HTU21D(F) RH/T Sensor IC
Digital Relative Humidity sensor with Temperature output
HPC199_5 HTU21D(F) Sensor Datasheet www.meas-spec.com April 2014
1/22
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 in
terms of size and intelligence, it is embedded in a reflow solderable Dual Flat No leads (DFN) package with a
small 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 is
made possible with the module for humidity and temperature digital outputs. These low power sensors are
designed 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 and
a 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 required
in standard conditions
Automotive: defogging, HVAC
Home Appliance
Instantaneous desaturation after long periods in
saturation phase
Medical
Printers
Compatible with automatized assembly processes,
including Pb free and reflow processes
Humidifier
Individual marking for compliance to stringent
traceability requirements
HTU21D(F) RH/T Sensor IC
Digital Relative Humidity sensor with Temperature output
HPC199_5 HTU21D(F) Sensor Datasheet www.meas-spec.com April 2014
2/22
NOMENCLATURE
HTU2XY(F)
HTU2XY Modules HTU2XYF Modules
PERFORMANCE SPECS
MAXIMUM 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 Range
Ta
-40 to +125
°C
VDD to GND
-0.3 to 3.6V
V
Digital I/O pins (DATA/SCK) to VDD
-0.3 to VDD+0.3
V
Input current on any pin
-10 to +10
mA
Peak conditions: less than 10% of the operating time
Exposure to absolute maximum rating conditions for extended periods may affect the sensor reliability.
Humidity accuracy :
X = 0: +/-5%RH tolerance @55%RH
= 1: +/-3%RH tolerance @55%RH
Output Sensor:
Y = D for Digital, I2C protocol
= P for PWM interface, analog output
With embedded PTFE filter: HTU2XYF
HTU21D(F) RH/T Sensor IC
Digital Relative Humidity sensor with Temperature output
HPC199_5 HTU21D(F) Sensor Datasheet www.meas-spec.com April 2014
3/22
ELECTRICAL AND GENERAL ITEMS
(@T = 25°C, @Vdd = 3V)
Characteristics
Symbol
Min
Typ
Max
Unit
Voltage Supply
VDD
1.5
3.0
3.6
V
Current
consumption (1)
Sleep mode
idd
0.02
0.14
µA
Measuring
300
450
500
µA
Power Dissipation
Sleep mode
0.06
0.5
µW
Average 8bit (2)
2.7
µW
Communication
digital 2-wire interface, I²C protocol
Heater
VDD=3V
5.5mW/ΔT=+0.5-1.5°C
Storage
-40°C/125°C
(1) Conditions: Vdd = 3V, SCK= 400kHz at 25°C
(2) Conditions: Vdd = 3V, SCK= 400kHz, Temp<60°C, duty cycle <10%
SENSOR PERFORMANCE
RELATIVE HUMIDITY
(@T = 25°C, @Vdd = 3V)
Characteristics
Symbol
Min
Typ
Max
Unit
Resolution
12 bits
0.04
%RH
8 bits
0.7
%RH
Humidity Operating Range
RH
0
100
%RH
Relative Humidity Accuracy
@25°C (20%RH to 80%RH)
typ
±2
%RH
max
See graph 1
%RH
Replacement
fully interchangeable
Temperature coefficient (from 0°C to 80°C)
Tcc
-0.15
%RH/°C
Humidity Hysteresis
±1
%RH
Measuring Time (1)
12 bits
14
16
ms
11 bits
7
8
ms
10 bits
4
5
ms
8 bits
2
3
ms
PSRR
±10
LSB
Recovery time after 150 hours of condensation
t
10
s
Long term drift
0.5
%RH/yr
Response Time (at 63% of signal) from 33 to 75%RH (2)
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
HTU21D(F) RH/T Sensor IC
Digital Relative Humidity sensor with Temperature output
HPC199_5 HTU21D(F) Sensor Datasheet www.meas-spec.com April 2014
4/22
GRAPH 1 : RELATIVE HUMIDITY ERROR BUDGET CONDITIONS AT 25°C
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:
CoeffTempTRHRH actualactualTdTcompensate )25(
RHactualT Ambient humidity in %RH, computed from HTU21D(F) sensor
Tactual Humidity cell temperature in °C, computed from HTU21D(F) sensor
CoeffTemp Temperature coefficient of the HTU21D(F) in %RH/°C
0
1
2
3
4
5
6
7
8
9
10
0 10 20 30 40 50 60 70 80 90 100
Delta Relative Humidity (%RH)
Relative Humidity (%RH)
Maximal Tolerance
Typical Tolerance
HTU21D(F) RH/T Sensor IC
Digital Relative Humidity sensor with Temperature output
HPC199_5 HTU21D(F) Sensor Datasheet www.meas-spec.com April 2014
5/22
TEMPERATURE
Characteristics
Symbol
Min
Typ
Max
Unit
Resolution
14 bit
0.01
°C
12 bit
0.04
°C
Temperature Operating Range
T
-40
+125
°C
Temperature Accuracy @25°C
typ
±0.3
°C
max
See graph 2
°C
Replacement
fully interchangeable
Measuring time (1)
14 bit
44
50
ms
13 bit
22
25
ms
12 bit
11
13
ms
11 bit
6
7
ms
PSSR
±25
LSB
Long term drift
0.04
°C/yr
Response Time (at 63% of signal) from 15°C to 45°C (2)
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
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.
0
0,2
0,4
0,6
0,8
1
1,2
1,4
1,6
1,8
2
-40 -20 0 20 40 60 80 100 120
Delta Temperature (°C)
Temperature (°C)
Maximal Tolerance
Typical Tolerance
HTU21D(F) RH/T Sensor IC
Digital Relative Humidity sensor with Temperature output
HPC199_5 HTU21D(F) Sensor Datasheet www.meas-spec.com April 2014
6/22
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 in
IR/convection reflow ovens) must be handled in a manner consistent with IPC/JEDEC J-STD-033 or an
equivalent documented procedure. IPC-1601 provides humidity control, handling and packing of PCBs.
The HTU21D(F) sensor 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 (24
hours 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.
HTU21D(F) RH/T Sensor IC
Digital Relative Humidity sensor with Temperature output
HPC199_5 HTU21D(F) Sensor Datasheet www.meas-spec.com April 2014
7/22
Storage Conditions and Handling Instructions
It is recommended to store HTU21D(F) sensor in its original packaging at following conditions: Temperature
shall be in the range of -40°C – 125°C.
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 the
relative 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 transfer
from 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.
HTU21D(F) RH/T Sensor IC
Digital Relative Humidity sensor with Temperature output
HPC199_5 HTU21D(F) Sensor Datasheet www.meas-spec.com April 2014
8/22
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 result
in 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 should
be 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 other
pins)
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 SPECIFICATION
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.
N°
Function
Comment
1
DATA
Data bit-stream
2
GND
Ground
3
NC
Must be left unconnected
4
NC
Must be left unconnected
5
VDD
Supply Voltage
6
SCK
Selector for RH or Temp
PAD
Ground or unconnected
HTU21D(F) RH/T Sensor IC
Digital Relative Humidity sensor with Temperature output
HPC199_5 HTU21D(F) Sensor Datasheet www.meas-spec.com April 2014
9/22
Serial clock input (SCK)
SCK is used to synchronize the communication between microcontroller and HTU21D(F) sensor. Since the
interface 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 falling
edge of SCK, the DATA value may be changed. For safe communication DATA shall be valid tSU and tHD before
the rising and after the falling edge of SCK, respectively. For reading data from the HTU21D(F) sensor, DATA is
valid 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
Symbol
Min
Typ
Max
Unit
Low level output
voltage
VDD=3V
-4mA<IOL<0mA
VOL
0
-
0.4
V
High level output voltage
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 time
tSCKLH
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
HTU21D(F) RH/T Sensor IC
Digital Relative Humidity sensor with Temperature output
HPC199_5 HTU21D(F) Sensor Datasheet www.meas-spec.com April 2014
10/22
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 byte
by pulling the DATA pin low (ACK bit) after the falling edge of the 8th SCK clock. After the issue of a
measurement command (0xE3 for temperature, 0xE5 for relative humidity), the MCU must wait for the
measurement to complete. The basic commands are given in the table below:
HTU21D(F) RH/T Sensor IC
Digital Relative Humidity sensor with Temperature output
HPC199_5 HTU21D(F) Sensor Datasheet www.meas-spec.com April 2014
11/22
Hold/No Hold master modes
There are two different operation modes to communicate with the HTU21D(F) sensor: Hold Master mode and
No Hold Master mode.
In the first case, the SCK line is blocked (controlled by HTU21D(F) sensor) during measurement process while
in the second case the SCK line remain open for other communication while the sensor is processing the
measurement.
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 and
that 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 the
measurement type (‘0’: temperature, ‘1’: humidity). Bit 0 is currently not assigned.
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
S
1
0
0
0
0
0
0
0
ACK
1
1
1
0
0
1
0
1
ACK
I²C address + write
Command (see table p.9)
19
20
21
22
23
24
25
26
27
S
1
0
0
0
0
0
0
1
ACK
Measurement
I²C address + read
Hold during measurement
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
Data (MSB)
Data (LSB)
Status
46
47
48
49
50
51
52
53
54
1
0
0
1
0
1
1
1
NACK
P
Checksum
Hold Master communication sequence
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 master
Write user register
0xE6
Read user register
0xE7
Soft Reset
0xFE
HTU21D(F) RH/T Sensor IC
Digital Relative Humidity sensor with Temperature output
HPC199_5 HTU21D(F) Sensor Datasheet www.meas-spec.com April 2014
12/22
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
S
1
0
0
0
0
0
0
0
ACK
1
1
1
1
0
1
0
1
ACK
I²C address + write
Command (see table p.9)
19
20
21
22
23
24
25
26
27
Measurement
S
1
0
0
0
0
0
0
1
NACK
measuring
I²C address + read
19
20
21
22
23
24
25
26
27
Measurement
S
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
Data (MSB)
Data (LSB)
Status
46
47
48
49
50
51
52
53
54
1
0
0
1
0
1
1
1
NACK
P
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 checksum
transmission.
For No Hold Master sequence, if measurement is not completed upon “read” command, sensor does not
provide ACK on bit 27 (more of these iterations are possible). If bit 45 is changed to NACK followed by stop
condition, checksum transmission is omitted.
In those examples, the HTU21D(F) sensor output is SRH = ‘0111’1100’1000’0000 (0x7C80). For the calculation
of 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. Upon
reception of this command, the HTU21D(F) sensor system reinitializes and starts operation according to the
default settings with the exception of the heater bit in the user register. The soft reset takes less than 15ms.
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
S
1
0
0
0
0
0
0
0
ACK
1
1
1
1
1
1
1
0
ACK
P
I²C address + write
Soft Reset Command
Grey blocks are controlled by HTU21D(F) sensor.
HTU21D(F) RH/T Sensor IC
Digital Relative Humidity sensor with Temperature output
HPC199_5 HTU21D(F) Sensor Datasheet www.meas-spec.com April 2014
13/22
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. Please
use soft reset instead as it contains OTP reload.
Bit
#Bits
Description/Coding
Default
7,0
2
Measurement resolution
Bit 7
Bit 0
RH
Temp
0
0
12 bits
14 bits
0
1
8 bits
12 bits
1
0
10 bits
13 bits
1
1
11 bits
11 bits
‘00’
6
1
Status: End of Battery(1)
‘0’: VDD>2.25V
‘1’: VDD<2.25V
‘0’
3, 4, 5
3
Reserved
‘0’
2
1
Enable on-chip heater
‘0’
1
1
Disable OTP reload
‘1’
(1) This status bit is updated after each measurement
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.
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
S
1
0
0
0
0
0
0
0
ACK
1
1
1
0
0
1
1
1
ACK
I²C address + write
Read Register Command
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
S
1
0
0
0
0
0
0
1
ACK
0
0
0
0
0
0
1
0
NACK
I²C address + read
Register content
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
S
1
0
0
0
0
0
0
0
ACK
1
1
1
0
0
1
1
0
ACK
I²C address + write
Write Register Command
55
56
57
58
59
60
61
62
63
0
0
0
0
0
0
1
1
ACK
P
Register Content to be written
Grey blocks are controlled by HTU21D(F) sensor.
HTU21D(F) RH/T Sensor IC
Digital Relative Humidity sensor with Temperature output
HPC199_5 HTU21D(F) Sensor Datasheet www.meas-spec.com April 2014
14/22
CRC Checksum
HTU21D(F) sensor provides a CRC-8 checksum for error detection. The polynomial used is X8 + X5 + X4 + 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 against
presumed 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 are
simple 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, the
function 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
X8 + X5 + X4 + 1
Initialization
0x00
Protected data
Read data
Final Operation
none
CRC calculation
To compute an n-bit binary CRC, line the bits representing the input in a row, and position the (n+1)-bit pattern
representing 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, the
divisor 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-hand
end of the input row.
HTU21D(F) RH/T Sensor IC
Digital Relative Humidity sensor with Temperature output
HPC199_5 HTU21D(F) Sensor Datasheet www.meas-spec.com April 2014
15/22
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 the
division 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 time
with the check value added instead of zeroes. The remainder should equal zero if there are no detectable
errors.
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 are
transferred in two byte packages, i.e. in frames of 8-bit length where the most significant bit (MSB) is transferred
first (left aligned). Each byte is followed by an acknowledge bit. The two status bits, the last bits of LSB, must be
set to ‘0’ before calculating physical values.
To accommodate/adapt any process variation (nominal capacitance value of the humidity die), tolerances of the
sensor 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 this
value.
-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:
16
2
1256RH
S
RH
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:
16
2
72.17585.46 Temp
S
Temp
HTU21D(F) RH/T Sensor IC
Digital Relative Humidity sensor with Temperature output
HPC199_5 HTU21D(F) Sensor Datasheet www.meas-spec.com April 2014
16/22
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 and
temperature increases, relative humidity will decrease.
Dew point temperature of the air is calculated using Ambient Relative Humidity and Temperature measurements
from HTU21D(F) sensor with following formulas given below:
Partial Pressure (PPTamb) formula from Ambient Temperature:
Dew point Temperature (Td) formula from Partial Pressure (PPTamb):
PPTamb Partial Pressure in mmHg at ambient temperature (Tamb)
RHamb Ambient humidity in %RH, computed from HTU21D(F) sensor
Tamb Humidity cell temperature in °C, computed from HTU21D(F) sensor
Td Calculated Dew Point in °C
A, B, C Constants: A=8.1332; B=1762.39; C=235.66
C
A
PP
RH
B
T
Tamb
amb
d
100
log10
)(
10 CTamb
B
A
Tamb
PP
HTU21D(F) RH/T Sensor IC
Digital Relative Humidity sensor with Temperature output
HPC199_5 HTU21D(F) Sensor Datasheet www.meas-spec.com April 2014
17/22
PACKAGE OUTLINE
HTU21D Sensor Dimensions
HTU21D(F) RH/T Sensor IC
Digital Relative Humidity sensor with Temperature output
HPC199_5 HTU21D(F) Sensor Datasheet www.meas-spec.com April 2014
18/22
HTU21DF Sensor Dimensions
Dimensions are given in mm, tolerances are ±0.1mm. The die pad (thermal center pad) is internally connected
to GND.
Packaging Type
HTU21D(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 lead
frame 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
HTU21D(F) RH/T Sensor IC
Digital Relative Humidity sensor with Temperature output
HPC199_5 HTU21D(F) Sensor Datasheet www.meas-spec.com April 2014
19/22
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
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 and
will be provided upon justified request.
Laser marking on HTU21D(F) sensor
Reels are also labeled, as displayed below and give additional traceability information.
HTU21D(F) RH/T Sensor IC
Digital Relative Humidity sensor with Temperature output
HPC199_5 HTU21D(F) Sensor Datasheet www.meas-spec.com April 2014
20/22
With: XX: Sensor Type (21 for HTU21D(F))
O: Output mode (D = Digital, P = PWM)
(F): Sensor with PTFE membrane (only for HTU21DF)
NN: Product revision number
TTTTTTTTT: MEAS Traceability Code
YY: Two last digits of the year
DDD: Day of the year
QQQQ: 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 is
shown in the picture below.
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.
USER DIRECTION
OF UNREELING
HTU21D(F) RH/T Sensor IC
Digital Relative Humidity sensor with Temperature output
HPC199_5 HTU21D(F) Sensor Datasheet www.meas-spec.com April 2014
21/22
ORDERING INFORMATION
** HTU21D – I.C 21D RH/T DIGITAL MODULE**
PACKAGE: TAPE AND REEL M.P.Q OF 400 PIECES, 1500 PIECES OR 5000 PIECES
HPP845E031R4 - I.C 21D RH/T DIGITAL MODULE in tape and reel of 400 pieces
HPP845E031R1 - I.C 21D RH/T DIGITAL MODULE in tape and reel of 1500 pieces
HPP845E031R5 - I.C 21D RH/T DIGITAL MODULE in tape and reel of 5000 pieces
** HTU21DF – I.C 21DF RH/T DIGITAL MODULE WITH PTFE MEMBRANE **
PACKAGE: TAPE AND REEL M.P.Q OF 400 PIECES, 1500 PIECES OR 5000 PIECES
HPP845E131R4 - I.C 21DF RH/T DIGITAL MODULE in tape and reel of 400 pieces
HPP845E131R1 - I.C 21DF RH/T DIGITAL MODULE in tape and reel of 1500 pieces
HPP845E131R5 - I.C 21DF RH/T DIGITAL MODULE in tape and reel of 5000 pieces
** I.C 21D DEMOKIT – HPP845KIT **
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.
HTU21D(F) RH/T Sensor IC
Digital Relative Humidity sensor with Temperature output
HPC199_5 HTU21D(F) Sensor Datasheet www.meas-spec.com April 2014
22/22
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
Edition
Comments
Who
Date
0
Document creation
D. LE GALL
April 12
A
General update
D. LE GALL-ZIRILLI
February 13
2
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.
D. LE GALL-ZIRILLI
July 13
3
Correction of I²C communication reading and writing,
correction of soldering peak temperature
M.ROBERT
October 2013
4
Obsolescence of HTU21S (SDM interface) version
External package dimensions update
M.ROBERT
January 2014
5
Part number and designation modification
M.ROBERT
April 2014
The information in this sheet has been carefully reviewed and is believed to be accurate; however, no responsibility is assumed for
inaccuracies. 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.
