Humidity and Temperature Sensor IC
Digital output, I2C interface
Low power consumption
Excellent long-term stability
DFN type package – reflow solderable
The SHT21 humidity and temperature sensor of Sensirion Every sensor is individually calibrated and tested. Lot
has become an industry standard in terms of form factor identification is printed on the sensor and an electronic
and intelligence: Embedded in a reflow solderable Dual identification code is stored on the chip – which can be
Flat No leads (DFN) package of 3 x 3mm foot print and read out by command. Furthermore, the resolution of
1.1mm height it provides calibrated, linearized sensor SHT2x can be changed by command (8/12bit up to
signals in digital, I2C format. 12/14bit for RH/T) and a checksum helps to improve
The SHT2x sensors contain a capacitive type humidity communication reliability.
sensor, a band gap temperature sensor and specialized With this set of features and the proven reliability and
analog and digital integrated circuit – all on a single long-term stability, the SHT2x sensors offer an
CMOSens® chip. This yields in an unmatched sensor outstanding performance-to-price ratio. For testing SHT2x
performance in terms of accuracy and stability as well as two evaluation kits EK-H4 and EK-H5 are available.
minimal power consumption.
Dimensions Sensor Chip
3.0 SHT21 features a generation 4C CMOSens® chip.
0.3 typ Besides the capacitive relative humidity sensor and the
2.4 max band gap temperature sensor, the chip contains an
3.0 amplifier, A/D converter, OTP memory and a digital
2.2 1.1 Material Contents
0.2 While the sensor itself is made of Silicon the sensors’
Bottom housing consists of a plated Cu lead-frame and green
0.4 0.3 View epoxy-based mold compound. The device is fully RoHS
NC VDD SCL and WEEE compliant, e.g. free of Pb, Cd and Hg.
1.5 2.4 Additional Information and Evaluation Kits
Additional information such as Application Notes is
1.0 1.0 NC VSS SDA available from the web page www.sensirion.com/sht21.
For more information please contact Sensirion via
Figure 1: Drawing of SHT21 sensor package, dimensions are For SHT2x two Evaluation Kits are available: EK-H4, a
given in mm (1mm = 0.039inch), tolerances are ±0.1mm. The
die pad (center pad) is internally connected to VSS. The NC four-channel device with Viewer Software, that also serves
pads must be left floating. VSS = GND, SDA = DATA. for data-logging, and a simple EK-H5 directly connecting
Numbering of E/O pads starts at lower right corner (indicated by one sensor via USB port to a computer.
notch in die pad) and goes clockwise (compare Table 2).
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Relative Humidity1234 Temperature567
Parameter Condition Value Units Parameter Condition Value Units
Resolution 1 12 bit 0.04 %RH Resolution 1 14 bit 0.01 °C
8 bit 0.7 %RH 12 bit 0.04 °C
Accuracy tolerance 2 typ 2 %RH Accuracy tolerance 2 typ 0.3 °C
max see Figure 2 %RH max see Figure 3
Repeatability 0.1 %RH Repeatability 0.1 °C
Hysteresis 1 %RH Operating Range extended 4 -40 to 125 °C
Nonlinearity <0.1 %RH Response Time 7 63% 5 to 30 s
Response time 3 63% 8 s Long Term Drift 8 Typ. < 0.02 °C/yr
Operating Range extended 4 0 to 100 %RH
Long Term Drift 5 Typ. < 0.25 %RH/yr
DRH (%RH) DT (°C)
± 10 ± 2.0
maximum accuracy maximum accuracy
±8 typical accuracy ± 1.5 typical accuracy
±2 ± 0.5
±0 ± 0.0
0 10 20 30 40 50 60 70 80 90 100 -40 -20 0 20 40 60 80 100 120
Relative Humidity (%RH) Temperature (°C)
Figure 2 Typical and maximal tolerance at 25°C for relative Figure 3 Typical and maximal tolerance for temperature sensor
humidity. For extensive information see Users Guide, Sect. 1.2. in °C.
Parameter Condition min typ max Units Packaging Information
Supply Voltage, VDD 2.1 3.0 3.6 V Sensor Type Packaging Quantity Order Number
sleep mode 0.15 0.4 µA Tape & Reel 400 1-100707-01
Supply Current, IDD 6 measuring 200 300 330 µA SHT21 Tape & Reel 1500 1-100645-01
sleep mode 0.5 1.2 µW Tape & Reel 5000 1-100694-01
Power Dissipation 6 measuring 0.6 0.9 1.0 mW
average 8bit 3.2 µW This datasheet is subject to change and may be amended
Heater VDD = 3.0 V 5.5mW, DT = + 0.5-1.5°C without prior notice
Communication 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. %RH/y. Value may be higher in environments with vaporized solvents, out-
2 Accuracies are tested at Outgoing Quality Control at 25°C and 3.0V. Values gassing tapes, adhesives, packaging materials, etc. For more details please
exclude hysteresis and long term drift and are applicable to non-condensing refer to Handling Instructions.
environments only. 6 Min and max values of Supply Current and Power Dissipation are based on
3 Time for achieving 63% of a step function, valid at 25°C and 1m/s airflow. fixed VDD = 3.0V and T<60°C. The average value is based on one 8bit
4 Normal operating range: 0-80%RH, beyond this limit sensor may read a measurement per second.
reversible offset with slow kinetics (+3%RH after 60h at humidity >80%RH). For 7 Response time depends on heat conductivity of sensor substrate.
more details please see Section 1.1 of the Users Guide. 8 Max. value is < 0.04°C/y.
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Users Guide SHT21
1 Extended Specification 1.3 Electrical Specification
For details on how Sensirion is specifying and testing Current consumption as given in Table 1 is dependent on
accuracy performance please consult Application Note temperature and supply voltage VDD. For estimations on
“Statement on Sensor Specification”. energy consumption of the sensor Figures 6 and 7 may be
consulted. Please note that values given in these Figures
1.1 Operating Range are of typical nature and the variance is considerable.
The sensor works stable within recommended Normal Supply Current IDD (μA) 8
Range – see Figure 4. Long term exposure to conditions 7
outside Normal Range, especially at humidity >80%RH, 6
may temporarily offset the RH signal (+3%RH after 60h). 5
After return into the Normal Range it will slowly return 4
towards calibration state by itself. Prolonged exposure to 3
extreme conditions may accelerate ageing. 2
Relative Humidity (%) 100 1
80 0 20 40 60 80 100 120
60 Temperature (°C)
40 Range Range Figure 6 Typical dependency of supply current (sleep mode)
versus temperature at VDD = 3.0V. Please note that the
20 variance of these data can be above ±25% of displayed value.
-40 -20 0 20 40 60 80 100 120 Supply Current IDD (nA) 20
Temperature (°C) 18
Figure 4 Operating Conditions 16
1.2 RH accuracy at various temperatures 12
Typical RH accuracy at 25°C is defined in Figure 2. For 10
other temperatures, typical accuracy has been evaluated 8
to be as displayed in Figure 5. 6
2.1 2.3 2.5 2.7 2.9 3.1 3.3 3.5
100 ±3.5 ±3 ±3 ±3 ±3 ±3 ±3.5 ±4 ±4 Supply Voltage (VDD)
Relative Humidity [%RH] 90 ±3.5 ±3 ±2.5 ±2.5 ±2.5 ±2.5 ±3 ±3.5 ±4
80 ±3 ±2.5 ±2 ±2 ±2 ±2.5 ±2.5 ±3 ±3.5 Figure 7 Typical dependency of supply current (sleep mode)
70 ±3 ±2.5 ±2 ±2 ±2 ±2 ±2.5 ±2.5 ±3 versus supply voltage at 25°C. Please note that deviations may
60 ±2.5 ±2 ±2 ±2 ±2 ±2 ±2 ±2.5 ±2.5 be up to ±50% of displayed value. Values at 60°C scale with a
factor of about 15 (compare Table 1).
50 ±2.5 ±2 ±2 ±2 ±2 ±2 ±2 ±2 ±2.5
40 ±2.5 ±2 ±2 ±2 ±2 ±2 ±2 ±2 ±2
30 ±2.5 ±2 ±2 ±2 ±2 ±2 ±2 ±2 ±2
20 ±2.5 ±2.5 ±2 ±2 ±2 ±2 ±2 ±2 ±2
10 ±3 ±3 ±2.5 ±2.5 ±2.5 ±2.5 ±2.5 ±2.5 ±2.5
0 ±3.5 ±3.5 ±3 ±3 ±3 ±3 ±3 ±3 ±3
0 10 20 30 40 50 60 70 80
Figure 5 Typical accuracy of relative humidity measurements
given in %RH for temperatures 0 – 80°C.
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2 Application Information centered on the thermal land area. It can also be split in
2.1 Soldering Instructions two openings.
The DFN’s die pad (centre pad) and perimeter I/O pads Due to the low mounted height of the DFN, “no clean”
are fabricated from a planar copper lead-frame by over- type 3 solder paste11 is recommended as well as Nitrogen
molding leaving the die pad and I/O pads exposed for purge during reflow.
mechanical and electrical connection. Both the I/O pads TP tP
and die pad should be soldered to the PCB. In order to
prevent oxidation and optimize soldering, the bottom side Temperature TL
of the sensor pads is plated with Ni/Pd/Au. tL
On the PCB the I/O lands9 should be 0.2mm longer than TS (max)
the package I/O pads. Inward corners may be rounded to
match the I/O pad shape. The I/O land width should match
the DFN-package I/O-pads width 1:1 and the land for the
die pad should match 1:1 with the DFN package – see preheating critical zone
Figure 8. Time
The solder mask10 design for the land pattern preferably is Figure 9 Soldering profile according to JEDEC standard. TP <=
of type Non-Solder Mask Defined (NSMD) with solder 260°C and tP < 30sec for Pb-free assembly. TL < 220°C and tL <
mask openings larger than metal pads. For NSMD pads, 150sec. Ramp-up/down speeds shall be < 5°C/sec.
the solder mask opening should be about 120μm to It is important to note that the diced edge or side faces of
150μm larger than the pad size, providing a 60μm to 75μm the I/O pads may oxidise over time, therefore a solder fillet
design clearance between the copper pad and solder may or may not form. Hence there is no guarantee for
mask. Rounded portions of package pads should have a solder joint fillet heights of any kind.
matching rounded solder mask-opening shape to minimize For soldering SHT2x, standard reflow soldering ovens may
the risk of solder bridging. For the actual pad dimensions, be used. The sensor is qualified to withstand soldering
each pad on the PCB should have its own solder mask profile according to IPC/JEDEC J-STD-020 with peak
opening with a web of solder mask between adjacent temperatures at 260°C during up to 30sec for Pb-free
pads. assembly in IR/Convection reflow ovens (see Figure 9).
0.2 For manual soldering contact time must be limited to 5
0.7 0.4 seconds at up to 350°C.
Immediately after the exposure to high temperatures the
1.5 2.4 sensor may temporarily read a negative humidity offset
(typ. -1 to -2 %RH after reflow soldering). This offset
0.2 slowly disappears again by itself when the sensor is
exposed to ambient conditions (typ. within 1-3 days). If RH
1.0 1.0 testing is performed immediately after reflow soldering,
this offset should be considered when defining the test
Figure 8 Recommended metal land pattern for SHT2x. Values limits.
in mm. Die pad (centre pad) may be left floating or be connected In no case, neither after manual nor reflow soldering, a
to ground, NC pads shall be left floating. The outer dotted line board wash shall be applied. Therefore, and as mentioned
represents the outer dimension of the DFN package. above, it is strongly recommended to use “no-clean” solder
For solder paste printing a laser-cut, stainless steel stencil paste. In case of applications with exposure of the sensor
with electro-polished trapezoidal walls and with 0.125mm to corrosive gases or condensed water (i.e. environments
stencil thickness is recommended. For the I/O pads the with high relative humidity) the soldering pads shall be
stencil apertures should be 0.1mm longer than PCB pads sealed (e.g. conformal coating) to prevent loose contacts
and positioned with 0.1mm offset away from the centre of or short cuts.
the package. The die pad aperture should cover about 70 2.2 Storage Conditions and Handling Instructions
– 90% of the pad area – say up to 1.4mm x 2.3mm
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 11 Solder types are related to the solder particle size in the paste: Type 3 covers
covering the connecting lines. the size range of 25 – 45 µm (powder type 42).
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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 to
be handled with care. Chemical vapors at high
concentration in combination with long exposure times
may offset the sensor reading.
For this reason it is recommended to store the sensors in Figure 10 Top view of example of mounted SHT2x with slits
original packaging including the sealed ESD bag at milled into PCB to minimize heat transfer.
following conditions: Temperature shall be in the range of
10°C – 50°C and humidity at 20 – 60%RH (sensors that 2.4 Light
are not stored in ESD bags). For sensors that have been The SHT2x is not light sensitive. Prolonged direct
removed from the original packaging we recommend to exposure to sunshine or strong UV radiation may age the
store them in ESD bags made of metal-in PE-HD12. sensor.
In manufacturing and transport the sensors shall be
prevented of high concentration of chemical solvents and 2.5 Materials Used for Sealing / Mounting
long exposure times. Out-gassing of glues, adhesive tapes Many materials absorb humidity and will act as a buffer
and stickers or out-gassing packaging material such as increasing response times and hysteresis. Materials in the
bubble foils, foams, etc. shall be avoided. Manufacturing vicinity of the sensor must therefore be carefully chosen.
area shall be well ventilated. Recommended materials are: Any metals, LCP, POM
For more detailed information please consult the (Delrin), PTFE (Teflon), PEEK, PP, PB, PPS, PSU, PVDF,
document “Handling Instructions” or contact Sensirion. PVF.
2.3 Temperature Effects For sealing and gluing (use sparingly): Use high filled
Relative humidity reading strongly depends on epoxy for electronic packaging (e.g. glob top, underfill),
temperature. Therefore, it is essential to keep humidity and Silicone. Out-gassing of these materials may also
sensors at the same temperature as the air of which the contaminate the sensor (see Section 2.2). Therefore try to
relative humidity is to be measured. In case of testing or add the sensor as a last manufacturing step to the
qualification the reference sensor and test sensor must assembly, store the assembly well ventilated after
show equal temperature to allow for comparing humidity manufacturing or bake at >50°C for 24h to outgas
readings. contaminants before packing.
If the sensor shares a PCB with electronic components 2.6 Wiring Considerations and Signal Integrity
that produce heat it should be mounted in a way that Carrying the SCL and SDA signal parallel and in close
prevents heat transfer or keeps it as low as possible. proximity (e.g. in wires) for more than 10cm may result in
Measures to reduce heat transfer can be ventilation, cross talk and loss of communication. This may be
reduction of copper layers between the sensor and the resolved by routing VDD and/or VSS between the two
rest of the PCB or milling a slit into the PCB around the SDA signals and/or using shielded cables. Furthermore,
sensor – see Figure 10. slowing down SCL frequency will possibly improve signal
Furthermore, there are self-heating effects in case the integrity. Power supply pins (VDD, VSS) must be
measurement frequency is too high. To keep self-heating decoupled with a 100nF capacitor – see next Section.
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.
12 For example, 3M antistatic bag, product “1910” with zipper.
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of MCUs. See Table 4 and Table 5 for detailed I/O
3 Interface Specifications characteristic of the sensor.
Pin Name Comment 4 Electrical Characteristics
1 SDA Serial Data, bidirectional 4 3 4.1 Absolute Maximum Ratings
2 VSS Ground
5 VDD Supply Voltage 5 2 The electrical characteristics of SHT2x are defined in
6 SCL Serial Clock, bidirectional 6 1 Table 1. The absolute maximum ratings as given in Table
3,4 NC Not Connected 3 are stress ratings only and give additional information.
Functional operation of the device at these conditions is
Table 2 SHT2x pin assignment, NC remain floating (top view) not implied. Exposure to absolute maximum rating
conditions for extended periods may affect the sensor
3.1 Power Pins (VDD, VSS) reliability (e.g. hot carrier degradation, oxide breakdown).
The supply voltage of SHT2x must be in the range of 2.1 – Parameter min max Units
3.6V, recommended supply voltage is 3.0V. Power supply VDD to VSS -0.3 5 V
pins Supply Voltage (VDD) and Ground (VSS) must be Digital I/O Pins (SDA, SCL)
decoupled with a 100nF capacitor, that shall be placed as to VSS -0.3 VDD + 0.3 V
close to the sensor as possible – see Figure 11. Input Current on any Pin -100 100 mA
3.2 Serial clock (SCL) Table 3 Electrical absolute maximum ratings
SCL is used to synchronize the communication between
microcontroller (MCU) and the sensor. Since the interface ESD immunity is qualified according to JEDEC JESD22-
consists of fully static logic there is no minimum SCL A114 method (Human Body Model at 4kV), JEDEC
frequency. JESD22-A115 method (Machine Model 200V) and ESDA
ESD-STM5.3.1-1999 and AEC-Q100-011 (Charged
3.3 Serial SDA (SDA) Device Model, 750V corner pins, 500V other pins). Latch-
The SDA pin is used to transfer data in and out of the up immunity is provided at a force current of 100mA with
sensor. For sending a command to the sensor, SDA is Tamb = 125°C according to JEDEC JESD78. For exposure
valid on the rising edge of SCL and must remain stable beyond named limits the sensor needs additional
while SCL is high. After the falling edge of SCL the SDA protection circuit.
value may be changed. For safe communication SDA shall
be valid tSU and tHD before the rising and after the falling 4.2 Input / Output Characteristics
edge of SCL, respectively – see Figure 12. For reading The electrical characteristics such as power consumption,
data from the sensor, SDA is valid tVD after SCL has gone low and high level input and output voltages depend on
low and remains valid until the next falling edge of SCL. the supply voltage. For proper communication with the
sensor it is essential to make sure that signal design is
VDD strictly within the limits given in Table 4 & 5 and Figure 12.
MCU (master) RP RP Parameter Conditions min typ max Units
SCL IN SCL Output Low VDD = 3.0 V, 0 - 0.4 V
SCL OUT C = 100nF Voltage, VOL -4 mA < IOL < 0mA
SHT2x Output Sink
(slave) - - -4 mA
SDA IN SDA Current, IOL
SDA OUT Input Low 30%
Voltage, VIL 0 - VDD V
Input High 70% - VDD V
Figure 11 Typical application circuit, including pull-up resistors Voltage, VIH VDD
RP and decoupling of VDD and VSS by a capacitor. Input Current VDD = 3.6 V, - - ±1 uA
VIN = 0 V to 3.6 V
To avoid signal contention the micro-controller unit (MCU) Table 4 DC characteristics of digital input/output pads. VDD =
must only drive SDA and SCL low. External pull-up 2.1V to 3.6V, T = -40°C to 125°C, unless otherwise noted.
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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1/fSCL 5.1 Start Up Sensor
tSCLH tSCLL tR tF
70% As a first step, the sensor is powered up to the chosen
SCL 30% supply voltage VDD (between 2.1V and 3.6V). After
SDA valid write power-up, the sensor needs at most 15ms, while SCL is
tSU tHD high, for reaching idle state, i.e. to be ready accepting
DATA IN commands from the master (MCU). Current consumption
SDA 70% during start up is 350µA maximum. Whenever the sensor
30% is powered up, but not performing a measurement or
SDA valid read communicating, it is automatically in idle state (sleep
tVD tF tR mode).
30% 5.2 Start / Stop Sequence
Each transmission sequence begins with Start condition
Figure 12 Timing Diagram for Digital Input/Output Pads, (S) and ends with Stop condition (P) as displayed in Figure
abbreviations are explained in Table 5. SDA directions are seen 13 and Figure 14.
from the sensor. Bold SDA line is controlled by the sensor, plain
SDA line is controlled by the micro-controller. Note that SDA SCL 70%
valid read time is triggered by falling edge of anterior toggle. 30%
Parameter min typ max Units
SCL frequency, fSCL 0 - 0.4 MHz SDA 70%
SCL High Time, tSCLH 0.6 - - µs 30%
SCL Low Time, tSCLL 1.3 - - µs Figure 13 Transmission Start condition (S) - a high to low
SDA Set-Up Time, tSU 100 - - ns transition on the SDA line while SCL is high. The Start condition
SDA Hold Time, tHD 0 - 900 ns is a unique state on the bus created by the master, indicating to
SDA Valid Time, tVD 0 - 400 ns the slaves the beginning of a transmission sequence (bus is
considered busy after a Start).
SCL/SDA Fall Time, tF 0 - 100 ns
SCL/SDA Rise Time, tR 0 - 300 ns SCL 70%
Capacitive Load on Bus Line, CB 0 - 400 pF 30%
Table 5 Timing specifications of digital input/output pads for I2C
fast mode. Entities are displayed in Figure 12. VDD = 2.1V to SDA 70%
3.6V, T = -40°C to 125°C, unless otherwise noted. For further 30%
information regarding timing, please refer to Figure 14 Transmission Stop condition (P) - a low to high
http://www.standardics.nxp.com/support/i2c/. transition on the SDA line while SCL is high. The Stop condition
5 Communication with Sensor is a unique state on the bus created by the master, indicating to
the slaves the end of a transmission sequence (bus is
SHT21 communicates with I2C protocol. For information on considered free after a Stop).
I2C beyond the information in the following Sections 5.3 Sending a Command
please refer to the following website:
http://www.standardics.nxp.com/support/i2c/. After sending the Start condition, the subsequent I2C
Please note that all sensors are set to the same I2C header consists of the 7-bit I2C device address ‘1000’000’
address, as defined in Section 5.3. and an SDA direction bit (Read R: ‘1’, Write W: ‘0’). The
sensor indicates the proper reception of a byte by pulling
Furthermore, please note, that Sensirion provides an the SDA pin low (ACK bit) after the falling edge of the 8th
exemplary sample code on its home page – compare SCL clock. After the issue of a measurement command
www.sensirion.com/sht21. (‘1110’0011’ for temperature, ‘1110’0101’ for relative
Please note that in case VDD is set to 0 V (GND), e.g. in humidity’), the MCU must wait for the measurement to
case of a power off of the SHT2x, the SCL and SDA pads complete. The basic commands are summarized in Table
are also pulled to GND. Consequently, the I2C bus is 6.
blocked while VDD of the SHT2x is set to 0 V.
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done by sending a Start condition followed by the I2C
Command Comment Code header (1000’0001) as shown in Figure 16. If the internal
Trigger T measurement hold master 1110’0011 processing is finished, the sensor acknowledges the poll of
Trigger RH measurement hold master 1110’0101 the MCU and data can be read by the MCU. If the
measurement processing is not finished the sensor
Trigger T measurement no hold master 1111’0011 answers no ACK bit and the Start condition must be
Trigger RH measurement no hold master 1111’0101 issued once more.
Write user register 1110’0110 When using the no hold master mode it is recommended
Read user register 1110’0111 to include a wait period of 20 µs after the reception of the
Soft reset 1111’1110 sensor’s ACK bit (bit 18 in Figure 16) and before the Stop
Table 6 Basic command set, RH stands for relative humidity, condition.
and T stands for temperature For both modes, since the maximum resolution of a
Hold master or no hold master modes are explained in measurement is 14 bit, the two last least significant bits
next Section. (LSBs, bits 43 and 44) are used for transmitting status
information. Bit 1 of the two LSBs indicates the
5.4 Hold / No Hold Master Mode measurement type (‘0’: temperature, ‘1’ humidity). Bit 0 is
There are two different operation modes to communicate currently not assigned.
with the sensor: Hold Master mode or No Hold Master 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18
mode. In the first case the SCL line is blocked (controlled S 1 0 0 0 0 0 0 0 AC K 1 1 1 1 0 1 0 1 AC K wait P
by sensor) during measurement process while in the latter I2C address + write Command (see Table 6) 20µs
case the SCL line remains open for other communication
while the sensor is processing the measurement. No hold 19 20 21 22 23 24 25 26 27
master mode allows for processing other I2C Measurement S 1 0 0 0 0 0 0 1 NACK P
communication tasks on a bus while the sensor is measuring I2C address + read
measuring. A communication sequence of the two modes
is displayed in Figure 15 and Figure 16, respectively. 19 20 21 22 23 24 25 26 27
In the hold master mode, the SHT2x pulls down the SCL Measurement S 1 0 0 0 0 0 0 1 AC K
line while measuring to force the master into a wait state. continue measuring I2C address + read
By releasing the SCL line the sensor indicates that internal 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45
processing is terminated and that transmission may be AC K AC K
continued. 0 1 1 0 0 0 1 1 0 1 0 1 0 0 1 0
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 Data (MSB) Data (LSB) Stat.
S 1 0 0 0 0 0 0 0 AC K 1 1 1 0 0 1 0 1 AC K 46 47 48 49 50 51 52 53 54
I2C address + write Command (see Table 6) 0 1 1 0 0 1 0 0 NACK P
19 20 21 22 23 24 25 26 27 Checksum
S 1 0 0 0 0 0 0 1 AC K Measurement Figure 16 No Hold master communication sequence – grey
I2C address + read Hold during measurement blocks are controlled by SHT2x. If measurement is not
28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 completed upon “read” command, sensor does not provide ACK
AC K AC K on bit 27 (more of these iterations are possible). If bit 45 is
0 1 1 0 0 0 1 1 0 1 0 1 0 0 1 0 changed to NACK followed by Stop condition (P) checksum
Data (MSB) Data (LSB) Stat. transmission is omitted.
46 47 48 49 50 51 52 53 54 In the examples given in Figure 15 and Figure 16 the
0 1 1 0 0 1 0 0 NACK P sensor output is SRH = ‘0110’0011’0101’0000’. For the
Checksum calculation of physical values Status Bits must be set to ‘0’
– see Chapter 6.
Figure 15 Hold master communication sequence – grey blocks The maximum duration for measurements depends on the
are controlled by SHT2x. Bit 45 may be changed to NACK type of measurement and resolution chosen – values are
followed by Stop condition (P) to omit checksum transmission. displayed in Table 7. Maximum values shall be chosen for
In no hold master mode, the MCU has to poll for the the communication planning of the MCU.
termination of the internal processing of the sensor. This is
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before every measurement. This feature is disabled per
default and is not recommended for use. Please use Soft
Resolution RH typ RH max T typ T max Units Reset instead – it contains OTP Reload.
14 bit 66 85 ms
13 bit 33 43 ms Bit # Bits Description / Coding Default
12 Bit 22 29 17 22 ms 7, 0 2 Measurement resolution ‘00’
11 bit 12 15 9 11 ms
10 bit 7 9 ms RH T
8 bit 3 4 ms ‘00’ 12 bit 14 bit
‘01’ 8 bit 12 bit
Table 7 Measurement times for RH and T measurements at ‘10’ 10 bit 13 bit
different resolutions. Typical values are recommended for ‘11’ 11 bit 11 bit
calculating energy consumption while maximum values shall be 6 1 Status: End of battery15 ‘0’
applied for calculating waiting times in communication. ‘0’: VDD > 2.25V
Please note: I2C communication allows for repeated Start ‘1’: VDD < 2.25V
conditions (S) without closing prior sequence with Stop 3, 4, 5 3 Reserved
condition (P) – compare Figures 15, 16 and 18. Still, any 2 1 Enable on-chip heater ‘0’
sequence with adjacent Start condition may alternatively 1 1 Disable OTP Reload ‘1’
be closed with a Stop condition. Table 8 User Register. Cut-off value for End of Battery signal
5.5 Soft Reset may vary by ±0.1V. Reserved bits must not be changed. “OTP
This command (see Table 6) is used for rebooting the reload” = ‘0’ loads default settings after each time a
measurement command is issued.
sensor system without switching the power off and on
again. Upon reception of this command, the sensor An example for I2C communication reading and writing the
system reinitializes and starts operation according to the User Register is given in Figure 18.
default settings – with the exception of the heater bit in the
user register (see Sect. 5.6). The soft reset takes less than 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18
15ms. S 1 0 0 0 0 0 0 0 AC K 1 1 1 0 0 1 1 1 AC K
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 I2C address + write Read Register
S 1 0 0 0 0 0 0 0 AC K 1 1 1 1 1 1 1 0 AC K P 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36
I2C address + write Soft Reset S 1 0 0 0 0 0 0 1 AC K 0 0 0 0 0 0 1 0 NACK
Figure 17 Soft Reset – grey blocks are controlled by SHT2x. I2C address + read Register content
37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54
5.6 User Register S 1 0 0 0 0 0 0 0 AC K 1 1 1 0 0 1 1 0 AC K
The content of User Register is described in Table 8.
Please note that reserved bits must not be changed and I2C address + write Write Register
default values of respective reserved bits may change 55 56 57 58 59 60 61 62 63
over time without prior notice. Therefore, for any writing to 0 0 0 0 0 0 1 1 AC K P
the User Register, default values of reserved bits must be
read first. Thereafter, the full User Register string is Register content to be written
composed of respective default values of reserved bits Figure 18 Read and write register sequence – grey blocks are
and the remainder of accessible bits optionally with default controlled by SHT2x. In this example, the resolution is set to 8bit
or non-default values. / 12bit.
The end of battery alert is activated when the battery 5.7 CRC Checksum
power falls below 2.25V.
The heater is intended to be used for functionality SHT21 provides a CRC-8 checksum for error detection.
diagnosis – relative humidity drops upon rising The polynomial used is x8 + x5 + x4 +1. For more details
temperature. The heater consumes about 5.5mW and and implementation please refer to the application note
provides a temperature increase of about 0.5 – 1.5°C. “CRC Checksum Calculation for SHT2x”.
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
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5.8 Serial Number 7 Environmental Stability
SHT21 provides an electronic identification code. For
instructions on how to read the identification code please The SHT2x sensor series were tested based on AEC-
refer to the Application Note “Electronic Identification Q100 Rev. G qualification test method where applicable.
Code” – to be downloaded from the web page Sensor specifications are tested to prevail under the AEC-
www.sensirion.com/SHT21. Q100 temperature grade 1 test conditions listed in Table
6 Conversion of Signal Output Environment Standard Results17
Default resolution is set to 12 bit relative humidity and 14 HTOL 125°C, 408 hours Pass
bit temperature reading. Measured data are transferred in TC -50°C - 125°C, 1000 cycles Pass
two byte packages, i.e. in frames of 8 bit length where the UHST 130°C / 85%RH / ≈2.3bar, 96h Pass
most significant bit (MSB) is transferred first (left aligned).
Each byte is followed by an acknowledge bit. The two THB 85°C / 85%RH, 1000h Pass
status bits, the last bits of LSB, must be set to ‘0’ before HTSL 150°C, 1000h Pass
calculating physical values. In the example of Figure 15
and Figure 16, the transferred 16 bit relative humidity data ELFR 125°C, 48h Pass
is ‘0110’0011’0101’0000’ = 25424. ESD immunity HBM 4kV, MM 200V, CDM Pass
750V/500V (corner/other pins)
6.1 Relative Humidity Conversion Latch-up force current of ±100mA with Tamb Pass
With the relative humidity signal output SRH the relative = 125°C
humidity RH is obtained by the following formula (result in Table 9: Performed qualification test series. HTOL = High
%RH), no matter which resolution is chosen: Temperature Operating Lifetime, TC = Temperature Cycles,
RH 6 125 SRH UHST = Unbiased Highly accelerated Stress Test, THB =
216 Temperature Humidity Biased, HTSL = High Temperature
Storage Lifetime, ELFR = Early Life Failure Rate. For details on
In the example given in Figure 15 and Figure 16 the ESD see Sect. 4.1.
relative humidity results to be 42.5%RH. Sensor performance under other test conditions cannot be
The physical value RH given above corresponds to the guaranteed and is not part of the sensor specifications.
relative humidity above liquid water according to World Especially, no guarantee can be given for sensor
Meteorological Organization (WMO). For relative humidity performance in the field or for customer’s specific
above ice RHi the values need to be transformed from application.
relative humidity above water RHw at temperature t. The If sensors are qualified for reliability and behavior in
equation is given in the following, compare also extreme conditions, please make sure that they
Application Note “Introduction to Humidity”: experience same conditions as the reference sensor. It
ex p βw t p βi t should be taken into account that response times in
RHi RHw λw t ex λi t assemblies may be longer, hence enough dwell time for
the measurement shall be granted. For detailed
Units are %RH for relative humidity and °C for information please consult Application Note “Testing
temperature. The corresponding coefficients are defined Guide”.
as follows: βw = 17.62, λw = 243.12°C, βi = 22.46, λi =
272.62°C. 8 Packaging
6.2 Temperature Conversion 8.1 Packaging Type
The temperature T is calculated by inserting temperature SHT2x sensors are provided in DFN packaging (in
signal output ST into the following formula (result in °C), no analogy with QFN packaging). DFN stands for Dual Flat
matter which resolution is chosen: No leads.
T 46.85 175.72 ST The sensor chip is mounted to a lead frame made of Cu
216 and plated with Ni/Pd/Au. Chip and lead frame are over
molded by green epoxy-based mold compound. Please
note that side walls of sensors are diced and hence lead
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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frame at diced edge is not covered with respective
protective coating. The total weight of the sensor is 25mg.
8.2 Filter Cap and Sockets
For SHT2x a filter cap SF2 will is available. It is designed Device Type: 1-100PPP-NN
for fast response times and compact size. Please find the Description: Humidity & Temperature Sensor
datasheet on Sensirion’s web page. SHTxx
For testing of SHT2x sensors sockets, such as from Part Order No. 1-100PPP-NN or Customer Number
Plastronics, part number 10LQ50S13030 are Date of Delivery: DD.MM.YYYY
recommended (see e.g. www.locknest.com). Order Code: 46CCCC / 0
8.3 Traceability Information Figure 21: Second label on reel: For Device Type and Part
All SHT2x are laser marked with an alphanumeric, five- Order Number (See Packaging Information on page 2), Delivery
digit code on the sensor – see Figure 19. Date (also Date Code) is date of packaging of sensors (DD =
The marking on the sensor consists of two lines with five day, MM = month, YYYY = year), CCCC = Sensirion order
digits each. The first line denotes the sensor type number.
(SHT21). The first digit of the second line defines the 8.4 Shipping Package
output mode (D = digital, Sensibus and I2C, P = PWM, S = SHT2x are provided in tape & reel shipment packaging,
SDM). The second digit defines the manufacturing year (0 sealed into antistatic ESD bags. Standard packaging sizes
= 2010, 1 = 2011, etc.). The last three digits represent an are 400, 1500 and 5000 units per reel. For SHT21, each
alphanumeric tracking code. That code can be decoded by reel contains 440mm (55 pockets) header tape and
Sensirion only and allows for tracking on batch level 200mm (25 pockets) trailer tape.
through production, calibration and testing – and will be
provided upon justified request. The drawing of the packaging tapes with sensor
orientation is shown in Figure 22. The reels are provided in
sealed antistatic bags.
SHT21 8.0 Ø1.5 MIN
0.3 2.0 1.75
D0AC4 4.0 Ø1.5 MIN
R0.3 MAX 5.5
Figure 19 Laser marking on SHT21. For details see text. 3.3 12.0
Reels are also labeled, as displayed in Figure 20 and 1.3
Figure 21, and give additional traceability information. 3.3
Lot No.: XXO-NN-YRRRTTTTT R0.25
RoHS: Compliant Figure 22 Sketch of packaging tape and sensor orientation.
Header tape is to the right and trailer tape to the left on this
Lot No. sketch.
9 Compatibility to SHT1x / 7x protocol
Figure 20: First label on reel: XX = Sensor Type (21 for SHT21), SHT2x sensors may be run by communicating with the
O = Output mode (D = Digital, P = PWM, S = SDM), NN = Sensirion specific communication protocol used for SHT1x
product revision no., Y = last digit of year, RRR = number of and SHT7x. In case such protocol is applied please refer
sensors on reel divided by 10 (200 for 2000 units), TTTTT = to the communication chapter of datasheet SHT1x or
Traceability Code. 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 an
alternative to OTP Reload the soft reset may be used.
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Please note that even if SHT1x/7x protocol is applied the
timing values of Table 5 and Table 7 in this SHT2x
For the calculation of physical values the following
equation must be applied:
For relative humidity RH
RH 6 125 SRH
and for temperature T
T 46.85 175.72 ST
RES is the chosen respective resolution, e.g. 12 (12bit) for
relative humidity and 14 (14bit) for temperature.
www.sensirion.com Version 4 – May 2014 12/14
Date Version Page(s) Changes
6 May 2009 0.3 1–9 Initial preliminary release
21 January 2010 1.0 1 – 4, 7 – 10 Complete revision. For complete revision list please require respective document.
5 May 2010 1.1 1 – 12 Typical specification for temperature sensor. Elimination of errors. For detailed
information, please require complete change list at email@example.com.
9 May 2011 2 1 – 7, 10 – Updated temperature accuracy specifications, MSL and standards. Elimination of
13 errors. For detailed information, please require complete change list at
December 2011 3 1, 7-10 Tolerance of threshold value for low battery signal, minor text adaptations and
May 2014 4 1-4, 7-8, 9-10 Sensor window dimension updated, several minor adjustments
www.sensirion.com Version 4 – May 2014 13/14
Warning, Personal Injury
Do not use this product as safety or emergency stop devices or in such defects shall be found, to SENSIRION’s reasonable
any other application where failure of the product could result in satisfaction, to have arisen from SENSIRION’s faulty design,
personal injury. Do not use this product for applications other material, or workmanship;
than its intended and authorized use. Before installing, handling, the defective product shall be returned to SENSIRION’s factory at
using or servicing this product, please consult the data sheet and the Buyer’s expense; and
application notes. Failure to comply with these instructions could the warranty period for any repaired or replaced product shall be
result in death or serious injury. limited to the unexpired portion of the original period.
This warranty does not apply to any equipment which has not been
If the Buyer shall purchase or use SENSIRION products for any installed and used within the specifications recommended by
unintended or unauthorized application, Buyer shall defend, indemnify SENSIRION for the intended and proper use of the equipment.
and hold harmless SENSIRION and its officers, employees, EXCEPT FOR THE WARRANTIES EXPRESSLY SET FORTH
subsidiaries, affiliates and distributors against all claims, costs, HEREIN, SENSIRION MAKES NO WARRANTIES, EITHER EXPRESS
damages and expenses, and reasonable attorney fees arising out of, OR IMPLIED, WITH RESPECT TO THE PRODUCT. ANY AND ALL
directly or indirectly, any claim of personal injury or death associated WARRANTIES, INCLUDING WITHOUT LIMITATION, WARRANTIES
with such unintended or unauthorized use, even if SENSIRION shall be OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR
allegedly negligent with respect to the design or the manufacture of the PURPOSE, ARE EXPRESSLY EXCLUDED AND DECLINED.
product. SENSIRION is only liable for defects of this product arising under the
ESD Precautions conditions of operation provided for in the data sheet and proper use of
The inherent design of this component causes it to be sensitive to the goods. SENSIRION explicitly disclaims all warranties, express or
electrostatic discharge (ESD). To prevent ESD-induced damage and/or implied, for any period during which the goods are operated or stored
degradation, take customary and statutory ESD precautions when not in accordance with the technical specifications.
handling this product. SENSIRION does not assume any liability arising out of any application
See application note “ESD, Latchup and EMC” for more information. or use of any product or circuit and specifically disclaims any and all
liability, including without limitation consequential or incidental
Warranty damages. All operating parameters, including without limitation
SENSIRION warrants solely to the original purchaser of this product for recommended parameters, must be validated for each customer’s
a period of 12 months (one year) from the date of delivery that this applications by customer’s technical experts. Recommended
product shall be of the quality, material and workmanship defined in parameters can and do vary in different applications.
SENSIRION’s published specifications of the product. Within such SENSIRION reserves the right, without further notice, (i) to change the
period, if proven to be defective, SENSIRION shall repair and/or product specifications and/or the information in this document and (ii) to
replace this product, in SENSIRION’s discretion, free of charge to the improve reliability, functions and design of this product.
Buyer, provided that: Copyright © 2014, by SENSIRION.
notice in writing describing the defects shall be given to CMOSens® is a trademark of Sensirion
SENSIRION within fourteen (14) days after their appearance; All rights reserved
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