Si7021-A20-IM1 - Silicon Labs

Si7021-A20

I2C HUMIDITY AND TEMPERATURE SENSOR

Features

Applications

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.

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.

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

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

Patent Protected. Patents pending

Ordering Information:

See page 29.

Pin Assignments

DNC

SCL

VDD

5GND

SDA

3 4

DNC

Top View

Si7021-A20

2 Rev. 1.2

Functional Block Diagram

ADC

GND

Humidity

Sensor Control Logic

SCL

Si7021

Temp

Sensor

1.25V

Ref

I2C Interface SDA

Vdd

Calibration

Memory

Si7021-A20

Rev. 1.2 3

TABLE OF CONTENTS

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

5. I2C 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

Si7021-A20

4 Rev. 1.2

1. Electrical Specifications

Unless otherwise specified, all min/max specifications apply over the recommended operating conditions.

Table 1. Recommended Operating Conditions

Parameter Symbol Test Condition Min Typ Max Unit

Power Supply VDD 1.9 — 3.6 V

Operating Temperature TAI and Y grade –40 — +125 °C

Operating Temperature TAG 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

Output Voltage Low VOL SDA pin; IOL =2.5mA; VDD = 3.3 V — — 0.6 V

SDA pin; IOL =1.2mA;

VDD =1.9V

——0.4V

Current

Consumption

IDD RH conversion in progress — 150 180 μA

Temperature conversion in progress — 90 120 μA

Standby, –40 to +85 °C2—0.060.62μA

Standby, –40 to +125 °C2—0.063.8μA

Peak IDD during powerup3—3.54.0mA

Peak IDD during I2C operations4—3.54.0mA

Heater Current5IHEAT —3.1 to 94.2 —mA

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.

Si7021-A20

Rev. 1.2 5

Conversion Time1tCONV 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

Powerup Time tPU From VDD ≥1.9 V to ready for a

conversion, 25 °C

—1825

From VDD ≥1.9 V to ready for a

conversion, full temperature range

——80

After issuing a software reset

command

—515

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-to-

high transition

0.05 x VDD ——V

SCLK Frequency2fSCL — — 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.

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 Symbol Test Condition Min Typ Max Unit

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.

Si7021-A20

6 Rev. 1.2

Figure 1. I2C Interface Timing Dia gram

Stop Setup Time tSPS 0.6 — — µs

Bus Free Time tBUF Between Stop and Start 1.3 — — µs

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

Suppressed Pulse Width3tSPS 50 — — ns

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

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.

SCL

1/fSCL tSKH

SDA

tSKL

tSTH

D5 D4 D0 R/W ACK

tDS tDH

Start Bit Stop Bit

tBUF

tSTS tVD :

ACK

tSPS

tSP

Si7021-A20

Rev. 1.2 7

Table 4. Humidity Sensor

1.9 ≤ VDD ≤ 3.6 V; TA = 30 °C; default conversion time unless otherwise noted.

Parameter Symbol Test Condition Min Typ Max Unit

Operating Range1Non-condensing 0 — 100 %RH

Accuracy2, 3 0 – 80% RH — ±2 ±3 %RH

80 – 100% RH See Figure 2.

Repeatability/Noise 12-bit resolution — 0.025 —

%RH RMS

11-bit resolution — 0.05 —

10-bit resolution — 0.1 —

8-bit resolution — 0.2 —

Response Time4τ63%

1 m/s airflow, with cover — 18 — S

1 m/s airflow, without cover — 17 —

Drift vs. Temperature — 0.05 — %RH/°C

Hysteresis — ±1 — %RH

Long Term Stability3—<0.25 — %RH/yr

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.

Si7021-A20

8 Rev. 1.2

Figure 2. RH Accuracy at 30 °C

Si7021-A20

Rev. 1.2 9

Figure 3. Temperature Accuracy*

*Note: Applies only to I and Y grade devices beyond +85 °C.

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 Test Condition Min Typ Max Unit

Operating Range I and Y Grade –40 — +125 °C

G Grade –40 — +85 °C

Accuracy1–10 °C< tA< 85 °C — ±0.3 ±0.4 °C

–40 < tA< 125°C Figure3

Repeatability/Noise 14-bit resolution — 0.01 —

°C RMS

13-bit resolution — 0.02 —

12-bit resolution — 0.04 —

11-bit resolution — 0.08 —

Response Time2τ63% Unmounted device — 0.7 — s

Si7021-EB board — 5.1 —

Long Term Stability —  0.01 — °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.

Si7021-A20

10 Rev. 1.2

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 Symbol Test Condition 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

ESD Tolerance HBM — — 2 kV

CDM — — 1.25 kV

MM — — 250 V

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”.

Si7021-A20

Rev. 1.2 11

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.

Figure 4. Typical Application Circuit for Relative Humidity and Temperature Measurement

0.1µF

VDD

SCL

SDA

Si7021

SCL

SDA

1.9 to 3.6V

GND

10k

Si7021-A20

12 Rev. 1.2

3. Bill of Materials

Table 8. Typical Application Circuit BOM for Relative Humidity and Temperature Measurement

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

Si7021-A20

Rev. 1.2 13

4. Functional Description

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.

ADC

GND

Humidity

Sensor Control Logic

SCL

Si7021

Temp

Sensor

1.25V

Ref

I2C Interface SDA

Vdd

Calibration

Memory

Si7021-A20

14 Rev. 1.2

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

Accuracy of factory calibration

Margin for shifts that can occur during solder reflow

The accuracy specification does not include:

Hysteresis (typically ±1%)

Effects 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.

Si7021-A20

Rev. 1.2 15

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.

Si7021-A20

16 Rev. 1.2

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

Si7021-A20

Rev. 1.2 17

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.

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:

Baking the sensor at 125 °C for ≥ 12 hours

Hydration 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.

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.

Table 9. Specifications of Protective Cover

Parameter Value

Material PTFE

Operating Temperature –40 to 125 °C

Maximum Reflow Temperature 260 °C

IP Rating (per IEC 529) IP67

Si7021-A20

18 Rev. 1.2

5. I2C Interface

The Si7021 communicates with the host controller over a digital I2C interface. The 7-bit base slave address is

0x40.

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 10. I2C Slave Address Byte

A6 A5 A4 A3 A2 A1 A0 R/W

10000000

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

Si7021-A20

Rev. 1.2 19

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+x

5+x

4+ 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.

In the I2C sequence diagrams in the following sections, bits produced by the master and slave are color coded as

shown:

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).

Master Slave

Si7021-A20

20 Rev. 1.2

*Note: Device will NACK the slave address byte until conversion is complete.

Si7021-A20

Rev. 1.2 21

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:

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.

%RH 125RH_Code

65536

--------------------------------------- 6–=

SequencetoreadtemperaturevaluefrompreviousRHmeasurement

SSlave

AddressWA0xE0ASrSlave

AddressRAMSByte



ALSByteNAP

Si7021-A20

22 Rev. 1.2

The results of the temperature measurement may be converted to temperature in degrees Celsius (°C) using the

following expression:

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.

Temperature (C175.72Temp_Code

65536

--------------------------------------------------------46.85–=

Si7021-A20

Rev. 1.2 23

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.

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:

First access:

Sequencetoreadaregister

SSlave

Address WAReadReg

Cmd ASr

Slave

Address RAReadData NA P

Sequencetowritearegister

SSlaveAddress W AWriteRegCmd AWriteData AP

Master Slave

SSlaveAddress W ACK 0xFA ACK 0X0F ACK

SSlaveAddress R ACK

SNA_3 ACK CRC ACK SNA_2 ACK CRC ACK

SNA_1 ACK CRC ACK SNA_0 ACK CRC NACK P

2nd access:

SSlaveAddress W ACK 0xFC ACK 0XC9 ACK

SSlaveAddress R ACK

SNB_3 ACK SNB_2 ACK CRC ACK

SNB_1 ACK SNB_0 ACK CRC NACK P

Si7021-A20

24 Rev. 1.2

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:

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:

The values in this field are encoded as follows:

0xFF = Firmware version 1.0

0x20 = Firmware version 2.0

SNA_3 SNA_2 SNA_1 SNA_0 SNB_3 SNB_2 SNB_1 SNB_0

SSlave

Address WA0x84 A0xB8A S Slave

Address

RAFWREV NA P

Si7021-A20

Rev. 1.2 25

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 =3.3V.

Si7021-A20

26 Rev. 1.2

6. Control Registers

6.1. Register Descriptions

Reset Settings = 0011_1010

Table 14. Register Summary

User Register 1 RES1 VDDS RSVD RSVD RSVD HTRE RSVD RES0

Heater Control

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.

Bit D7 D6 D5 D4D3D2 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

Bit Name Function

D7; D0 RES[1:0] Measurement Resolution:

RH Temp

00: 12 bit 14 bit

01: 8 bit 12 bit

10: 10 bit 13 bit

11: 11 bit 11 bit

D6 VDDS VDD Status:

0: VDD OK

1: VDD Low

The minimum recommended operating voltage is 1.9 V. A transi-

tion 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.

D5, D4, D3 RSVD Reserved

D2 HTRE 1 = On-chip Heater Enable

0 = On-chip Heater Disable

D1 RSVD Reserved

Si7021-A20

Rev. 1.2 27

Reset Settings = 0000_0000

BitD7D6D5D4D3D2D1D0

Name RSVD Heater [3:0]

Type R/W R/W

Bit Name Function

D3:D0 HEATER[3:0] D3 D2 D1 D0 Heater Current

0000

3.09 mA

0001

9.18 mA

0010

15.24 mA

...

0100

27.39 mA

...

1000

51.69 mA

...

1 1 1 1 94.20 mA

D7,D6,

D5,D4

RSVD Reserved

Si7021-A20

28 Rev. 1.2

7. Pin Descriptions: Si7021 (Top View)

Pin Name Pin # Pin Description

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 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).

TGND Paddle This pad is connected to GND internally. This pad is the main thermal input to the on-

chip temperature sensor. The paddle should be soldered to a floating pad.

DNC

SCL

VDD

5GND

SDA

3 4

DNC

Si7021-A20

Rev. 1.2 29

8. Ordering Guide

Table 15. Device Ordering Guide

P/N Description Max. Accuracy Pkg Operating

Range (°C) Protective

Cover Packing

Format

Temp RH

Si7021-A20-GM Digital temperature/ humidity sensor ±0.4 °C ± 3% DFN 6 –40 to +85 °C N Cut Tape

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

Si7021-A20-

GM1R

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 Cut Tape

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 Cut Tape

Si7021-A20-YM0R Digital temperature/ humidity sensor –

automotive

±0.4 °C ± 3% DFN 6 –40 to +125 °C N Tape &

Reel

Si7021-A20-YM1 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

Note: The “A” denotes product revision A and “20” denotes firmware version 2.0.

Si7021-A20

30 Rev. 1.2

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 3.00 BSC.

D2 1.40 1.50 1.60

e 1.00 BSC.

E 3.00 BSC.

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.

Si7021-A20

Rev. 1.2 31

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 Min Nom Max

A——1.21

A1 0.00 0.02 0.05

A2 0.70 0.75 0.80

b 0.35 0.40 0.45

D 3.00 BSC.

D2 1.40 1.50 1.60

e 1.00 BSC.

E 3.00 BSC.

E2 2.30 2.40 2.50

F1 2.70 2.80 2.90

F2 2.70 2.80 2.90

h 0.76 0.83 0.90

L 0.35 0.40 0.45

R1 0.45 0.50 0.55

aaa 0.10

bbb 0.10

ccc 0.05

ddd 0.10

eee 0.05

Notes:

1. All dimensions are shown in millimeters (mm).

2. Dimensioning and Tolerancing per ANSI Y14.5M-1994.

Si7021-A20

32 Rev. 1.2

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

E1.00

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. The recommended card reflow profile is per the JEDEC/IPC J-STD-020

specification for Small Body Components.



Si7021-A20

Rev. 1.2 33

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

Si7021-A20

34 Rev. 1.2

12. Additional Reference Resources

AN607: Si70xx Humidity Sensor Designer’s Guide

Si7021-A20

Rev. 1.2 35

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.

Revision 1.1 to Revision 1.2

Updated diagram in "5.4. Firmware Revision" on

page 24.

Updated notes in Table 18, “PCB Land Pattern

Dimensions,” on page 32.

Changed packing format from tube to cut tape for all

non-tape & reel part numbers without protective filter

covers.

Disclaimer

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using or intending to use the Silicon Laboratories products. Characterization data, available modules and peripherals, memory sizes and memory addresses refer to each specific

device, and "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 make changes without further notice and limitation to product information, specifications, and descriptions herein, and does not give warranties as to the accuracy

or completeness of the included information. Silicon Laboratories shall have no liability for the consequences of use of the information supplied herein. This document does not imply

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