S i 7 0 2 1 - A 2 0 I

S i 7 0 2 1 - A 2 0 I
Si 7 0 2 1 - A20
I 2 C H UMIDITY A N D TEMPERATURE S ENSOR
Features






Precision Relative Humidity Sensor
 ± 3% RH (max), 0–80% RH
High Accuracy Temperature Sensor
±0.4 °C (max), –10 to 85 °C
0 to 100% RH operating range
Up to –40 to +125 °C operating
range
Wide operating voltage
(1.9 to 3.6 V)
Low Power Consumption
150 µA active current
60 nA standby current

Factory-calibrated

I2C Interface
Integrated on-chip heater
3x3 mm DFN Package
Excellent long term stability
Optional factory-installed cover
Low-profile
Protection during reflow
Excludes liquids and particulates




Ordering Information:
See page 29.
Applications
Pin Assignments




HVAC/R
Thermostats/humidistats
 Respiratory therapy
 White goods
 Indoor weather stations
Micro-environments/data centers
Automotive climate control and
defogging
 Asset and goods tracking
 Mobile phones and tablets
Top View
SDA
1
6
SCL
GND
2
5
VDD
DNC
3
4
DNC
Description
The Si7021 I2C Humidity and Temperature Sensor is a monolithic CMOS IC
integrating humidity and temperature sensor elements, an analog-to-digital
converter, signal processing, calibration data, and an I2C Interface. The patented
use of industry-standard, low-K polymeric dielectrics for sensing humidity enables
the construction of low-power, monolithic CMOS Sensor ICs with low drift and
hysteresis, and excellent long term stability.
The humidity and temperature sensors are factory-calibrated and the calibration
data is stored in the on-chip non-volatile memory. This ensures that the sensors
are fully interchangeable, with no recalibration or software changes required.
Patent Protected. Patents pending
The Si7021 is available in a 3x3 mm DFN package and is reflow solderable. It can
be used as a hardware- and software-compatible drop-in upgrade for existing RH/
temperature sensors in 3x3 mm DFN-6 packages, featuring precision sensing
over a wider range and lower power consumption. The optional factory-installed
cover offers a low profile, convenient means of protecting the sensor during
assembly (e.g., reflow soldering) and throughout the life of the product, excluding
liquids (hydrophobic/oleophobic) and particulates.
The Si7021 offers an accurate, low-power, factory-calibrated digital solution ideal
for measuring humidity, dew-point, and temperature, in applications ranging from
HVAC/R and asset tracking to industrial and consumer platforms.
Rev. 1.1 6/15
Copyright © 2015 by Silicon Laboratories
Si7021-A20
Si7021-A20
Functional Block Diagram
Vdd
Si7021
Humidity
Sensor
1.25V
Ref
ADC
Calibration
Memory
Control Logic
Temp
Sensor
I2C Interface
SDA
SCL
GND
2
Rev. 1.1
Si7021-A20
TABLE O F C ONTENTS
Section
Page
1. Electrical Specifications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .4
2. Typical Application Circuits . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
3. Bill of Materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
4. Functional Description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
4.1. Relative Humidity Sensor Accuracy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
4.2. Hysteresis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
4.3. Prolonged Exposure to High Humidity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
4.4. PCB Assembly . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
4.5. Protecting the Sensor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
4.6. Bake/Hydrate Procedure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
4.7. Long Term Drift/Aging . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
2
5. I C Interface . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
5.1. Issuing a Measurement Command . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
5.2. Reading and Writing User Registers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
5.3. Electronic Serial Number . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .23
5.4. Firmware Revision . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24
5.5. Heater . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25
6. Control Registers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .26
6.1. Register Descriptions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26
7. Pin Descriptions: Si7021 (Top View) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28
8. Ordering Guide . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29
9. Package Outline . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30
9.1. Package Outline: 3x3 6-pin DFN . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30
9.2. Package Outline: 3x3 6-pin DFN with Protective Cover . . . . . . . . . . . . . . . . . . . . . . 31
10. PCB Land Pattern and Solder Mask Design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32
11. Top Marking . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .33
11.1. Si7021 Top Marking . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33
11.2. Top Marking Explanation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33
12. Additional Reference Resources . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34
Document Change List . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .35
Contact Information . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .36
Rev. 1.1
3
Si7021-A20
1. Electrical Specifications
Unless otherwise specified, all min/max specifications apply over the recommended operating conditions.
Table 1. Recommended Operating Conditions
Symbol
Parameter
Power Supply
Test Condition
VDD
Min
Typ
Max
Unit
1.9
—
3.6
V
Operating Temperature
TA
I and Y grade
–40
—
+125
°C
Operating Temperature
TA
G grade
–40
—
+85
°C
Table 2. General Specifications
1.9 < VDD < 3.6 V; TA = –40 to 85 °C (G grade) or –40 to 125 °C (I/Y grade); default conversion time unless otherwise noted.
Parameter
Symbol
Test Condition
Min
Typ
Max
Unit
Input Voltage High
VIH
SCL, SDA pins
0.7xVDD
—
—
V
Input Voltage Low
VIL
SCL, SDA pins
—
—
0.3xVDD
V
Input Voltage Range
VIN
SCL, SDA pins with respect to GND
0.0
—
VDD
V
Input Leakage
IIL
SCL, SDA pins
—
—
1
μA
VOL
SDA pin; IOL = 2.5 mA; VDD = 3.3 V
—
—
0.6
V
SDA pin; IOL = 1.2 mA;
VDD = 1.9 V
—
—
0.4
V
RH conversion in progress
—
150
180
μA
Temperature conversion in progress
—
90
120
μA
Output Voltage Low
Current
Consumption
IDD
Standby, –40 to +85
°C2
Standby, –40 to +125 °C
—
0.06
0.62
μA
2
—
0.06
3.8
μA
3
—
3.5
4.0
mA
—
3.5
4.0
mA
—
3.1 to 94.2
—
mA
Peak IDD during powerup
Peak IDD during I2C operations4
Heater Current
5
IHEAT
Notes:
1. Initiating a RH measurement will also automatically initiate a temperature measurement. The total conversion time will
be tCONV(RH) + tCONV(T).
2. No conversion or I2C transaction in progress. Typical values measured at 25 °C.
3. Occurs once during powerup. Duration is <5 msec.
4. Occurs during I2C commands for Reset, Read/Write User Registers, Read EID, and Read Firmware Version. Duration is
<100 µs when I2C clock speed is >100 kHz (>200 kHz for 2-byte commands).
5. Additional current consumption when HTRE bit enabled. See Section “5.5. Heater” for more information.
4
Rev. 1.1
Si7021-A20
Table 2. General Specifications (Continued)
1.9 < VDD < 3.6 V; TA = –40 to 85 °C (G grade) or –40 to 125 °C (I/Y grade); default conversion time unless otherwise noted.
Parameter
Conversion Time1
Powerup Time
Symbol
Test Condition
Min
Typ
Max
tCONV
12-bit RH
—
10
12
11-bit RH
—
5.8
7
10-bit RH
—
3.7
4.5
8-bit RH
—
2.6
3.1
14-bit temperature
—
7
10.8
13-bit temperature
—
4
6.2
12-bit temperature
—
2.4
3.8
11-bit temperature
—
1.5
2.4
From VDD ≥ 1.9 V to ready for a
conversion, 25 °C
—
18
25
From VDD ≥ 1.9 V to ready for a
conversion, full temperature range
—
—
80
After issuing a software reset
command
—
5
15
tPU
Unit
ms
ms
Notes:
1. Initiating a RH measurement will also automatically initiate a temperature measurement. The total conversion time will
be tCONV(RH) + tCONV(T).
2. No conversion or I2C transaction in progress. Typical values measured at 25 °C.
3. Occurs once during powerup. Duration is <5 msec.
4. Occurs during I2C commands for Reset, Read/Write User Registers, Read EID, and Read Firmware Version. Duration is
<100 µs when I2C clock speed is >100 kHz (>200 kHz for 2-byte commands).
5. Additional current consumption when HTRE bit enabled. See Section “5.5. Heater” for more information.
Table 3. I2C Interface Specifications1
1.9  VDD  3.6 V; TA = –40 to +85 °C (G grade) or –40 to +125 °C (I/Y grade) unless otherwise noted.
Parameter
Symbol
Test Condition
Min
Typ
Max
Unit
Hysteresis
VHYS
High-to-low versus low-tohigh transition
0.05 x VDD
—
—
V
SCLK Frequency2
fSCL
—
—
400
kHz
SCL High Time
tSKH
0.6
—
—
µs
SCL Low Time
tSKL
1.3
—
—
µs
Start Hold Time
tSTH
0.6
—
—
µs
Start Setup Time
tSTS
0.6
—
—
µs
Notes:
1. All values are referenced to VIL and/or VIH.
2. Depending on the conversion command, the Si7021 may hold the master during the conversion (clock stretch). At
above 100 kHz SCL, the Si7021 may also hold the master briefly for user register and device ID transactions. At the
highest I2C speed of 400 kHz the stretching will be <50 µs.
3. Pulses up to and including 50 ns will be suppressed.
Rev. 1.1
5
Si7021-A20
Table 3. I2C Interface Specifications1 (Continued)
1.9  VDD  3.6 V; TA = –40 to +85 °C (G grade) or –40 to +125 °C (I/Y grade) unless otherwise noted.
Parameter
Symbol
Test Condition
Min
Typ
Max
Unit
0.6
—
—
µs
1.3
—
—
µs
Stop Setup Time
tSPS
Bus Free Time
tBUF
SDA Setup Time
tDS
100
—
—
ns
SDA Hold Time
tDH
100
—
—
ns
SDA Valid Time
tVD;DAT
From SCL low to data valid
—
—
0.9
µs
SDA Acknowledge Valid Time
tVD;ACK
From SCL low to data valid
—
—
0.9
µs
50
—
—
ns
Suppressed Pulse Width3
Between Stop and Start
tSPS
Notes:
1. All values are referenced to VIL and/or VIH.
2. Depending on the conversion command, the Si7021 may hold the master during the conversion (clock stretch). At
above 100 kHz SCL, the Si7021 may also hold the master briefly for user register and device ID transactions. At the
highest I2C speed of 400 kHz the stretching will be <50 µs.
3. Pulses up to and including 50 ns will be suppressed.
tSKH
1/fSCL
tSKL
tSP
SCL
tBUF
tSTH
tDS
D6
SDA
D5
tDH
D4
D0
tSPS
R/W
ACK
Start Bit
Stop Bit
tVD : ACK
tSTS
Figure 1. I2C Interface Timing Diagram
6
Rev. 1.1
Si7021-A20
Table 4. Humidity Sensor
1.9 ≤ VDD ≤ 3.6 V; TA = 30 °C; default conversion time unless otherwise noted.
Parameter
Operating
Accuracy
Symbol
Range1
2, 3
Test Condition
Min
Typ
Max
Unit
Non-condensing
0
—
100
%RH
0 – 80% RH
—
±2
±3
%RH
80 – 100% RH
Repeatability/Noise
See Figure 2.
12-bit resolution
—
0.025
—
11-bit resolution
—
0.05
—
10-bit resolution
—
0.1
—
8-bit resolution
—
0.2
—
1 m/s airflow, with cover
—
18
—
1 m/s airflow, without cover
—
17
—
Drift vs. Temperature
—
0.05
—
%RH/°C
Hysteresis
—
±1
—
%RH
Long Term Stability3
—
< 0.25
—
%RH/yr
Response Time4
τ63%
%RH RMS
S
Notes:
1. Recommended humidity operating range is 20% to 80% RH (non-condensing) over –10 °C to 60 °C. Prolonged
operation beyond these ranges may result in a shift of sensor reading, with slow recovery time.
2. Excludes hysteresis, long term drift, and certain other factors and is applicable to non-condensing environments only.
See Section “4.1. Relative Humidity Sensor Accuracy” for more details.
3. Drift due to aging effects at typical room conditions of 30 °C and 30% to 50% RH. May be impacted by dust, vaporized
solvents or other contaminants, e.g., out-gassing tapes, adhesives, packaging materials, etc. See Section “4.7. Long
Term Drift/Aging” .
4. Response time to a step change in RH. Time for the RH output to change by 63% of the total RH change.
Rev. 1.1
7
Si7021-A20
Figure 2. RH Accuracy at 30 °C
8
Rev. 1.1
Si7021-A20
Table 5. Temperature Sensor
1.9 ≤ VDD ≤ 3.6 V; TA = –40 to +85 °C (G grade) or –40 to +125 °C (I/Y grade) default conversion time, unless otherwise noted.
Parameter
Symbol
Operating Range
Accuracy1
Test Condition
Min
Typ
Max
Unit
I and Y Grade
–40
—
+125
°C
G Grade
–40
—
+85
°C
–10 °C< tA < 85 °C
—
±0.3
±0.4
°C
–40 < tA < 125 °C
Repeatability/Noise
Figure 3
14-bit resolution
—
0.01
—
13-bit resolution
—
0.02
—
12-bit resolution
—
0.04
—
11-bit resolution
—
0.08
—
Unmounted device
—
0.7
—
Si7021-EB board
—
5.1
—
—
 0.01
—
°C RMS
Response Time2
τ63%
Long Term Stability
s
°C/Yr
Notes:
1. 14b measurement resolution (default).
2. Time to reach 63% of final value in response to a step change in temperature. Actual response time will vary
dependent on system thermal mass and air-flow.
Figure 3. Temperature Accuracy*
*Note: Applies only to I and Y grade devices beyond +85 °C.
Rev. 1.1
9
Si7021-A20
Table 6. Thermal Characteristics
Parameter
Symbol
Test Condition
DFN-6
Unit
Junction to Air Thermal Resistance
JA
JEDEC 2-Layer board,
No Airflow
256
°C/W
Junction to Air Thermal Resistance
JA
JEDEC 2-Layer board,
1 m/s Airflow
224
°C/W
Junction to Air Thermal Resistance
JA
JEDEC 2-Layer board,
2.5 m/s Airflow
205
°C/W
Junction to Case Thermal Resistance
JC
JEDEC 2-Layer board
22
°C/W
Junction to Board Thermal Resistance
JB
JEDEC 2-Layer board
134
°C/W
Table 7. Absolute Maximum Ratings1
Parameter
Min
Typ
Max
Unit
Ambient temperature
under bias
–55
—
125
°C
Storage Temperature2
–65
—
150
°C
Voltage on I/O pins
–0.3
—
VDD+0.3 V
V
Voltage on VDD with
respect to GND
–0.3
—
4.2
V
HBM
—
—
2
kV
CDM
—
—
1.25
kV
MM
—
—
250
V
ESD Tolerance
Symbol
Test Condition
Notes:
1. Absolute maximum ratings are stress ratings only, operation at or beyond these conditions is not implied and may
shorten the life of the device or alter its performance.
2. Special handling considerations apply; see application note, “AN607: Si70xx Humidity Sensor Designer’s Guide”.
10
Rev. 1.1
Si7021-A20
2. Typical Application Circuits
The primary function of the Si7021 is to measure relative humidity and temperature. Figure 4 demonstrates the
typical application circuit to achieve these functions.
1.9 to 3.6V
0.1µF
10k 10k
5
VDD
Si7021
SCL
6
SDA
1
SCL
SDA
GND
2
Figure 4. Typical Application Circuit for Relative Humidity and Temperature Measurement
Rev. 1.1
11
Si7021-A20
3. Bill of Materials
Table 8. Typical Application Circuit BOM for Relative Humidity and Temperature Measurement
12
Reference
Description
Mfr Part Number
Manufacturer
R1
Resistor, 10 k, ±5%, 1/16 W, 0603
CR0603-16W-103JT
Venkel
R2
Resistor, 10 k, ±5%, 1/16 W, 0603
CR0603-16W-103JT
Venkel
C1
Capacitor, 0.1 µF, 16 V, X7R, 0603
C0603X7R160-104M
Venkel
U1
IC, Digital Temperature/humidity Sensor
Si7021-A20-GM
Silicon Labs
Rev. 1.1
Si7021-A20
4. Functional Description
Vdd
Si7021
Humidity
Sensor
1.25V
Ref
ADC
Calibration
Memory
Control Logic
Temp
Sensor
I2C Interface
SDA
SCL
GND
Figure 5. Si7021 Block Diagram
The Si7021 is a digital relative humidity and temperature sensor that integrates temperature and humidity sensor
elements, an analog-to-digital converter, signal processing, calibration, polynomial non-linearity correction, and an
I2C interface all in a single chip. The Si7021 is individually factory-calibrated for both temperature and humidity,
with the calibration data stored in on-chip non-volatile memory. This ensures that the sensor is fully
interchangeable, with no recalibration or changes to software required. Patented use of industry-standard CMOS
and low-K dielectrics as a sensor enables the Si7021 to achieve excellent long term stability and immunity to
contaminants with low drift and hysteresis. The Si7021 offers a low-power, high-accuracy, calibrated and stable
solution ideal for a wide range of temperature, humidity, and dew-point applications including medical and
instrumentation, high-reliability automotive and industrial systems, and cost-sensitive consumer electronics.
While the Si7021 is largely a conventional mixed-signal CMOS integrated circuit, relative humidity sensors in
general and those based on capacitive sensing using polymeric dielectrics have unique application and use
requirements that are not common to conventional (non-sensor) ICs. Chief among those are:
The
need to protect the sensor during board assembly, i.e., solder reflow, and the need to subsequently
rehydrate the sensor.
The need to protect the senor from damage or contamination during the product life-cycle.
The impact of prolonged exposure to extremes of temperature and/or humidity and their potential effect on
sensor accuracy.
The effects of humidity sensor “memory”.
Each of these items is discussed in more detail in the following sections.
Rev. 1.1
13
Si7021-A20
4.1. Relative Humidity Sensor Accuracy
To determine the accuracy of a relative humidity sensor, it is placed in a temperature and humidity controlled
chamber. The temperature is set to a convenient fixed value (typically 25–30 °C) and the relative humidity is swept
from 20 to 80% and back to 20% in the following steps: 20% – 40% – 60% – 80% – 80% – 60% – 40% – 20%. At
each set-point, the chamber is allowed to settle for a period of 60 minutes before a reading is taken from the
sensor. Prior to the sweep, the device is allowed to stabilize to 50%RH. The solid trace in Figure 6, “Measuring
Sensor Accuracy Including Hysteresis,” shows the result of a typical sweep.
Figure 6. Measuring Sensor Accuracy Including Hysteresis
The RH accuracy is defined as the dotted line shown in Figure 6, which is the average of the two data points at
each relative humidity set-point. In this case, the sensor shows an accuracy of 0.25%RH. The Si7021 accuracy
specification (Table 4) includes:
Unit-to-unit
and lot-to-lot variation
of factory calibration
Margin for shifts that can occur during solder reflow
The accuracy specification does not include:
Accuracy
Hysteresis
(typically ±1%)
from long term exposure to very humid conditions
Contamination of the sensor by particulates, chemicals, etc.
Other aging related shifts ("Long-term stability")
Variations due to temperature (see Drift vs. Temperature in Table 4). RH readings will typically vary with
temperature by less than  0.05%  C.
Effects
14
Rev. 1.1
Si7021-A20
4.2. Hysteresis
The moisture absorbent film (polymeric dielectric) of the humidity sensor will carry a memory of its exposure
history, particularly its recent or extreme exposure history. A sensor exposed to relatively low humidity will carry a
negative offset relative to the factory calibration, and a sensor exposed to relatively high humidity will carry a
positive offset relative to the factory calibration. This factor causes a hysteresis effect illustrated by the solid trace
in Figure 6. The hysteresis value is the difference in %RH between the maximum absolute error on the decreasing
humidity ramp and the maximum absolute error on the increasing humidity ramp at a single relative humidity
setpoint and is expressed as a bipolar quantity relative to the average error (dashed trace). In the example of
Figure 6, the measurement uncertainty due to the hysteresis effect is ±1.0%RH.
4.3. Prolonged Exposure to High Humidity
Prolonged exposure to high humidity will result in a gradual upward drift of the RH reading. The shift in sensor
reading resulting from this drift will generally disappear slowly under normal ambient conditions. The amount of
shift is proportional to the magnitude of relative humidity and the length of exposure. In the case of lengthy
exposure to high humidity, some of the resulting shift may persist indefinitely under typical conditions. It is generally
possible to substantially reverse this affect by baking the device (see Section “4.6. Bake/Hydrate Procedure” ).
4.4. PCB Assembly
4.4.1. Soldering
Like most ICs, Si7021 devices are shipped from the factory vacuum-packed with an enclosed desiccant to avoid
any RH accuracy drift during storage and to prevent any moisture-related issues during solder reflow. The following
guidelines should be observed during PCB assembly:
Si7021
devices are compatible with standard board assembly processes. Devices should be soldered
using reflow per the recommended card reflow profile. See Section “10. PCB Land Pattern and Solder
Mask Design” for the recommended card reflow profile.
A "no clean" solder process is recommended to minimize the need for water or solvent rinses after
soldering. Cleaning after soldering is possible, but must be done carefully to avoid impacting the
performance of the sensor. See “AN607: Si70xx Humidity Sensor Designer’s Guide” for more information
on cleaning.
It is essential that the exposed polymer sensing film be kept clean and undamaged. This can be
accomplished by careful handling and a clean, well-controlled assembly process. When in doubt or for
extra protection, a heat-resistant, protective cover such as Kapton™ KPPD-1/8 polyimide tape can be
installed during PCB assembly.
Si7021s may be ordered with a factory-fitted, solder-resistant protective cover. This cover provides protection
during PCB assembly or rework but without the time and effort required to install and remove the Kapton tape. It
can be left in place for the lifetime of the product, preventing liquids, dust or other contaminants from coming into
contact with the polymer sensor film. See Section “8. Ordering Guide” for a list of ordering part numbers that
include the cover.
Rev. 1.1
15
Si7021-A20
4.4.2. Rehydration
The measured humidity value will generally shift slightly after solder reflow. A portion of this shift is permanent and
is accounted for in the accuracy specifications in Table 4. After soldering, an Si7021 should be allowed to
equilibrate under controlled RH conditions (room temperature, 45–55%RH) for at least 48 hours to eliminate the
remainder of the shift and return the device to its specified accuracy performance.
4.4.3. Rework
To maintain the specified sensor performance, care must be taken during rework to minimize the exposure of the
device to excessive heat and to avoid damage/contamination or a shift in the sensor reading due to liquids, solder
flux, etc. Manual touch-up using a soldering iron is permissible under the following guidelines:
The
exposed polymer sensing film must be kept clean and undamaged. A protective cover is
recommended during any rework operation (Kapton® tape or the factory installed cover).
Flux must not be allowed to contaminate the sensor; liquid flux is not recommended even with a cover in
place. Conventional lead-free solder with rosin core is acceptable for touch-up as long as a cover is in
place during the rework.
If possible, avoid water or solvent rinses after touch-up. Cleaning after soldering is possible, but must be
done carefully to avoid impacting the performance of the sensor. See AN607 for more information on
cleaning.
Minimize the heating of the device. Soldering iron temperatures should not exceed 350 °C and the contact
time per pin should not exceed five seconds.
Hot air rework is not recommended. If a device must be replaced, remove the device by hot air and solder
a new part in its place by reflow following the guidelines above.
*Note: All trademarks are the property of their respective owners.
Figure 7. Si7021 with Factory-Installed Protective Cover
16
Rev. 1.1
Si7021-A20
4.5. Protecting the Sensor
Because the sensor operates on the principal of measuring a change in capacitance, any changes to the dielectric
constant of the polymer film will be detected as a change in relative humidity. Therefore, it is important to minimize
the probability of contaminants coming into contact with the sensor. Dust and other particles as well as liquids can
affect the RH reading. It is recommended that a cover is employed in the end system that blocks contaminants but
allows water vapor to pass through. Depending on the needs of the application, this can be as simple as plastic or
metallic gauze for basic protection against particulates or something more sophisticated such as a hydrophobic
membrane providing up to IP67 compliant protection.
The Si7021 may be ordered with a factory-fitted, solder-resistant cover that can be left in place for the lifetime of
the product. It is very low-profile, hydrophobic and oleophobic. See Section “8. Ordering Guide” for a list of ordering
part numbers that include the cover. A dimensioned drawing of the IC with the cover is included in Section “9.
Package Outline” . Other characteristics of the cover are listed in Table 9.
Table 9. Specifications of Protective Cover
Parameter
Value
Material
PTFE
Operating Temperature
–40 to 125 °C
Maximum Reflow Temperature
IP Rating (per IEC 529)
260 °C
IP67
4.6. Bake/Hydrate Procedure
After exposure to extremes of temperature and/or humidity for prolonged periods, the polymer sensor film can
become either very dry or very wet, in each case the result is either high or low relative humidity readings. Under
normal operating conditions, the induced error will diminish over time. From a very dry condition, such as after
shipment and soldering, the error will diminish over a few days at typical controlled ambient conditions, e.g.,
48 hours of 45 ≤ %RH ≤ 55. However, from a very wet condition, recovery may take significantly longer. To
accelerate recovery from a wet condition, a bake and hydrate cycle can be implemented. This operation consists of
the following steps:
the sensor at 125 °C for ≥ 12 hours
at 30 °C in 75% RH for ≥ 10 hours
Following this cycle, the sensor will return to normal operation in typical ambient conditions after a few days.
Baking
Hydration
4.7. Long Term Drift/Aging
Over long periods of time, the sensor readings may drift due to aging of the device. Standard accelerated life
testing of the Si7021 has resulted in the specifications for long-term drift shown in Table 4 and Table 5. This
contribution to the overall sensor accuracy accounts only for the long-term aging of the device in an otherwise
benign operating environment and does not include the effects of damage, contamination, or exposure to extreme
environmental conditions.
Rev. 1.1
17
Si7021-A20
5. I2C Interface
The Si7021 communicates with the host controller over a digital I2C interface. The 7-bit base slave address is
0x40.
Table 10. I2C Slave Address Byte
A6
A5
A4
A3
A2
A1
A0
R/W
1
0
0
0
0
0
0
0
Master I2C devices communicate with the Si7021 using a command structure. The commands are listed in the I2C
command table. Commands other than those documented below are undefined and should not be sent to the
device.
Table 11. I2C Command Table
Command Description
Command Code
Measure Relative Humidity, Hold Master Mode
0xE5
Measure Relative Humidity, No Hold Master Mode
0xF5
Measure Temperature, Hold Master Mode
0xE3
Measure Temperature, No Hold Master Mode
0xF3
Read Temperature Value from Previous RH Measurement
0xE0
Reset
0xFE
Write RH/T User Register 1
0xE6
Read RH/T User Register 1
0xE7
Write Heater Control Register
0x51
Read Heater Control Register
0x11
Read Electronic ID 1st Byte
0xFA 0x0F
Read Electronic ID 2nd Byte
0xFC 0xC9
Read Firmware Revision
0x84 0xB8
18
Rev. 1.1
Si7021-A20
5.1. Issuing a Measurement Command
The measurement commands instruct the Si7021 to perform one of two possible measurements; Relative Humidity
or Temperature. The procedure to issue any one of these commands is identical. While the measurement is in
progress, the option of either clock stretching (Hold Master Mode) or Not Acknowledging read requests (No Hold
Master Mode) is available to indicate to the master that the measurement is in progress; the chosen command
code determines which mode is used.
Optionally, a checksum byte can be returned from the slave for use in checking for transmission errors. The
checksum byte will follow the least significant measurement byte if it is acknowledged by the master. The
checksum byte is not returned if the master “not acknowledges” the least significant measurement byte. The
checksum byte is calculated using a CRC generator polynomial of x8 + x5 + x4 + 1, with an initialization of 0x00.
The checksum byte is optional after initiating an RH or temperature measurement with commands 0xE5, 0xF5,
0xE3, and 0xF3. The checksum byte is required for reading the electronic ID with commands 0xFA 0x0F and 0xFC
0xC9. For all other commands, the checksum byte is not supported.
Table 12. I2C Bit Descriptions
Name
Symbol
Description
START
S
SDA goes low while SCL high.
STOP
P
SDA goes high while SCL high.
Repeated START
Sr
SDA goes low while SCL high. It is allowable to generate a STOP before the
repeated start. SDA can transition to high before or after SCL goes high in
preparation for generating the START.
READ
R
Read bit = 1
WRITE
W
Write bit = 0
All other bits
—
SDA value must remain high or low during the entire time SCL is high (this is
the set up and hold time in Figure 1).
In the I2C sequence diagrams in the following sections, bits produced by the master and slave are color coded as
shown:
Master
Slave
Rev. 1.1
19
Si7021-A20
*Note: Device will NACK the slave address byte until conversion is complete.
20
Rev. 1.1
Si7021-A20
5.1.1. Measuring Relative Humidity
Once a relative humidity measurement has been made, the results of the measurement may be converted to
percent relative humidity by using the following expression:
 RH_Code
%RH = 125
--------------------------------------–6
65536
Where:
%RH is the measured relative humidity value in %RH
RH_Code is the 16-bit word returned by the Si7021
A humidity measurement will always return XXXXXX10 in the LSB field.
Due to normal variations in RH accuracy of the device as described in Table 4, it is possible for the measured value
of %RH to be slightly less than 0 when the actual RH level is close to or equal to 0. Similarly, the measured value
of %RH may be slightly greater than 100 when the actual RH level is close to or equal to 100. This is expected
behavior, and it is acceptable to limit the range of RH results to 0 to 100%RH in the host software by truncating
values that are slightly outside of this range.
5.1.2. Measuring Temperature
Each time a relative humidity measurement is made a temperature measurement is also made for the purposes of
temperature compensation of the relative humidity measurement. If the temperature value is required, it can be
read using command 0xE0; this avoids having to perform a second temperature measurement. The measure
temperature commands 0xE3 and 0xF3 will perform a temperature measurement and return the measurement
value, command 0xE0 does not perform a measurement but returns the temperature value measured during the
relative humidity measurement.
The checksum output is not available with the 0xE0 command.
SequencetoreadtemperaturevaluefrompreviousRHmeasurement
S
Slave
Address
W
A
A
LSByte
NA
P
0xE0
A
Sr
Slave
Address
R
A
MSByte
Rev. 1.1
21
Si7021-A20
The results of the temperature measurement may be converted to temperature in degrees Celsius (°C) using the
following expression:
175.72 Temp_Code- – 46.85
Temperature (C  = ------------------------------------------------------65536
Where:
Temperature (°C) is the measured temperature value in °C
Temp_Code is the 16-bit word returned by the Si7021
A temperature measurement will always return XXXXXX00 in the LSB field.
22
Rev. 1.1
Si7021-A20
5.2. Reading and Writing User Registers
There is one user register on the Si7021 that allows the user to set the configuration of the Si7021. The procedure
for accessing that register is described below.
The checksum byte is not supported after reading a user register.
Sequence to read a register
Slave S
Address
W
A
Read Reg Cmd
A
Slave Sr
Address
R
A
Read Data
NA
P
Sequence to write a register
S
Slave Address
W
A
Write Reg Cmd
A
Write Data
A
P
5.3. Electronic Serial Number
The Si7021 provides a serial number individualized for each device that can be read via the I2C serial interface.
Two I2C commands are required to access the device memory and retrieve the complete serial number. The
command sequence, and format of the serial number response is described in the figure below:
Master
Slave
First access:
S
Slave Address
W
ACK
0xFA
ACK
0X0F
ACK
S
Slave Address
R
ACK
SNA_3
ACK
CRC
ACK
SNA_2
ACK
CRC
ACK
SNA_1
ACK
CRC
ACK
SNA_0
ACK
CRC
NACK
S
Slave Address
W
ACK
0xFC
ACK
0XC9
ACK
S
Slave Address
R
ACK
SNB_3
ACK
SNB_2
ACK
CRC
ACK
SNB_1
ACK
SNB_0
ACK
CRC
NACK
P
2nd access:
Rev. 1.1
P
23
Si7021-A20
The format of the complete serial number is 64-bits in length, divided into 8 data bytes. The complete serial number
sequence is shown below:
SNA_3
SNA_2
SNA_1
SNA_0
SNB_3
SNB_2
SNB_1
SNB_0
The SNB3 field contains the device identification to distinguish between the different Silicon Labs relative humidity
and temperature devices. The value of this field maps to the following devices according to this table:
0x00 or 0xFF engineering samples
0x0D=13=Si7013
0x14=20=Si7020
0x15=21=Si7021
5.4. Firmware Revision
The internal firmware revision can be read with the following I2C transaction:
S
Slave Address
W
A
0x84
R
A
A
FWREV
The values in this field are encoded as follows:
0xFF = Firmware version 1.0
0x20 = Firmware version 2.0
24
Rev. 1.1
0xB8
A
NA
A
P
S
Slave Address
Si7021-A20
5.5. Heater
The Si7021 contains an integrated resistive heating element that may be used to raise the temperature of the
sensor. This element can be used to test the sensor, to drive off condensation, or to implement dew-point
measurement when the Si7021 is used in conjunction with a separate temperature sensor such as another Si7021
(the heater will raise the temperature of the internal temperature sensor).
The heater can be activated using HTRE, bit 2 in User Register 1. Turning on the heater will reduce the tendency
of the humidity sensor to accumulate an offset due to "memory" of sustained high humidity conditions. Several
different power levels are available. The various settings are adjusted using the Heater Control Register and are
described in the following table.
Table 13. Heater Control Settings
HEATER[3:0] Typical Current
Draw* (mA)
0000
3.09
0001
9.18
0010
15.24
...
...
0100
27.39
...
...
1000
51.69
...
...
1111
94.20
*Note: Assumes VDD = 3.3 V.
Rev. 1.1
25
Si7021-A20
6. Control Registers
Table 14. Register Summary
Register
Bit 7
Bit 6
Bit 5
Bit 4
Bit 3
Bit 2
Bit 1
Bit 0
User Register 1
RES1
VDDS
RSVD
RSVD
RSVD
HTRE
RSVD
RES0
Heater Control
Register
RSVD
HEATER[3:0]
Notes:
1. Any register not listed here is reserved and must not be written. The result of a read operation on these bits is
undefined.
2. Except where noted, reserved register bits will always read back as “1,” and are not affected by write operations. For
future compatibiliy, it is recommended that prior to a write operation, registers should be read. Then the values read
from the RSVD bits should be written back unchanged during the write operation.
6.1. Register Descriptions
Register 1. User Register 1
Bit
D7
D6
D5
D4
D3
D2
D1
D0
Name
RES1
VDDS
RSVD
RSVD
RSVD
HTRE
RSVD
RES0
Type
R/W
R
R/W
R/W
R/W
R/W
R/W
Reset Settings = 0011_1010
Bit
Name
D7; D0
RES[1:0]
D6
VDDS
Function
Measurement Resolution:
RH
00:
12 bit
01:
8 bit
10:
10 bit
11:
11 bit
VDD Status:
0:
1:
Temp
14 bit
12 bit
13 bit
11 bit
VDD OK
VDD Low
The minimum recommended operating voltage is 1.9 V. A transition of the VDD status bit from 0 to 1 indicates that VDD is
between 1.8 V and 1.9 V. If the VDD drops below 1.8 V, the
device will no longer operate correctly.
26
D5, D4, D3
RSVD
Reserved
D2
HTRE
1 = On-chip Heater Enable
0 = On-chip Heater Disable
D1
RSVD
Reserved
Rev. 1.1
Si7021-A20
Register 2. Heater Control Register
Bit
D7
D6
D5
D4
D3
D2
D1
Name
RSVD
Heater [3:0]
Type
R/W
R/W
D0
Reset Settings = 0000_0000
Bit
D3:D0
Name
HEATER[3:0]
Function
Heater Current
D3
D2
D1
D0
0
0
0
0
3.09 mA
0
0
0
1
9.18 mA
0
0
1
0
15.24 mA
0
27.39 mA
0
51.69 mA
1
94.20 mA
...
0
1
0
...
1
0
0
...
1
D7,D6,
D5,D4
RSVD
1
1
Reserved
Rev. 1.1
27
Si7021-A20
7. Pin Descriptions: Si7021 (Top View)
SDA
1
6
SCL
GND
2
5
VDD
DNC
3
4
DNC
Pin Name
Pin #
SDA
1
I2C data
GND
2
Ground. This pin is connected to ground on the circuit board through a trace. Do not
connect directly to GND plane.
VDD
5
Power. This pin is connected to power on the circuit board.
SCL
6
I2C clock
DNC
3,4
TGND
Paddle
28
Pin Description
These pins should be soldered to pads on the PCB for mechanical stability; they can be
electrically floating or tied to VDD (do not tie to GND).
This pad is connected to GND internally. This pad is the main thermal input to the onchip temperature sensor. The paddle should be soldered to a floating pad.
Rev. 1.1
Si7021-A20
8. Ordering Guide
Table 15. Device Ordering Guide
P/N
Description
Max. Accuracy
Pkg
Operating
Range (°C)
Protective
Cover
Packing
Format
Temp
RH
Digital temperature/ humidity sensor
±0.4 °C
± 3%
DFN 6
–40 to +85 °C
N
Tube
Si7021-A20-GMR Digital temperature/ humidity sensor
±0.4 °C
± 3%
DFN 6
–40 to +85 °C
N
Tape &
Reel
Si7021-A20-GM1 Digital temperature/ humidity sensor
±0.4 °C
± 3%
DFN 6
–40 to +85 °C
Y
Cut Tape
Digital temperature/ humidity sensor
±0.4 °C
± 3%
DFN 6
–40 to +85 °C
Y
Tape &
Reel
Si7021-A20-IM
Digital temperature/ humidity sensor –
industrial temp range
±0.4 °C
± 3%
DFN 6
–40 to +125 °C
N
Tube
Si7021-A20-IMR
Digital temperature/ humidity sensor –
industrial temp range
±0.4 °C
± 3%
DFN 6
–40 to +125 °C
N
Tape &
Reel
Si7021-A20-IM1
Digital temperature/ humidity sensor –
industrial temp range
±0.4 °C
± 3%
DFN 6
–40 to +125 °C
Y
Cut Tape
Si7021-A20-IM1R Digital temperature/ humidity sensor –
industrial temp range
±0.4 °C
± 3%
DFN 6
–40 to +125 °C
Y
Tape &
Reel
Si7021-A20-YM0
Digital temperature/ humidity sensor –
automotive
±0.4 °C
± 3%
DFN 6
–40 to +125 °C
N
Tube
Si7021-A20-YM0R Digital temperature/ humidity sensor –
automotive
±0.4 °C
± 3%
DFN 6
–40 to +125 °C
N
Tape &
Reel
Digital temperature/ humidity sensor –
automotive
±0.4 °C
± 3%
DFN 6
–40 to +125 °C
Y
Cut Tape
Si7021-A20-YM1R Digital temperature/ humidity sensor –
automotive
±0.4 °C
± 3%
DFN 6
–40 to +125 °C
Y
Tape &
Reel
Si7021-A20-GM
Si7021-A20GM1R
Si7021-A20-YM1
Note: The “A” denotes product revision A and “20” denotes firmware version 2.0.
Rev. 1.1
29
Si7021-A20
9. Package Outline
9.1. Package Outline: 3x3 6-pin DFN
Figure 10. 3x3 6-pin DFN
Table 16. 3x3 6-pin DFN Package Diagram Dimensions
Dimension
Min
Nom
Max
A
0.70
0.75
0.80
A1
0.00
0.02
0.05
b
0.35
0.40
0.45
D
D2
3.00 BSC.
1.40
1.50
e
1.00 BSC.
E
3.00 BSC.
1.60
E2
2.30
2.40
2.50
H1
0.85
0.90
0.95
H2
1.39
1.44
1.49
L
0.35
0.40
0.45
aaa
0.10
bbb
0.10
ccc
0.05
ddd
0.10
eee
0.05
fff
0.05
Notes:
1. All dimensions shown are in millimeters (mm).
2. Dimensioning and Tolerancing per ANSI Y14.5M-1994.
30
Rev. 1.1
Si7021-A20
9.2. Package Outline: 3x3 6-pin DFN with Protective Cover
Figure 8 illustrates the package details for the Si7021 with the optional protective cover. The table below lists the
values for the dimensions shown in the illustration.
Figure 8. 3x3 6-pin DFN with Protective Cover
Table 17. 3x3 6-pin DFN with Protective Cover Package Diagram Dimensions
Dimension
A
A1
A2
b
D
D2
e
E
E2
F1
F2
h
L
R1
aaa
bbb
ccc
ddd
eee
Min
—
0.00
0.70
0.35
1.40
2.30
2.70
2.70
0.76
0.35
0.45
Nom
—
0.02
0.75
0.40
3.00 BSC.
1.50
1.00 BSC.
3.00 BSC.
2.40
2.80
2.80
0.83
0.40
0.50
0.10
0.10
0.05
0.10
0.05
Max
1.21
0.05
0.80
0.45
1.60
2.50
2.90
2.90
0.90
0.45
0.55
Notes:
1. All dimensions are shown in millimeters (mm).
2. Dimensioning and Tolerancing per ANSI Y14.5M-1994.
Rev. 1.1
31
Si7021-A20
10. PCB Land Pattern and Solder Mask Design
Figure 9. Si7021 PCB Land Pattern
Table 18. PCB Land Pattern Dimensions
Symbol
mm
C1
2.90
E
1.00
P1
1.60
P2
2.50
X1
0.45
Y1
0.85
Notes:
General
1. All dimensions shown are at Maximum Material Condition (MMC). Least Material
Condition (LMC) is calculated based on a Fabrication Allowance of 0.05 mm.
2. This Land Pattern Design is based on the IPC-7351 guidelines.
Solder Mask Design
3. All metal pads are to be non-solder mask defined (NSMD). Clearance between the
solder mask and the metal pad is to be 60 µm minimum, all the way around the
pad.
Stencil Design
4. A stainless steel, laser-cut and electro-polished stencil with trapezoidal walls
should be used to assure good solder paste release.
5. The stencil thickness should be 0.125 mm (5 mils).
6. The ratio of stencil aperture to land pad size should be 1:1 for all perimeter pins.
7. A 2x1 array of 1.00 mm square openings on 1.30 mm pitch should be used for the
center ground pad to achieve a target solder coverage of 50%.
Card Assembly
8. A No-Clean, Type-3 solder paste is recommended.
9. The recommended card reflow profile is per the JEDEC/IPC J-STD-020
specification for Small Body Components.
32
Rev. 1.1
Si7021-A20
11. Top Marking
11.1. Si7021 Top Marking
11.2. Top Marking Explanation
Mark Method:
Laser
Font Size
0.30 mm
Pin 1 Indicator:
Circle = 0.30 mm Diameter
Upper-Left Corner
Line 1 Marking:
TTTT = Mfg Code
Rev. 1.1
33
Si7021-A20
12. Additional Reference Resources
AN607:
34
Si70xx Humidity Sensor Designer’s Guide
Rev. 1.1
Si7021-A20
DOCUMENT CHANGE LIST
Revision 0.9 to Revision 0.91

Updated Table 2 on page 4.
Revision 0.91 to Revision 1.0

Updated document revision to 1.0.
Revision 1.0 to Revision 1.1

Updated Footnote 2 in Table 3
Updated Section “4.5. Protecting the Sensor”
 Updated Table 9

Corrected a typo in the I2C sequence for no-hold
mode in Section “5. I2C Interface”
 Corrected a typo in Table 12
 Updated Table 17 dimensions F1 and F2

Rev. 1.1
35
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"Typical" parameters provided can and do vary in different applications. Application examples described herein are for illustrative purposes only. Silicon Laboratories reserves the right to
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