
calculation of physical values Status Bits must be set to ‘0’– see Chapter 6.

46 47 48 49 50 51 52 53 54

0 1 1 0 0 1 0 0

5

Checksum28 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

4

0 1 1 0 0 1 0 0

Command (see Table 6)

I2C address + read3

S 1 0 0 0 0 0 0 019 20 21 22 23 24 25 26 27

S 1 0 0 0 0 0 0 1

2

Data (MSB)

10 11 12 13 14 15 16 17 18

ACK

2

ACK

1

1

NACK

5.4 Hold / No Hold Master Mode
There are two different operation modes to communicate
with the sensor: Hold Master mode or No Hold Mastermode. In the first case the SCL line is blocked (controlled
by sensor) during measurement process while in the lattercase the SCL line remains open for other communication
while the sensor is processing the measurement. No holdmaster mode allows for processing other I2C
communication tasks on a bus while the sensor ismeasuring. A communication sequence of the two modes
is displayed in Figure 15 and Figure 16, respectively.

ACK

Hold master or no hold master modes are explained innext Section.

For both modes, since the maximum resolution of ameasurement is 14 bit, the two last least significant bits
(LSBs, bits 43 and 44) are used for transmitting status
information. Bit 1 of the two LSBs indicates themeasurement type (‘0’: temperature, ‘1’ humidity). Bit 0 iscurrently not assigned.

ACK

Table 6 Basic command set, RH stands for relative humidity,
and T stands for temperature

When using the no hold master mode it is recommended
to include a wait period of 20 µs after the reception of thesensor’s ACK bit (bit 18 in Figure 16) and before the Stopcondition.

ACK

Code
1110’0011
1110’0101
1111’0011
1111’0101
1110’0110
1110’0111
1111’1110

ACK

Comment
hold masterhold masterno hold masterno hold master

NACK

Command
Trigger T measurement
Trigger RH measurement
Trigger T measurement
Trigger RH measurement
Write user register
Read user register
Soft reset

done by sending a Start condition followed by the I2C
header (1000’0001) as shown in Figure 16. If the internal
processing is finished, the sensor acknowledges the poll of
the MCU and data can be read by the MCU. If themeasurement processing is not finished the sensor
answers no ACK bit and the Start condition must be
issued once more.

The maximum duration for measurements depends on thetype of measurement and resolution chosen – values are
displayed in Table 7. Maximum values shall be chosen forthe communication planning of the MCU.

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Datasheet SHT21

Please note: I2C communication allows for repeated Start
conditions (S) without closing prior sequence with Stopcondition (P) – compare Figures 15, 16 and 18. Still, any
sequence with adjacent Start condition may alternatively
be closed with a Stop condition.
5.5 Soft ResetThis command (see Table 6) is used for rebooting thesensor system without switching the power off and on
again. Upon reception of this command, the sensor
system reinitializes and starts operation according to thedefault settings – with the exception of the heater bit in theuser register (see Sect. 5.6). The soft reset takes less than
15ms.
3

4

5

6

7

8

S 1 0 0 0 0 0 0 0
I2C address + write

9

1 1 1 1 1 1 1 0

6

13, 4, 5
2
1

3
1
1

RH
12 bit8 bit
10 bit11 bitSoft Reset

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 risingtemperature. The heater consumes about 5.5mW andprovides 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,

‘0’

‘0’
‘1’

An example for I2C communication reading and writing theUser Register is given in Figure 18.
1

2

3

4

5

6

7

8

S 1 0 0 0 0 0 0 09

10 11 12 13 14 15 16 17 18

1 1 1 0 0 1 1 1
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

5.6 User Register
The content of User Register is described in Table 8.
Please note that reserved bits must not be changed anddefault values of respective reserved bits may changeover time without prior notice. Therefore, for any writing to
the User Register, default values of reserved bits must beread first. Thereafter, the full User Register string iscomposed of respective default values of reserved bits
and the remainder of accessible bits optionally with default
or non-default values.

T
14 bit12 bit13 bit11 bitStatus: End of battery15‘0’: VDD > 2.25V‘1’: VDD < 2.25V
Reserved
Enable on-chip heater
Disable OTP Reload

I2C address + write

P

Default‘00’

Table 8 User Register. Cut-off 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 ameasurement command is issued.

Figure 17 Soft Reset – grey blocks are controlled by SHT2x.

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Description / Coding
Measurement resolution
‘00’
‘01’
‘10’
‘11’

10 11 12 13 14 15 16 17 18

ACK

2

ACK

1

# Bits2

ACK

Table 7 Measurement times for RH and T measurements at
different resolutions. Typical values are recommended for
calculating energy consumption while maximum values shall beapplied for calculating waiting times in communication.

Bit
7, 0

0 0 0 0 0 0 1 0

I2C address + read

NACK

Units
ms
ms
ms
ms
ms
ms

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

1 1 1 0 0 1 1 0

I2C address + write

ACK

29
15
9
4

T max
85
43
22
11

ACK

22
12
7
3

T typ
66
33
17
9

ACK

RH maxACK

RH typWrite Register

55 56 57 58 59 60 61 62 63

0 0 0 0 0 0 1 1

ACKResolution
14 bit13 bit12 Bit11 bit10 bit8 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.

P

Register content to be written

Figure 18 Read and write register sequence – grey blocks arecontrolled by SHT2x. In this example, the resolution is set to 8bit
/ 12bit.

5.7 CRC Checksum
SHT21 provides a CRC-8 checksum for error detection.
The polynomial used is x8 + x5 + x4 +1. For more details
and implementation please refer to the application note
“CRC Checksum Calculation for SHT2x”.

15 This status bit is updated after each measurement

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Datasheet SHT21

5.8 Serial Number
SHT21 provides an electronic identification code. For
instructions on how to read the identification code pleaserefer to the Application Note “Electronic Identification
Code” – to be downloaded from the web pagewww.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 themost 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 SRH the relative
humidity RH is obtained by the following formula (result in%RH), no matter which resolution is chosen:

RH   6  125 

SRH
216

In the example given in Figure 15 and Figure 16 therelative humidity results to be 42.5%RH.
The physical value RH given above corresponds to therelative humidity above liquid water according to World
Meteorological Organization (WMO). For relative humidity
above ice RHi the values need to be transformed from
relative humidity above water RHw at temperature t. The
equation is given in the following, compare also
Application Note “Introduction to Humidity”:
 β t 
RHi  RHw  ex p w  λw  t 

 β t 
ex p i 
 λi  t 

Units are %RH for relative humidity and °C fortemperature. The corresponding coefficients are defined
as follows: βw = 17.62, λw = 243.12°C, βi = 22.46, λi =272.62°C.

7 Environmental Stability
The SHT2x sensor series were tested based on AECQ100 Rev. G qualification test method where applicable.
Sensor specifications are tested to prevail under the AECQ100 temperature grade 1 test conditions listed in Table
916.
Environment Standard
HTOL
125°C, 408 hours
TC-50°C - 125°C, 1000 cycles

Results17
Pass
Pass

UHST

130°C / 85%RH / ≈2.3bar, 96h

Pass

THB

85°C / 85%RH, 1000h

Pass

HTSL

150°C, 1000h

Pass

ELFR

125°C, 48hPass

ESD immunity HBM 4kV, MM 200V, CDM
Pass
750V/500V (corner/other pins)
Latch-upforce current of ±100mA with Tamb Pass
= 125°C
Table 9: Performed qualification test series. HTOL = High
Temperature Operating Lifetime, TC = Temperature Cycles,
UHST = Unbiased Highly accelerated Stress Test, THB =Temperature Humidity Biased, HTSL = High Temperature
Storage Lifetime, ELFR = Early Life Failure Rate. For details on
ESD see Sect. 4.1.

Sensor performance under other test conditions cannot be
guaranteed and is not part of the sensor specifications.
Especially, no guarantee can be given for sensor
performance in the field or for customer’s specificapplication.
If sensors are qualified for reliability and behavior in
extreme conditions, please make sure that theyexperience same conditions as the reference sensor. It
should be taken into account that response times inassemblies may be longer, hence enough dwell time forthe measurement shall be granted. For detailed
information please consult Application Note “Testing
Guide”.

8 Packaging

6.2 Temperature Conversion
The temperature T is calculated by inserting temperature
signal output ST into the following formula (result in °C), no
matter which resolution is chosen:

8.1

Packaging TypeS
T   46.85  175.72  16T
2

The sensor chip is mounted to a lead frame made of Cu
and plated with Ni/Pd/Au. Chip and lead frame are overmolded by green epoxy-based mold compound. Please
note that side walls of sensors are diced and hence lead

SHT2x sensors are provided in DFN packaging (inanalogy with QFN packaging). DFN stands for Dual Flat
No leads.

16 Temperature range is -40 to 125°C (AEC-Q100 temperature grade 1).
17 According to accuracy and long term drift specification given on Page 2.

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Datasheet SHT21

frame at diced edge is not covered with respective
protective coating. The total weight of the sensor is 25mg.

8.3 Traceability Information
All SHT2x are laser marked with an alphanumeric, fivedigit code on the sensor – see Figure 19.
The marking on the sensor consists of two lines with five
digits each. The first line denotes the sensor type
(SHT21). The first digit of the second line defines theoutput mode (D = digital, Sensibus and I2C, P = PWM, S =SDM). The second digit defines the manufacturing year (0= 2010, 1 = 2011, etc.). The last three digits represent analphanumeric tracking code. That code can be decoded by
Sensirion only and allows for tracking on batch level
through production, calibration and testing – and will be
provided upon justified request.

1-100PPP-NN
Humidity & Temperature Sensor
SHTxx
Part Order No. 1-100PPP-NN or Customer Number
Date of Delivery: DD.MM.YYYY
Order Code:46CCCC / 0

Figure 21: Second label on reel: For Device Type and Part
Order Number (See Packaging Information on page 2), Delivery
Date (also Date Code) is date of packaging of sensors (DD =day, MM = month, YYYY = year), CCCC = Sensirion ordernumber.

8.4 Shipping Package
SHT2x are provided in tape & reel shipment packaging,sealed into antistatic ESD bags. Standard packaging sizes
are 400, 1500 and 5000 units per reel. For SHT21, eachreel contains 440mm (55 pockets) header tape and200mm (25 pockets) trailer tape.
The drawing of the packaging tapes with sensor
orientation is shown in Figure 22. The reels are provided insealed antistatic bags.
8.0

SHT21
D0AC4

2.0
4.0

0.3

Ø1.5 MIN
Ø1.5 MIN

3.3

Reels are also labeled, as displayed in Figure 20 and
Figure 21, and give additional traceability information.
Lot No.:Quantity:
RoHS:

5.5

R0.3 MAX

Figure 19 Laser marking on SHT21. For details see text.

XXO-NN-YRRRTTTTT
RRRR
Compliant

12.0

For testing of SHT2x sensors sockets, such as from
Plastronics, part number 10LQ50S13030 arerecommended (see e.g. www.locknest.com).

Device Type:
Description:

1.75

8.2 Filter Cap and Sockets
For SHT2x a filter cap SF2 will is available. It is designed
for fast response times and compact size. Please find thedatasheet on Sensirion’s web page.

1.3
3.3

0.25
R0.25

Figure 22 Sketch of packaging tape and sensor orientation.
Header tape is to the right and trailer tape to the left on thissketch.

Lot No.

9 Compatibility to SHT1x / 7x protocol
Figure 20: First label on reel: XX = Sensor Type (21 for SHT21),
O = Output mode (D = Digital, P = PWM, S = SDM), NN =product revision no., Y = last digit of year, RRR = number of
sensors on reel divided by 10 (200 for 2000 units), TTTTT =Traceability Code.

SHT2x sensors may be run by communicating with theSensirion specific communication protocol used for SHT1xand SHT7x. In case such protocol is applied please referto the communication chapter of datasheet SHT1x or
SHT7x. Please note that reserved status bits of user
register must not be changed.
Please understand that with the SHT1x/7x communication
protocol only functions described in respective datasheets
can be used with the exception of the OTP Reload
function that is not set to default on SHT2x. As analternative to OTP Reload the soft reset may be used.

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Datasheet SHT21

Please note that even if SHT1x/7x protocol is applied thetiming values of Table 5 and Table 7 in this SHT2x
datasheet apply.
For the calculation of physical values the following
equation must be applied:
For relative humidity RH

RH   6  125 

SRH
2 RES

and for temperature T

T   46.85  175.72 

ST
2 RES

RES is the chosen respective resolution, e.g. 12 (12bit) for
relative humidity and 14 (14bit) for temperature.

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Datasheet SHT21

Revision History
Date
6 May 200921 January 2010
5 May 2010

Version
0.3
1.0
1.1

9 May 2011

2

December 20113

May 2014

4

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Page(s)1–9
1 – 4, 7 – 10
1 – 12

Changes
Initial preliminary release
Complete revision. For complete revision list please require respective document.
Typical specification for temperature sensor. Elimination of errors. For detailed
information, please require complete change list at info@sensirion.com.
1 – 7, 10 – Updated temperature accuracy specifications, MSL and standards. Elimination of
13
errors. For detailed information, please require complete change list at
info@sensirion.com.
1, 7-10Tolerance of threshold value for low battery signal, minor text adaptations andcorrections.
1-4, 7-8, 9-10 Sensor window dimension updated, several minor adjustments

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Datasheet SHT21
Warning, Personal Injury

Important Notices
Do not use this product as safety or emergency stop devices or in
any other application where failure of the product could result inpersonal injury. Do not use this product for applications otherthan its intended and authorized use. Before installing, handling,using or servicing this product, please consult the data sheet andapplication notes. Failure to comply with these instructions could
result in death or serious injury.
If the Buyer shall purchase or use SENSIRION products for any
unintended or unauthorized application, Buyer shall defend, indemnify
and hold harmless SENSIRION and its officers, employees,
subsidiaries, affiliates and distributors against all claims, costs,
damages and expenses, and reasonable attorney fees arising out of,
directly or indirectly, any claim of personal injury or death associatedwith such unintended or unauthorized use, even if SENSIRION shall beallegedly negligent with respect to the design or the manufacture of the
product.
ESD Precautions
The inherent design of this component causes it to be sensitive toelectrostatic discharge (ESD). To prevent ESD-induced damage and/or
degradation, take customary and statutory ESD precautions whenhandling this product.
See application note “ESD, Latchup and EMC” for more information.
Warranty
SENSIRION warrants solely to the original purchaser of this product fora period of 12 months (one year) from the date of delivery that thisproduct shall be of the quality, material and workmanship defined in
SENSIRION’s published specifications of the product. Within such
period, if proven to be defective, SENSIRION shall repair and/or
replace this product, in SENSIRION’s discretion, free of charge to the
Buyer, provided that:
notice in writing describing the defects shall be given to
SENSIRION within fourteen (14) days after their appearance;such defects shall be found, to SENSIRION’s reasonablesatisfaction, to have arisen from SENSIRION’s faulty design,material, or workmanship;the defective product shall be returned to SENSIRION’s factory at
the Buyer’s expense; and
the warranty period for any repaired or replaced product shall belimited to the unexpired portion of the original period.
This warranty does not apply to any equipment which has not been
installed and used within the specifications recommended by
SENSIRION for the intended and proper use of the equipment.
EXCEPT FOR THE WARRANTIES EXPRESSLY SET FORTHHEREIN, SENSIRION MAKES NO WARRANTIES, EITHER EXPRESSOR IMPLIED, WITH RESPECT TO THE PRODUCT. ANY AND ALL
WARRANTIES, INCLUDING WITHOUT LIMITATION, WARRANTIESOF MERCHANTABILITY OR FITNESS FOR A PARTICULARPURPOSE, ARE EXPRESSLY EXCLUDED AND DECLINED.
SENSIRION is only liable for defects of this product arising under theconditions of operation provided for in the data sheet and proper use of
the goods. SENSIRION explicitly disclaims all warranties, express or
implied, for any period during which the goods are operated or stored
not in accordance with the technical specifications.
SENSIRION does not assume any liability arising out of any applicationor use of any product or circuit and specifically disclaims any and all
liability, including without limitation consequential or incidental
damages. All operating parameters, including without limitation
recommended parameters, must be validated for each customer’s
applications by customer’s technical experts. Recommended
parameters can and do vary in different applications.
SENSIRION reserves the right, without further notice, (i) to change theproduct specifications and/or the information in this document and (ii) toimprove reliability, functions and design of this product.
Copyright © 2014, by SENSIRION.
CMOSens® is a trademark of Sensirion
All rights reserved

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