HTU21D(F) Sensor
Digital Relative Humidity sensor with Temperature output



DFN type packageRelative Humidity and Temperature Digital Output, I²C
interface

Fully calibrated

Lead free sensor, reflow solderable



Low power consumptionFast response time

DESCRIPTION
The HTU21D(F) is a new digital humidity sensor with temperature output by MEAS. Setting new standards interms of size and intelligence, it is embedded in a reflow solderable Dual Flat No leads (DFN) package with asmall 3 x 3 x 0.9 mm footprint. This sensor provides calibrated, linearized signals in digital, I²C format.
HTU21D(F) digital humidity sensors are dedicated humidity and temperature plug and play transducers for OEM
applications where reliable and accurate measurements are needed. Direct interface with a micro-controller ismade possible with the module for humidity and temperature digital outputs. These low power sensors aredesigned for high volume and cost sensitive applications with tight space constraints.
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 be read out by command. Low battery can be detected anda checksum improves communication reliability. The resolution of these digital humidity sensors can be changed
by command (8/12bit up to 12/14bit for RH/T).
With MEAS’ improvements and miniaturization of this sensor, the performance-to-price ratio has been improved– and eventually, any device should benefit from its cutting edge energy saving operation mode.
Optional PTFE filter/membrane (F) protects HTU21D digital humidity sensors against dust and water immersion,as well as against contamination by particles. PTFE filter/membranes preserve a high response time. The white
PTFE filter/membrane is directly stuck on the sensor housing.

FEATURESAPPLICATIONS

Full interchangeability with no calibration requiredin standard conditions
Instantaneous desaturation after long periods insaturation phase
Compatible with automatized assembly processes,
including Pb free and reflow processes
Individual marking for compliance to stringent
traceability requirements

HPC199_3 HTU21D(F) Sensor Datasheet

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
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

Automotive: defogging, HVAC
Home Appliance
MedicalPrinters
Humidifier

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HTU21D(F) Sensor
Digital Relative Humidity sensor with Temperature output

NOMENCLATUREHTU2XY(F)With embedded PTFE filter: HTU2XYF
Output Sensor:
Y = D for Digital, I2C protocol= P for PWM interface, analog output= S for SDM interface convertible to analog output
Humidity accuracy :X = 0: +/-5%RH tolerance @55%RH= 1: +/-3%RH tolerance @55%RH

HTU2XY Modules

HTU2XYF Modules

PERFORMANCE SPECSMAXIMUM RATINGS
Ratings

Symbol

Value

Unit

Storage Temperature

Tstg-40 to 125°C

Supply Voltage (Peak)

Vcc

3.8V

Vdc

Humidity Operating Range

RH

0 to 100%RH

Temperature Operating RangeTa-40 to +125°C

VDD to GND
Digital I/O pins (DATA/SCK) to VDD
Input current on any pin
Peak conditions: less than 10% of the operating time

-0.3 to 3.6V

V

-0.3 to VDD+0.3

V-10 to +10mA

Exposure to absolute maximum rating conditions for extended periods may affect the sensor reliability.

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HTU21D(F) Sensor
Digital Relative Humidity sensor with Temperature output

ELECTRICAL AND GENERAL ITEMS(@T = 25°C, @Vdd = 3V)
Characteristics

SymbolMin

Voltage Supply

VDD

1.5

Current
consumption

Sleep mode
(1)

idd

Measuring

Typ

300

Sleep mode

Power Dissipation

Average 8bit

(2)

Max

Unit

3.0

3.6

V

0.02

0.14

µA

450

500

µA

0.06

0.5

µW

2.7

CommunicationµW

digital 2-wire interface, I²C protocol

Heater

5.5mW/ΔT=+0.5-1.5°C

VDD=3V

Storage(1)
Conditions: Vdd = 3V, SCK= 400kHz at 25°C
(2)
Conditions: Vdd = 3V, SCK= 400kHz, Temp<60°C, duty cycle <10%-40°C/125°C

SENSOR PERFORMANCERELATIVE HUMIDITY(@T = 25°C, @Vdd = 3V)
Characteristics
Resolution

Symbol

Min

Typ

Max

Unit

12 bits

0.04%RH

8 bits0.7%RH

Humidity Operating Range

RH

0

100

%RH

Relative Humidity Accuracy

typ

±2

%RH

@25°C (20%RH to 80%RH)

max

See graph 1%RH

Replacement

fully interchangeable

Temperature coefficient (from 0°C to 80°C)

Tcc

Humidity Hysteresis

Measuring Time

(1)

-0.15
±1

%RH/°C%RH

12 bits

14

16

ms

11 bits

78

ms

10 bits

45

ms

8 bits

2

3

ms

±10

LSB

PSRR
Recovery time after 150 hours of condensationt

Long term drift

10

s

0.5

%RH/yr(2)

Response Time (at 63% of signal) from 33 to 75%RH
τRH
5
10
s
(1)
Typical values are recommended for calculating energy consumption while maximum values shall be applied for
calculating waiting times in communication.
(2)
At 1m/s air flow

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HTU21D(F) Sensor
Digital Relative Humidity sensor with Temperature output

GRAPH 1 : RELATIVE HUMIDITY ERROR BUDGET CONDITIONS AT 25°C

Delta Relative Humidity (%RH)

10
Maximal Tolerance
Typical Tolerance

9

8
7
6
5
4

3
2
1
0
0

10

20

30
40
50
60
70
Relative Humidity (%RH)

80

90

100

 HTU21D(F) sensors are specified for optimum accuracy measurements within 5 to 95%RH.
 Operation out of this range (< 5% or > 95% RH, including condensation) is however possible.

TEMPERATURE COEFFICIENT COMPENSATION EQUATION
Using the following temperature coefficient compensation equation will guarantee Relative Humidity accuracy given p.3, from
0°C to 80°C:

RH compensatedT  RH actualT  (25  Tactual )  CoeffTemp
RHactualT
TactualCoeffTemp

Ambient humidity in %RH, computed from HTU21D(F) sensor
Humidity cell temperature in °C, computed from HTU21D(F) sensor
Temperature coefficient of the HTU21D(F) in %RH/°C

HPC199_3 HTU21D(F) Sensor Datasheet

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HTU21D(F) Sensor
Digital Relative Humidity sensor with Temperature output

TEMPERATURE
Characteristics

SymbolResolution

Max

Unit

0.01

°C

12 bit

0.04

°C

T

-40

+125

typ
max

Replacement

Measuring timeTyp

14 bitTemperature Operating Range
Temperature Accuracy @25°C

Min

°C

±0.3

°C

See graph 2°Cfully interchangeable(1)

14 bit44

50

ms

13 bit22

25

ms

12 bit1113

ms

11 bit

6

7

ms

±25

LSB

PSSR
Long term drift

0.04

°C/yr

(2)

Response Time (at 63% of signal) from 15°C to 45°C
τT
10
s
(1)
Typical values are recommended for calculating energy consumption while maximum values shall be applied for
calculating waiting times in communication.
(2)
At 1m/s air flow

GRAPH 2 : TEMPERATURE ERROR BUDGET

Maximal Tolerance
Typical Tolerance
Delta Temperature (°C)

2
1,8
1,6
1,4
1,2
1
0,8
0,6
0,4
0,2
0
-40

-20

0

20
40
60
Temperature (°C)

80

100

120

USER GUIDE HTU21D(F)APPLICATION INFORMATION

Soldering instructions: Lead free reflow soldering recommended process

For soldering HTU21D(F) sensor standard reflow soldering ovens may be used.

HPC199_3 HTU21D(F) Sensor Datasheet

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HTU21D(F) Sensor
Digital Relative Humidity sensor with Temperature output

HTU21D(F) sensor as a humidity sensitive component (as classified by IPC/JEDEC J-STD-020 or equivalent
documented procedure with peak temperature at 260°C during up to 30 seconds for Pb-free assembly inIR/convection reflow ovens) must be handled in a manner consistent with IPC/JEDEC J-STD-033 or anequivalent documented procedure. IPC-1601 provides humidity control, handling and packing of PCBs.
The HTU21D(F) s ensor is qualified to withstand one lead free reflow soldering recommended process profile
below according to JEDEC standard.

Mount parts within 24 hours after printing solder paste to avoid potential dry up.
For manual soldering, contact time must be limited to 5 seconds at up to 350°C.
For the design of the HTU21D(F) sensor footprint, it is recommended to use dimensions according to figure
below.

Recommended footprint for HTU21D(F) sensors. Values in mm.

No specific conditioning of devices is necessary after soldering process, either manual or reflow soldering.
Optimized performance in case of metrological measurements can be reached with stabilization of devices (24hours at 25°C / 55%RH). Similar process is advised after exposure of the devices to extreme relative humidity
conditions.
In no case, neither after manual nor reflow soldering, a board wash shall be applied. Therefore, it is strongly
recommended to use a “no-clean” solder paste. In case of applications with exposure of the sensor to corrosive
gases or condensed water (i.e. environments with high relative humidity) the soldering pads shall be sealed(e.g. conformal coating) to prevent loose contacts or short cuts.

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HTU21D(F) Sensor
Digital Relative Humidity sensor with Temperature output



Storage Conditions and Handling Instructions

It is recommended to store HTU21D(F) sensor in its original packaging at following conditions: Temperatureshall be in the range of -40°C – 125°C.
Temperature Effects

Relative humidity reading strongly depends on temperature. Therefore, it isessential 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 must show
equal temperature to allow for comparing humidity readings.
The HTU21D(F) sensor should be mounted in a way that prevents heat transferfrom electronic sensor or that keeps it as low as possible. Advice can be ventilation,
reduction of copper layers between the HTU21D(F) sensor and the rest of the PCB
or milling a slit into the PCB around the sensor (1mm minimum width).
Example of HTU21D(F) sensor mounting
with slits mills to minimize heat transfer
Materials Used for Sealing / Mounting

For sealing and gluing (use sparingly), use high filled epoxy for electronic packaging and silicone. For any
specific material please request to humidity.application@meas-spec.com.
Window must remain uncovered.

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HTU21D(F) Sensor
Digital Relative Humidity sensor with Temperature output


Wiring Considerations and Signal Integrity

Carrying the SCK and DATA signal parallel and in close proximity (e.g. in wires) for more than 10 cm may resultin cross talk and loss of communication.
This may be resolved by routing VDD and/or GND between the two data signals and/or using shielded cables.
Furthermore, slowing down SCK frequency will possibly improve signal integrity.
Power supply pins (VDD, GND) must be bypassed with a 100nF capacitor if wires are used. Capacitor shouldbe placed as close as possible to the sensor.


ESD (ElectroStatic Discharge)

ESD immunity is qualified according to: JEDEC JESD22-A114 method (Human Body Model at ±4kV) for pads & open window
 JEDEC JESD22-A115 method (Machine Model ±200V)
 ESDA ESD-STM5.3.1-1999 and AEC-Q100-011 (charged device model, 750V corner pins, 500V otherpins)
Latch-up immunity is provided at a force current of ±100mA with Tamb=25°C according to JEDEC JESD78. For
exposure beyond named limits the sensor need additional protection circuit.
INTERFACE SPECIFICATIONN°

Function

1

DATA

Comment
Data bit-stream

2

GND

Ground3

NC

Must be left unconnected

4NCMust be left unconnected

5

VDD

Supply Voltage

6

SCK

Selector for RH or Temp
Ground or unconnected

PAD

Typical application circuit, including pull-up resistor Rp and decoupling of VDD and GND by a capacitor.
Power Pins (VDD, GND)

The supply voltage of HTU21D(F) sensors must be in the range of 1.5VDC - 3.6VDC. Recommended supply
voltage is 3VDC (regulated).
However the typical application circuit includes a pull-up resistor R on data wire and a 100nF decoupling
capacitor between VDD and GND, placed as close as possible to the sensor.
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HTU21D(F) Sensor
Digital Relative Humidity sensor with Temperature output



Serial clock input (SCK)SCK is used to synchronize the communication between microcontroller and HTU21D(F) sensor. Since theinterface consists of fully static logic there is no minimum SCK frequency.
Serial data (DATA)The DATA pin is used to transfer data in and out of the device. For sending a command to the HTU21D(F)sensor, DATA is valid on the rising edge of SCK and must remain stable while SCK is high. After the fallingedge of SCK, the DATA value may be changed. For safe communication DATA shall be valid t SU and tHD before
the rising and after the falling edge of SCK, respectively. For reading data from the HTU21D(F) sensor, DATA isvalid tVD after SCK has gone low and remains valid until the next falling edge of SCK.
An external pull-up resistor (e.g. 10kΩ) on SCK is required to pull the signal high only for open collector or open
drain technology microcontrollers. In most of the cases, pull-up resistors are internally included in I/O circuits of
microcontrollers.
ELECTRICAL CHARACTERISTICS
Input/output DC characteristics(VDD=3V, Temperature=25°C unless otherwise noted)
Characteristics
Low level output

VDD=3V

voltage-4mA<IOL<0mA

High level output voltage

Symbol

Min

Typ

Max

Unit

VOL

0

-

0.4

V

VOH

70%VDD-

VDD

V

Low level input voltage

VIL

0

-

30%VDD

V

High level input voltage

VIH

70%VDD

-

VDD

V



Timing specifications of digital input/output pads for I²C fast mode

Characteristics

Symbol

Min

Typ

Max

Unit

SCK frequency

fSCK

0

-

0.4

MHz

SCK high timetSCKLH

0.6

-

-

µs

SCK low time

tSCLL

1.3

-

-

µs

DATA set-up time

tSU

100

-

-

ns

DATA hold-time

tHD

0

-

900

ns

DATA valid-tile

tVD

0

-

400

ns

SCK/DATA fall time

tF

0

-

100

ns

SCK/DATA rise time

tR

0

-

300

ns

Capacitive load on bus line

CB

0

-

500

pF

HPC199_3 HTU21D(F) Sensor Datasheet

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HTU21D(F) Sensor
Digital Relative Humidity sensor with Temperature output
Timing diagram for digital input/output pads

DATA directions are seen from the HTU21D(F) sensor. DATA line in bold is controlled by the sensor.
DATA valid read time is triggered by falling edge of anterior toggle.
COMMUNICATION PROTOCOL WITH HTU21D(F) SENSOR
Start-up sensor

The HTU21D(F) sensor requires a voltage supply between 1.5V and 3.6V. After power up, the device needs at
most 15ms while SCK is high for reaching idle state (sleep mode), i.e to be ready accepting commands from the
MCU. No command should be sent before that time. Soft reset is recommended at start, refer p.11.
Start sequence (S)To initiate transmission, a start bit has to be issued. It consists of a lowering of the DATA line
while SCK is high followed by lowering SCK.


Stop sequence (P)To stop transmission, a stop bit has to be issued. It consists of a heightening of the DATA line
while SCK is high preceded by a heightening of the SCK.

HTU21D(F) SENSOR LIST OF COMMANDS AND REGISTER ADRESSES
For sample source code, please request to humidity.application@meas-spec.com.
Sending a command

After sending the start condition, the subsequent I²C header consist of a 7-bit I²C device address 0x40 and a
DATA direction bit (‘0’ for Write access : 0x80). The HTU21D(F) sensor indicates the proper reception of a byteth
by pulling the DATA pin low (ACK bit) after the falling edge of the 8 SCK clock. After the issue of ameasurement command (0xE3 for temperature, 0xE5 for relative humidity), the MCU must wait for themeasurement to complete. The basic commands are given in the table below:

HPC199_3 HTU21D(F) Sensor Datasheet

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HTU21D(F) Sensor
Digital Relative Humidity sensor with Temperature output

Command

Code

Comment

Trigger Temperature Measurement

0xE3

Hold master

Trigger Humidity Measurement

0xE5

Hold master

Trigger Temperature Measurement

0xF3

No Hold master

Trigger Humidity Measurement

0xF5

No Hold masterWrite user register

0xE6

Read user register

0xE7

Soft Reset

0xFE

Hold/No Hold master modes

There are two different operation modes to communicate with the HTU21D(F) sensor: Hold Master mode andNo Hold Master mode.
In the first case, the SCK line is blocked (controlled by HTU21D(F) sensor) during measurement process whilein the second case the SCK line remain open for other communication while the sensor is processing themeasurement.
No Hold Master mode allows for processing other I²C communication tasks on a bus while the HTU21D(F)sensor is measuring. A communication sequence of the two modes is available below.
In the Hold Master mode, the HTU21D(F) pulls down the SCK line while measuring to force the master into a
wait state. By releasing the SCK line, the HTU21D(F) sensor indicates that internal processing is completed andthat transmission may be continued.
In the No Hold Master mode, the MCU has to poll for the termination of the internal processing of the
HTU21D(F) sensor. This is done by sending a start condition followed by the I²C header (‘1’ for Read access:0x81) as shown below. If the internal processing is finished, the HTU21D(F) sensor acknowledges the poll of the
MCU and data can be read by the MCU. If the measurement processing is not finished, the HTU21D(F) sensor
answers no ACK bit and start condition must be issued once more.
For both modes, since the maximum resolution of the measurement is 14 bits, 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 is currently not assigned.

S

1

2

1

0

3

4

5

6

7

8

9

10

11

12

13

14

15

16

17

18

0

0

0

0

0

0

ACK

1

1

1

0

0

1

0

1

ACK

I²C address + write

S

Command (see table p.9)

19

20

21

22

23

24

25

26

27

1

0

0

0

0

0

0

1

ACK

28

29

30

0

1

1

Measurement

I²C address + read

Hold during measurement

31

32

33

34

35

36

37

38

1

1

1

0

0

ACK

1

0

Data (MSB)

39

40

41

42

43

44

45

0

0

0

0

1

0

ACK

Data (LSB)

46

47

48

49

50

51

52

53

54

1

0

0

1

0

1

1

1

NACK

StatusP

Checksum

Hold Master communication sequence

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Digital Relative Humidity sensor with Temperature output

S

1

2

3

4

5

6

7

8

9

10

11

12

13

14

15

16

17

18

1

0

0

0

0

0

0

0

ACK

1

1

1

1

0

1

0

1

ACK

19

20

1

0

I²C address + write

Command (see table p.9)

MeasurementSmeasuring

21

22

23

24

25

26

27

0

0

0

0

0

1

NACK

I²C address + read

Measurement

S

19

20

21

22

23

24

25

26

27

1

0

0

0

0

0

0

1

ACK

continue measuring

I²C address + read

28

29

30

31

32

33

34

35

36

37

38

39

40

41

42

43

44

45

0

1

1

1

1

1

0

0

ACK

1

0

0

0

0

0

1

0

ACK

46

47

48

1

0

0

Data (MSB)

Data (LSB)

49

50

51

52

53

54

1

0

1

1

1

NACK

StatusP

Checksum

No Hold Master communication sequence
Grey blocks are controlled by HTU21D(F) sensor.
For Hold Master sequence, bit 45 may be changed to NACK followed by a stop condition to omit checksumtransmission.
For No Hold Master sequence, if measurement is not completed upon “read” command, sensor does notprovide ACK on bit 27 (more of these iterations are possible). If bit 45 is changed to NACK followed by stop
condition, checksum transmission is omitted.
In those examples, the HTU21D(F) sensor output is SRH = ‘0111’1100’1000’0000 (0x7C80). For the calculationof physical values status bits must be set to ‘0’. Refer to “Conversion of signal outputs” section p.14.
The maximum duration for measurement depends on the type of measurement and resolution chosen.
Maximum values shall be chosen for the communication planning of the MCU. Refer to the table p.3 and p.4
regarding measuring time specifications.
I²C communication allows for repeated start conditions without closing prior sequence with stop condition.
Soft reset

This command is used for rebooting the HTU21D(F) sensor switching the power off and on again. Uponreception of this command, the HTU21D(F) sensor system reinitializes and starts operation according to thedefault settings with the exception of the heater bit in the user register. The soft reset takes less than 15ms.

S

1

2

1

0

3

4

5

6

7

8

9

10

11

0

0

0

0

0

0

ACK

1

1

I²C address + write

12

13

14

15

16

17

18

1

1

1

1

1

0

ACK

P

Soft Reset Command

Grey blocks are controlled by HTU21D(F) sensor.

HPC199_3 HTU21D(F) Sensor Datasheet

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HTU21D(F) Sensor
Digital Relative Humidity sensor with Temperature output
User register

The content of user register is described in the table below. 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 user
register, default values of reserved bits must be read first.
The “End of Battery” alert/status 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 load the entire OTP settings to the register, with the exception of the heater
bit, before every measurement. This feature is disabled per default and it is not recommended for use. Pleaseuse soft reset instead as it contains OTP reload.
Bit

#Bits

Description/Coding

7,02

Measurement resolution
Bit 7
Bit 0
RH
0
0
12 bits
0
1
8 bits1
0
10 bits
1
1
11 bits

6

Default‘00’
Temp
14 bits
12 bits
13 bits
11 bits

(1)

1

‘0’

Status: End of Battery‘0’: VDD>2.25V‘1’: VDD<2.25V
3, 4, 5
3
Reserved
2
1
Enable on-chip heater
1
1Disable OTP reload(1)
This status bit is updated after each measurement

‘0’
‘0’
‘1’

Cut-off value for “End of Battery” signal may vary by ±0.1V.
Reserved bits must not be changed.
OTP reload active loads default settings after each time a measurement command is issued.
I²C communication reading and writing the user register example

In this example, the resolution is set to 8 bits / 12 bits (for RH/Temp) from default configuration.

S

1

2

3

4

5

6

7

8

9

10

11

12

13

14

15

16

17

18

1

0

0

0

0

0

0

0

ACK

1

1

1

0

0

1

1

1

ACK

19

20

21

22

23

24

25

26

27

28

29

30

31

32

33

34

35

36

1

0

0

0

0

0

0

1

ACK

0

0

0

0

0

0

1

0

NACK

I²C address + write

S

Read Register Command

I²C address + readS

Register content

37

38

39

40

41

42

43

44

45

46

47

48

49

50

51

52

53

54

1

0

0

0

0

0

0

0

ACK

1

1

1

0

0

1

1

0

ACK

55

56

57

58

59

60

61

62

63

0

0

0

0

0

0

1

1

ACK

I²C address + write

Write Register Command

P

Register Content to be written

Grey blocks are controlled by HTU21D(F) sensor.

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HTU21D(F) Sensor
Digital Relative Humidity sensor with Temperature output


CRC Checksum
8

5

4

HTU21D(F) sensor provides a CRC-8 checksum for error detection. The polynomial used is X + X + X + 1.
Basic Considerations
CRC stands for Cyclic Redundancy Check. It is one of the most effective error detection schemes and requires
a minimal amount of resources.
The types of errors that are detectable with CRC that is implemented in HTU21D(F) sensors are:
 Any odd number of errors anywhere within the data transmission
 All double-bit errors anywhere within the transmission
 Any cluster of errors that can be contained within an 8-bit window (1-8 bits incorrect)
 Most larger clusters of errors
A CRC is an error-detecting code commonly used in digital networks and storage devices to detect accidental
changes to raw data.
Blocks of data entering these systems get a short check value attached, based on the remainder of a polynomial
division of their contents; on retrieval the calculation is repeated, and corrective action can be taken againstpresumed data corruption if the check values do not match.
CRCs are so called because the check (data verification) value is a redundancy (it expands the message
without adding information) and the algorithm is based on cyclic codes. CRCs are popular because they aresimple to implement in binary hardware, easy to analyze mathematically, and particularly good at detecting
common errors caused by noise in transmission channels. Because the check value has a fixed length, thefunction that generates it is occasionally used as a hash function.
CRC for HTU21D(F) sensors using I²C Protocol
When HTU21D(F) sensors are run by communicating with the standard I²C protocol, an 8-bit CRC can be used
to detect transmission errors. The CRC covers all read data transmitted by the sensor. CRC properties for
HTU21D(F) sensors communicating with I²C protocol are listed in the table below.
CRC with I²C protocol
Generator polynomial
Initialization
Protected data
Final Operation

8

5

4

X +X +X +1
0x00
Read datanone

CRC calculation
To compute an n-bit binary CRC, line the bits representing the input in a row, and position the (n+1)-bit patternrepresenting the CRC's divisor (called a "polynomial") underneath the left-hand end of the row.
This is first padded with zeroes corresponding to the bit length n of the CRC.
If the input bit above the leftmost divisor bit is 0, do nothing. If the input bit above the leftmost divisor bit is 1, thedivisor is XORed into the input (in other words, the input bit above each 1-bit in the divisor is toggled). The
divisor is then shifted one bit to the right, and the process is repeated until the divisor reaches the right-handend of the input row.

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Digital Relative Humidity sensor with Temperature output

Since the left most divisor bit zeroed every input bit it touched, when this process ends the only bits in the input
row that can be nonzero are the n bits at the right-hand end of the row. These n bits are the remainder of thedivision step, and will also be the value of the CRC function.
The validity of a received message can easily be verified by performing the above calculation again, this timewith the check value added instead of zeroes. The remainder should equal zero if there are no detectableerrors.
CRC examples
The input message 11011100 (0xDC) will have as result 01111001 (0x79).
The input message 01101000 00111010 (0x683A: 24.7°C) will have as result 01111100 (0x7C).
The input message 01001110 10000101 (0x4E85: 32.3%RH) will have as result 01101011 (0x6B).

CONVERSION OF SIGNAL OUTPUTS
Default resolution is set to 12-bit relative humidity and 14-bit temperature readings. Measured data aretransferred in two byte packages, i.e. in frames of 8-bit length where the most significant bit (MSB) is transferredfirst (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.
To accommodate/adapt any process variation (nominal capacitance value of the humidity die), tolerances of thesensor above 100%RH and below 0%RH must be considered. As a consequence: 118%RH corresponds to 0xFF which is the maximum RH digital output that can be sent out from the
ASIC. RH output can reach 118%RH and above this value, there will have a clamp of the RH output to thisvalue.
 -6%RH corresponds to 0x00 which is the minimum RH digital output that can be sent out from the ASIC.
RH output can reach -6%RH and below this value, there will have a clamp of the RH output to this value.



Relative Humidity conversion

With the relative humidity signal output SRH, the relative humidity is obtained by the following formula (result in%RH), no matter which resolution is chosen:

RH  6  125 

S RH
216

In the example given p.10, the transferred 16-bit relative humidity data is 0x7C80: 31872. The relative humidity
results to be 54.8%RH.

Temperature conversion

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

Temp  46.85  175.72 

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Digital Relative Humidity sensor with Temperature output

APPLICATION: DEW POINT TEMPERATURE MEASUREMENT
The dew point is the temperature at which the water vapor in the air becomes saturated and condensation
begins.
The dew point is associated with relative humidity. A high relative humidity indicates that the dew point is closer
to the current air temperature. Relative humidity of 100% indicates that the dew point is equal to the current
temperature (and the air is maximally saturated with water). When the dew point stays constant andtemperature increases, relative humidity will decrease.
Dew point temperature of the air is calculated using Ambient Relative Humidity and Temperature measurementsfrom HTU21D(F) sensor with following formulas given below:

Partial Pressure (PPTamb) formula from Ambient Temperature:

PPTamb  10



B
 A

 (Tamb C ) 

Dew point Temperature (Td) formula from Partial Pressure (PPTamb):





B
Td    C
PPTamb  log 10  RH amb  100   A





PPTamb
RHamb
Tamb
Td
A, B, C

Partial Pressure in mmHg at ambient temperature (T amb)
Ambient humidity in %RH, computed from HTU21D(F) sensor
Humidity cell temperature in °C, computed from HTU21D(F) sensor
Calculated Dew Point in °C
Constants: A=8.1332; B=1762.39; C=235.66

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PACKAGE OUTLINEHTU21D Sensor Dimensions



HTU21DF Sensor Dimensions

Dimensions are given in mm, tolerances are ±0.1mm. The die pad (thermal center pad) is internally connected
to GND.

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Digital Relative Humidity sensor with Temperature output
Packaging TypeHTU21D(F) sensors are provided in DFN packaging. DFN stands for Dual Flat No leads.
The HTU21D(F) sensor chip is mounted to a lead frame made of Cu and plated with Ni/Pd/Au. Chip and leadframe are over molded by green epoxy-based mold compound. Please note that side walls of sensors are diced
and hence lead frame at diced edge is not covered with respective protective coating.
The total weight of the sensor is 0.025g.

Traceability Information

All HTU21D(F) sensors are laser marked with an alphanumeric, five-digit code on the sensor as pictured below.
The marking on the HTU21D(F) sensor consists of two lines with five digits each: The first line denotes the sensor type: HTU21.
 The second line denotes several information as:
o The first digit of the second line defines the output mode: D = digital and I²C
 P = PWM
 S = SDM
o The second digit defines the manufacturing year: 2 = 2012, 3 = 2013, etc.
o The last three digits represent an alphanumeric tracking code. That code can be decoded by
MEAS only and allows for tracking on batch level through production, calibration and testing andwill be provided upon justified request.

Laser marking on HTU21D(F) sensor

Reels are also labeled, as displayed below and give additional traceability information.

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Digital Relative Humidity sensor with Temperature output

With:
XX:
O:
(F):
NN:
TTTTTTTTT:
YY:
DDD:
QQQQ:



Sensor Type (21 for HTU21D(F))
Output mode (D = Digital, P = PWM, S = SDM)
Sensor with PTFE membrane (only for HTU21DF)
Product revision number
MEAS Traceability CodeTwo last digits of the year
Day of the year
Quantity per real (400, 1500 or 5000 units)

Tape and Reel Packaging

HTU21D(F) sensors are shipped in tape & reel packaging, sealed into antistatic ESD bags.
Standard packaging sizes are 400, 1500 and 5000 units per reel. Each reel contains 440mm (55 pockets)header tape and 200mm (25 pockets) trailer tape. The drawing of the packaging tapes with sensor orientation isshown in the picture below.

USER DIRECTION
OF UNREELINGPackaging reels

For 400 and 1500 units: outside diameter of 7” (178mm) and a 1/2” (13mm) diameter arbor hole.
For 5000 units: outside diameter of 13” (330mm) and a 1/2” (13mm) diameter arbor hole.

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Digital Relative Humidity sensor with Temperature output

ORDERING INFORMATION** HTU21D – DIGITAL TEMPERATURE AND RELATIVE HUMIDITY MODULE **PACKAGE: TAPE AND REEL M.P.Q OF 400 PIECES, 1500 PIECES OR 5000 PIECES

HPP828E031R4 - HTU21D in tape and reel of 400 pieces
HPP828E031R1 - HTU21D in tape and reel of 1500 pieces
HPP828E031R5 - HTU21D in tape and reel of 5000 pieces** HTU21DF – DIGITAL TEMPERATURE AND RELATIVE HUMIDITY MODULE WITH PTFE MEMBRANE **PACKAGE: TAPE AND REEL M.P.Q OF 400 PIECES, 1500 PIECES OR 5000 PIECES

HPP828E131R4 - HTU21DF in tape and reel of 400 pieces
HPP828E131R1 - HTU21DF in tape and reel of 1500 pieces
HPP828E131R5 - HTU21DF in tape and reel of 5000 pieces** HTU21D DEMOKIT – HPP828KIT **This is a USB device for MEAS Model HTU21D Digital Relative Humidity & Temperature sensor demonstration.
Supporting up to 4 sensor acquisitions at the same time, it shows the consistency of different sensors and test
sensor functions conveniently.
For detailed information, please request to humidity.application@meas-spec.com.

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Digital Relative Humidity sensor with Temperature output

Customer Service contact details
Measurement Specialties, Inc - MEAS France
Impasse Jeanne Benozzi
CS 83 163
31027 Toulouse Cedex 3
FRANCE
Tel:+33 (0)5 820.822.02
Fax:+33 (0)5.820.821.51
Sales: humidity.sales@meas-spec.com
MEAS Website: http://www.meas-spec.com/humidity-sensors.aspx

Revision
0A

2

3

Comments
Document creation
General update
HTU21DF product with embedded PTFE membrane reference
added, Storage conditions after soldering process updated(typing error), ESD performances updated, complementary
information on RH output signal in “Conversion of signal
outputs” paragraph, information on tape and reel packaging
added, HTU21D demokit availability information added.
Correction of I²C communication reading and writing,correction of soldering peak temperature

Who
D. LE GALL
D. LE GALL-ZIRILLI

Date
April 12February 13

D. LE GALL-ZIRILLI

July 13

M.ROBERT

October 2013

The information in this sheet has been carefully reviewed and is believed to be accurate; however, no responsibility is assumed forinaccuracies. Furthermore, this information does not convey to the purchaser of such devices any license under the patent rights to the
manufacturer. Measurement Specialties, Inc. reserves the right to make changes without further notice to any product herein. Measurement
Specialties, Inc. makes no warranty, representation or guarantee regarding the suitability of its product for any particular purpose, nor does
Measurement Specialties, Inc. assume any liability arising out of the application or use of any product or circuit and specifically disclaims
any and all liability, including without limitation consequential or incidental damages. Typical parameters can and do vary in different
applications. All operating parameters must be validated for each customer application by customer’s technical experts. Measurement
Specialties, Inc. does not convey any license under its patent rights nor the rights of others.

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