SENSIRION SCD42 User manual

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▪Large supply voltage range: 2.4 –5.5 V
▪Accuracy: ± 75 ppm @ 400 ppm –1’000 ppm
▪Digital I2C interface
▪Integrated temperature and humidity sensor
SCD42
Breaking the size barrier in CO2sensing
Products
Details
SCD42-D-R2
Accuracy: ± 75 ppm @ 400 –1’000 ppm
Full product list on page 20
Functional Block Diagram
Device Overview
Product Summary
The SCD42 is Sensirion’s next generation miniature CO2
sensor. This sensor builds on the photoacoustic sensing
principle and Sensirion’s patented PAsens® and
CMOSens® technology to offer high accuracy at an
unmatched price and smallest form factor. SMD assembly
allows cost- and space-effective integration of the sensor
combined with maximal freedom of design. On-chip signal
compensation is realized with the build-in SHT4x humidity
and temperature sensor.
CO2is a key indicator for indoor air quality as high levels
compromise humans’ cognitive performance and well-
being. The SCD42 enables smart ventilation systems to
regulate ventilation in the most energy-efficient and
human-friendly way. Moreover, indoor air quality monitors
and other connected devices based on the SCD42 can
help maintaining low CO2concentration for a healthy,
productive environment.
Features
▪Photoacoustic sensor technology PASens®
▪Smallest form factor: 10.1 x 10.1 x 6.5 mm3
▪Reflow solderable for cost effective assembly
▪Large output range: 0 ppm –40’000 ppm

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Table of Contents
1Sensor Performance .........................................................................................................................................3
1.1 CO2Sensing Performance ..........................................................................................................................3
1.2 Humidity Sensing Performance ...................................................................................................................3
1.3 Temperature Sensing Performance.............................................................................................................3
2Specifications ....................................................................................................................................................4
2.1 Electrical Specifications...............................................................................................................................4
2.2 Absolute Maximum Ratings.........................................................................................................................4
2.3 Interface Specifications ...............................................................................................................................5
2.4 Timing Specifications...................................................................................................................................6
2.5 Material Contents ........................................................................................................................................6
3Digital Interface Description.............................................................................................................................7
3.1 Power-Up and Communication Start ...........................................................................................................7
3.2 Data type & length.......................................................................................................................................7
3.3 Command Sequence Types........................................................................................................................7
3.4 SCD42 Command Overview .......................................................................................................................8
3.5 Basic Commands ........................................................................................................................................9
3.6 On-Chip Output Signal Compensation ......................................................................................................10
3.7 Field Calibration ........................................................................................................................................13
3.8 Advanced Features ...................................................................................................................................15
3.9 Checksum Calculation...............................................................................................................................17
4Mechanical specifications ..............................................................................................................................18
4.1 Package Outline ........................................................................................................................................18
4.2 Land Pattern..............................................................................................................................................18
4.3 Tape & Reel Package................................................................................................................................19
4.4 Moisture Sensitivity Level..........................................................................................................................19
4.5 Soldering Instructions................................................................................................................................20
4.6 Traceability................................................................................................................................................20
5Ordering Information.......................................................................................................................................20
6Revision History ..............................................................................................................................................20

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1Sensor Performance
1.1 CO2Sensing Performance
Default conditions of 25 °C, 50 % RH, ambient pressure 1013 mbar and 3.3 V supply voltage apply to values in the table
below, unless otherwise stated.
Parameter
Conditions
Value
CO2output range1
-
0 –40’000 ppm
SCD42 CO2measurement accuracy2
400 ppm –1’000 ppm
± 75 ppm
1’001 ppm – 2’000 ppm
± (40 ppm + 5% of reading)
Repeatability
Typical
± 10 ppm
Response time3
τ63%, typical
60 s
Accuracy drift per year with automatic self-
calibration algorithm enabled4
Typical
± (5 ppm + 0.5 % of reading)
Table 1: SCD42 CO2sensor specifications
1.2 Humidity Sensing Performance
5
Parameter
Conditions
Value
Humidity measurement range
-
0 %RH –100 %RH
Accuracy (typ.)
15 °C –35 °C, 20 %RH –65 %RH
± 6 % RH
-10 °C –60 °C, 0 %RH –100 %RH
± 9 % RH
Repeatability
Typical
± 0.4 %RH
Response time3
τ63%, typical
90 s
Accuracy drift
-
< 0.25 %RH / year
Table 2: SCD42 humidity sensor specifications
1.3 Temperature Sensing Performance5
Parameter
Conditions
Value
Temperature measurement range
-
- 10°C –60°C
Accuracy (typ.)
15 °C –35 °C
± 0.8 °C
-10 °C –60 °C
± 1.5 °C
Repeatability
-
± 0.1°C
Response time3
τ63%, typical
120 s
Accuracy drift
-
< 0.03 °C / year
Table 3: SCD42 temperature sensor specifications
1
Exposure to CO2concentrations smaller than 400 ppm can affect the accuracy of the sensor if the automatic self-calibration (ASC) is on.
2
Deviation to a high-precision reference. Accuracy is fulfilled by > 90% of the sensors after calibration. Rough handling, shipping and soldering reduces the accuracy of the
sensor. Accuracy is restored with FRC or ASC recalibration features. Accuracy is based on tests with gas mixtures having a tolerance of ± 1.5%.
3
Time for achieving 63% of a respective step function when operating the SCD42 Evaluation Kit with default measurement mode. Response time depends on design-in,
signal update rate and environment of the sensor in the final application.
4
For proper function of ASC field-calibration algorithm SCD42 has to be exposed to air with CO2concentration 400 ppm regularly. Maximum accuracy drift per year estimated
from stress tests is ± (5 ppm + 2 % of reading). Higher drift values may occur if the sensor is not handled according to its handling instructions.
5
Design-in of the SCD42 in final application, self-heating of the sensor and the environment impacts the accuracy of the RH/T sensor. To realize indicated specifications, the
temperature-offset of the SCD42 inside the customer device must be set correctly (see chapter 3.6).

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2Specifications
2.1 Electrical Specifications
Parameter
Symbol
Conditions
Min.
Typical
Max.
Units
Supply voltage DC6
VDD
2.4
3.3
5.5
V
Voltage ripple peak to peak
VRPP
Frequency ≥ 1 kHz
30
mV
Supply voltage variation7
VVar
Frequency < 1 kHz
±5
mV
Peak supply current8
Ipeak
VDD = 3.3 V
175
205
mA
VDD = 5 V
115
137
mA
Average supply current for periodic
measurement
IDD
VDD = 3.3 V
15
18
mA
VDD = 5 V
11
13
mA
Input high level voltage
VIH
0.65 x VDD
VDD
-
Input low level voltage
VIL
0.3 x VDD
-
Output low level voltage
VOL
3 mA sink current
0.66
V
Table 4 SCD42 electrical specifications
2.2 Absolute Maximum Ratings
Stress levels beyond those listed in Table 5 may cause permanent damage to the device. Exposure to minimum/maximum
rating conditions for extended periods may affect sensor performance and reliability of the device.
Parameter
Conditions
Value
Temperature operating conditions
0 –50°C
Humidity operating conditions9
Non-condensing
0 –95 %RH
MSL Level10
1
DC supply voltage
- 0.3 V –6.0 V
Max voltage on pins SDA, SCL, GND
–0.3 V to VDD+0.3 V
Input current on pins SDA, SCL, GND
- 280 mA to 100 mA
Short term storage temperature11
- 40°C –70°C
Recommended storage temperature
10 °C –30 °C
ESD HBM
2 kV
ESD CDM
500 V
Maintenance Interval
Maintenance free when ASC field-
calibration algorithm12 is used.
None
Sensor lifetime13
Typical operating conditions
> 10 years
Table 5: SCD42 operation conditions, lifetime and maximum ratings
6
Do not change supply voltage during operation
7
Accuracy can be impaired by excess supply voltage variation caused by external load or temperature variations
8
Power supply should be designed with respect to peak current.
9
Accuracy can be reduced at relative humidity levels lower than 10 %.
10
Exceeding MSL 1 storage conditions (30 °C, 85 % RH max.) prior to sensor assembly can lead to degraded sensor performance or sensor failure in the reflow soldering
process.
11
Short term storage refers to temporary conditions during e.g. transport.
12
For proper function of ASC field-calibration algorithm the SCD42 has to be exposed to clean air with 400 ppm CO2concentration regularly.
13
Sensor tested over simulated lifetime of > 10 years for indoor environment mission profile

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2.3 Interface Specifications
The SCD42 comes in an LGA package (Table 6). The package outline is schematically displayed in chapter 4.1. The landing
pattern of the SCD42 can be found in chapter 4.2.
Name
Comments
VDD
Supply voltage
VDDH
Supply voltage IR source, must
be connected to VDD on
customer PCB
GND
Ground contact
SDA
I2C serial data, bidirectional
SCL
I2C serial clock
DNC
Do not connect, pads must be
soldered to a floating pad on
the customer PCB
Table 6 Pin assignment (top view). The notched corner of the protection membrane serves as a
polarity mark to indicate pin 1 location.
VDD and VDDH are used to supply the sensor and must always be kept at the same voltage, i.e. should both be connected to
the same power supply. The combined maximum current drawn on VDD and VDDH is indicated in Table 4. Care should be
taken to choose a low noise power supply, which is adequately dimensioned for the relatively large peak currents. The power
supply should be designed such that voltage variations caused by external influences remain below 5 mV. Consequently, no
other components causing larger load variations should be supplied by the same voltage regulator. Sensirion recommends
integrating a low-dropout regulator, LDO, such as the Toshiba TCR2EF33,LM(CT.
SCL is used to synchronize the I2C communication between the master (microcontroller) and the slave (sensor). The SDA pin
is used to transfer data to and from the sensor. For safe communication, the timing specifications defined in the I2C manual
14
must be met. Both SCL and SDA lines should be connected to external pull-up resistors (e.g. Rp = 10 kΩ, see Figure 1). To
avoid signal contention, the microcontroller must only drive SDA and SCL low. For dimensioning resistor sizes please take bus
capacity and communication frequency into account (see example in Section 7.1 of NXPs I2C Manual for more details14). It
should be noted that pull-up resistors may be included in I/O circuits of microcontrollers.
Figure 1: Typical application circuit (for better clarity in the image, the positioning
of the pins does not reflect the positions on the real sensor). VDD and VDDH must
be connected to each other close to the sensor on the customer PCB.
14
NXP’s I2C-bus specification and user manual UM10204, Rev.6, 4 April 2014

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2.4 Timing Specifications
Table 7 list the timings of the ASIC part and does not reflect the availability or usefulness of the sensor readings. The SCD42
supports the I2C “standard-mode” as is described elsewhere (see footnote 14).
Parameter
Condition
Min.
Max.
Unit
Power-up time
After hard reset, VDD ≥2.25 V
-
1000
ms
Soft reset time
After re-initialization (i.e. reinit)
-
1000
ms
SCL clock frequency
-
0
100
kHz
Table 7 System timing specifications.
2.5 Material Contents
The device is fully REACH and RoHS compliant.

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3Digital Interface Description
All SCD42 commands and data are mapped to a 16-bit address space.
SCD42
Hex. Code
I2C address
0x62
Table 8 I2C device address.
3.1 Power-Up and Communication Start
The sensor starts powering-up after reaching the power-up threshold voltage VDD,Min = 2.25 V. After reaching this threshold
voltage, the sensor needs 1000 ms to enter the idle state. Once the idle state is entered it is ready to receive commands from
the master.
Each transmission sequence begins with a START condition (S) and ends with a STOP condition (P) as described in the I2C-
bus specification.
3.2 Data type & length
Data sent to and received from the sensor consists of a sequence of 16-bit commands and/or 16-bit words (each to be interpreted
as unsigned integer, most significant byte transmitted first). Each data word is immediately succeeded by an 8-bit CRC. In write
direction it is mandatory to transmit the checksum. In read direction it is up to the master to decide if it wants to process the
checksum (see chapter 3.9).
3.3 Command Sequence Types
The SCD42 features four different I2C command sequence types: “read I2C sequences”, “write I2C sequences”,“send I2C
command”and “send command and fetch result”sequences. Figure 2 illustrates how the I2C communication for the different
sequence types is built-up.
Figure 2: Command Sequence types: “write”sequence, “send command”sequence, “read”
sequence, and “send command and fetch result”sequence.
For “read”” or “send command and fetch results” sequences, after writing the address and/or data to the sensor and sending the
ACK bit, the sensor needs the execution time (see Table 9) to respond to the I2C read header with an ACK bit. Hence, it is
required to wait the command execution time before issuing the read header. Commands must not be sent while a previous
command is being processed.

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3.4 SCD42 Command Overview
Table 9: List of SCD42 sensor commands. Detailed description of SCD42 commands can be found
further down. *Column indicates whether command can be executed while a periodic measurement
is running.
Domain
Command
Hex.
Code
I2C sequence type
(see chapter 3.3)
Execution
time
[ms]
During
meas.*
Basic Commands
Chapter 3.5
start_periodic_measurement
0x21b1
send command
-
no
read_measurement
0xec05
read
1
yes
stop_periodic_measurement
0x3f86
send command
500
yes
On-chip output signal
compensation
Chapter 3.6
set_temperature_offset
0x241d
write
1
no
get_temperature_offset
0x2318
read
1
no
set_sensor_altitude
0x2427
write
1
no
get_sensor_altitude
0x2322
read
1
no
set_ambient_pressure
0xe000
write
1
yes
Field calibration
Chapter 3.7
perform_forced_recalibration
0x362f
send command and
fetch result
400
no
set_automatic_self_calibration_enabled
0x2416
write
1
no
get_automatic_self_calibration_enabled
0x2313
read
1
no
Advanced features
Chapter 3.8
persist_settings
0x3615
send command
800
no
get_serial_number
0x3682
read
1
no
perform_self_test
0x3639
read
10000
no
perform_factory_reset
0x3632
send command
1200
no
reinit
0x3646
send command
20
no

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3.5 Basic Commands
This section lists the basic SCD42 commands that are necessary to start a periodic measurement and subsequently read out
the sensor outputs.
The typical communication sequence between the I2C master (e.g., a microcontroller) and the SCD42 sensor is as follows:
1. The sensor is powered up
2. The I2C master sends a start_periodic_measurement command. Signal update interval is 5 seconds.
3. The I2C master periodically reads out data with the read measurement sequence.
4. To put the sensor back to idle mode, the I2C master sends a stop periodic measurement command.
While a periodic measurement is running, no other commands must be issued with the exception of read_measurement,
get_data_ready_status, stop_periodic_measurement and set_ambient_pressure.
3.5.1 start_periodic_measurement
Description: start periodic measurement, signal update interval is 5 seconds.
Table 10: start_periodic_measurement I2Csequence description
Write
(hexadecimal)
Input parameter: -
Response parameter: -
Max. command
duration [ms]
length [bytes]
signal conversion
length [bytes]
signal conversion
0x21b1
-
-
-
-
not applicable
Example: start periodic measurement
Write
0x21b1
(hexadecimal)
Command
3.5.2 read_measurement
Description: read sensor output. The measurement data can only be read out once per signal update interval as the buffer is
emptied upon read-out. If no data is available in the buffer, the sensor returns a NACK. To avoid a NACK response, the
get_data_ready_status can be issued to check data status (see chapter Error! Reference source not found. for further details). T
he I2C master can abort the read transfer with a NACK followed by a STOP condition after any data byte if the user is not
interested in subsequent data.
Table 11: read_measurment I2C sequence description
Write
(hexadecimal)
Input parameter: -
Response parameter: CO2, Temperature, Relative
Humidity
Max.
command
duration [ms]
length [bytes]
signal conversion
length [bytes]
signal conversion
0xec05
-
-
3
CO2 [ppm] = word[0]
1
3
T = - 45 + 175 * word[1] / 216
3
RH = 100 * word[2] / 216
Example: read sensor output (500 ppm, 25 °C, 37 % RH)
Write
0xec05
(hexadecimal)
Command
Wait
1 ms
command execution time
Response
0x01f4
0x7b
0x6667
0xa2
0x5eb9
0x3c
(hexadecimal)
CO2= 500 ppm
CRC of 0x01f4
Temp. = 25 °C
CRC of 0x6667
RH = 37 %
CRC of 0x5eb9

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3.5.3 stop_periodic_measurement
Description: stop periodic measurement to change the sensor configuration or to save power. Note that the sensor will only
respond to other commands after waiting 500 ms after issuing the stop_periodic_measurement command.
Table 12: stop_periodic_measurement I2C sequence description
Write
(hexadecimal)
Input parameter: -
Response parameter: -
Max. command
duration [ms]
length [bytes]
signal conversion
length [bytes]
signal conversion
0x3f86
-
-
-
-
500
Example: stop periodic measurement
Write
0x3f86
(hexadecimal)
Command
3.6 On-Chip Output Signal Compensation
The SCD42 features on-chip signal compensation to counteract pressure and temperature effects. Feeding the SCD42 with the
pressure or altitude enables highest accuracy of the CO2output signal across a large pressure range. Setting the temperature
offset improves the accuracy of the relative humidity and temperature output signal. Note that the temperature offset does not
impact the accuracy of the CO2output.
To change or read sensor settings, the SCD42 must be in idle mode. A typical sequence between the I2C master and the SCD42
is described as follows:
1. If the sensor is operated in a periodic measurement mode, the I2C master sends a stop_periodic_measurement
command.
2. The I2C master sends one or several commands to get or set the sensor settings.
3. If configurations shall be preserved after power-cycle events, the persist_settings command must be sent (see
chapter 3.8.1)
4. The I2C master sends a start measurement command to set the sensor in the operating mode again.
3.6.1 set_temperature_offset
Description: The temperature offset has no influence on the SCD42 CO2accuracy. Setting the temperature offset of the SCD42
inside the customer device correctly allows the user to leverage the RH and T output signal. Note that the temperature offset
can depend on various factors such as the SCD42 measurement mode, self-heating of close components, the ambient
temperature and air flow. Thus, the SCD42 temperature offset should be determined inside the customer device under its typical
operation conditions (including the operation mode to be used in the application) and in thermal equilibrium. Per default, the
temperature offset is set to 4° C. To save the setting to the EEPROM, the persist setting (see chapter 3.8.1) command must be
issued. Equation (1) shows how the characteristic temperature offset can be obtained.
𝑇𝑜𝑓𝑓𝑠𝑒𝑡_𝑎𝑐𝑡𝑢𝑎𝑙 = 𝑇
𝑆𝐶𝐷40 − 𝑇𝑅𝑒𝑓𝑒𝑟𝑒𝑛𝑐𝑒 + 𝑇𝑜𝑓𝑓𝑠𝑒𝑡_ 𝑝𝑟𝑒𝑣𝑖𝑜𝑢𝑠 (1)
Table 13: set_temperature_offset I2C sequence description
Write
(hexadecimal)
Input parameter: Offset temperature
Response parameter: -
Max. command
duration [ms]
length [bytes]
signal conversion
length [bytes]
signal conversion
0x241d
3
word[0] = Toffset [°C] * 216 / 175
-
-
1
Example: set temperature offset to 5.4 °C
Write
0x241d
0x07e6
0x48
(hexadecimal)
Command
Toffset = 5.4 °C
CRC of 0x7e6

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3.6.2 get_temperature_offset
Table 14: get_temperature_offset I2C sequence description
Write
(hexadecimal)
Input parameter: -
Response parameter: Offset temperature
Max.
command
duration [ms]
length [bytes]
signal conversion
length [bytes]
signal conversion
0x2318
-
-
3
Toffset [°C] = 175 * word[0] / 216
1
Example: temperature offset is 6.2 °C
Write
0x2318
(hexadecimal)
Command
Wait
1 ms
command execution time
Response
0x0912
0x63
(hexadecimal)
Toffset = 6.2 °C
CRC of 0x0912
3.6.3 set_sensor_altitude
Description: Reading and writing of the sensor altitude must be done while the SCD42 is in idle mode. Typically, the sensor
altitude is set once after device installation. To save the setting to the EEPROM, the persist setting (see chapter 3.8.1) command
must be issued. Per default, the sensor altitude is set to 0 meter above sea-level.
Table 15: set_sensor_altitude I2C sequence description
Write
(hexadecimal)
Input parameter: Sensor altitude
Response parameter: -
Max. command
duration [ms]
length [bytes]
signal conversion
length [bytes]
signal conversion
0x2427
3
word[0] = Sensor altitude [m]
-
-
1
Example: set sensor altitude to 1’950 m.a.s.l.
Write
0x2427
0x079e
0x09
(hexadecimal)
Command
Sensor altitude = 1’950 m
CRC of 0x79e
3.6.4 get_sensor_altitude
Table 16: get_sensor_altitude I2C sequence description
Write
(hexadecimal)
Input parameter: -
Response parameter: Sensor altitude
Max. command
duration [ms]
length [bytes]
signal conversion
length [bytes]
signal conversion
0x2322
-
-
3
Sensor altitude [m] = word[0]
1
Example: sensor altitude is 1’100 m.a.s.l.
Write
0x2322
(hexadecimal)
Command
Wait
1 ms
command execution time
Response
0x044c
0x42
(hexadecimal)
Sensor altitude = 1’100 m
CRC of 0x044c

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3.6.5 set_ambient_pressure
Description: The set_ambient_pressure command can be sent during periodic measurements to enable continuous pressure
compensation. Note that setting an ambient pressure using set_ambient_pressure overrides any pressure compensation based
on a previously set sensor altitude.
Table 17: set_ambient_pressure I2C sequence description
Write
(hexadecimal)
Input parameter: Ambient pressure
Response parameter: -
Max. command
duration [ms]
length [bytes]
signal conversion
length [bytes]
signal conversion
0xe000
3
word[0] = ambient P [Pa] /
100
-
-
1
Example: set ambient pressure to 98’700 Pa
Write
0xe000
0x03db
0x42
(hexadecimal)
Command
Ambient P = 98’700 Pa
CRC of 0x03db

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3.7 Field Calibration
To realize high initial and long-term accuracy, the SCD42 includes two field calibration features. Forced recalibration (FRC)
enables restoring highest accuracy with the assistance of a CO2reference value immediately. Typically, FRC is applied to
compensate for drifts originating from the sensor assembly process or other extensive stresses. Automatic self-calibration (ASC)
ensures highest long-term stability of the SCD42 without the need of manual action steps from the user. The automatic self-
calibration algorithm assumes that the sensor is exposed to the atmospheric CO2concentration of 400 ppm at least once per
week.
3.7.1 perform_forced_recalibration
Description: To successfully conduct an accurate forced recalibration, the following steps need to be carried out:
1. Operate the SCD42 in the operation mode later used in normal sensor operation for > 3 minutes in an environment
with homogenous and constant CO2concentration.
2. Issue stop_periodic_measurement. Wait 500 ms for the stop command to complete.
3. Subsequently issue the perform_forced_recalibration command and optionally read out the FRC correction (i.e. the
magnitude of the correction) after waiting for 400 ms for the command to complete.
•A return value of 0xffff indicates that the forced recalibration has failed.
Note that the sensor will fail to perform a forced recalibration if it was not operated before sending the command. Please make
sure that the sensor is operated at the voltage desired for the application when applying the forced recalibration sequence.
Table 18: perform_forced_recalibration I2C sequence description
Write
(hexadecimal)
Input parameter: Target CO2concentration
Response parameter: FRC-correction
Max.
command
duration [ms]
length [bytes]
signal conversion
length [bytes]
signal conversion
0x362f
3
word[0] = Target
concentration [ppm CO2]
3
FRC correction [ppm CO2] =
word[0] –0x8000
word[0] = 0xffff in case of
failed FRC
400
Example: perform forced recalibration, reference CO2concentration is 480 ppm
Write
0x362f
0x01e0
0xb4
(hexadecimal)
Command
Input: 480 ppm
CRC of 0x01e0
Wait
400 ms
command execution time
Response
0x7fce
0x7b
(hexadecimal)
Response: - 50 ppm
CRC of 0x7fce

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3.7.2 set_automatic_self_calibration_enabled
Description: Set the current state (enabled / disabled) of the automatic self-calibration. By default, ASC is enabled. To save the
setting to the EEPROM, the persist_setting (see chapter 3.8.1) command must be issued.
Table 19: set_automatic_self_calibration_enabled I2C sequence description.
Write
(hexadecimal)
Input parameter: ASC enabled
Response parameter: -
Max. command
duration [ms]
length [bytes]
signal conversion
length [bytes]
signal conversion
0x2416
3
word[0] = 1 → ASC enabled
word[0] = 0 → ASC disabled
-
-
1
Example: set automatic self-calibration status: enabled
Write
0x2416
0x0001
0xB0
(hexadecimal)
Command
ASC enabled
CRC of 0x0001
3.7.3 get_automatic_self_calibration_enabled
Table 20: get_automatic_self_calibration_enabled I2C sequence description
Write
(hexadecimal)
Input parameter: -
Response parameter: ASC enabled
Max. command
duration [ms]
length [bytes]
signal conversion
length [bytes]
signal conversion
0x2313
-
-
3
word[0] = 1 → ASC enabled
word[0] = 0 → ASC disabled
1
Example: read automatic self-calibration status: disabled
Write
0x2313
(hexadecimal)
Command
Wait
1 ms
command execution time
Response
0x0000
0x81
(hexadecimal)
ASC disabled
CRC of 0x0000

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3.8 Advanced Features
3.8.1 persist_settings
Description: Configuration settings such as the temperature offset, sensor altitude and the ASC enabled/disabled parameter
are by default stored in the volatile memory (RAM) only and will be lost after a power-cycle. The persist_settings command
stores the current configuration in the EEPROM of the SCD42, making them persistent across power-cycling. To avoid
unnecessary wear of the EEPROM, the persist_settings command should only be sent when persistence is required and if actual
changes to the configuration have been made. The EEPROM is guaranteed to endure at least 2000 write cycles before failure.
Note that field calibration history (i.e. FRC and ASC, see chapter 3.7) is automatically stored in a separate EEPROM
dimensioned for the specified sensor lifetime.
Table 21: persist_settings I2C sequence description
Write
(hexadecimal)
Input parameter: -
Response parameter: -
Max. command
duration [ms]
length [bytes]
signal conversion
length [bytes]
signal conversion
0x3615
-
-
-
-
800
Example: persist settings
Write
0x3615
(hexadecimal)
Command
3.8.2 get_serial_number
Description: Reading out the serial number can be used to identify the chip and to verify the presence of the sensor.
The get serial number command returns 3 words, and every word is followed by an 8-bit CRC checksum. Together, the 3 words
constitute a unique serial number with a length of 48 bits (big endian format).
Table 22: get_serial_number I2C sequence description
Write
(hexadecimal)
Input parameter: -
Response parameter: serial number
Max. command
duration [ms]
length [bytes]
signal conversion
length [bytes]
signal conversion
0x3682
-
-
9
Serial number = word[0] <<
32 | word[1] << 16 | word[2]
1
Example: serial number is 273’325’796’834’238
Write
0x3682
(hexadecimal)
Command
Wait
1 ms
command execution time
Response
0xf896
0x31
0x9f07
0xc2
0x3bbe
0x89
(hexadecimal)
word[0]
CRC of 0xf896
word[1]
CRC of 0x9f07
word[2]
CRC of 0x3bbe

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3.8.3 perform_self_test
Description: The perform_self_test feature can be used as an end-of-line test to check sensor functionality and the customer
power supply to the sensor.
Table 23: perform_self_test I2C sequence description
Write
(hexadecimal)
Input parameter: -
Response parameter: sensor status
Max. command
duration [ms]
length [bytes]
signal conversion
length [bytes]
signal conversion
0x3639
-
-
3
word[0] = 0 → no malfunction detected
word[0] ≠0 → malfunction detected
10000
Example: perform self-test, no malfunction detected
Write
0x3639
(hexadecimal)
Command
Wait
10000 ms
command execution time
Response
0x0000
0x81
(hexadecimal)
No malfunction detected
CRC of 0x0000
3.8.4 perfom_factory_reset
Description: The perform_factory_reset command resets all configuration settings stored in the EEPROM and erases the
FRC and ASC algorithm history.
Table 24: perform_factory_reset I2C sequence description
Write
(hexadecimal)
Input parameter: -
Response parameter: -
Max. command
duration [ms]
length [bytes]
signal conversion
length [bytes]
signal conversion
0x3632
-
-
-
-
1200
Example: perform factory reset
Write
0x3632
(hexadecimal)
Command
3.8.5 reinit
Description: The reinit command reinitializes the sensor by reloading user settings from EEPROM. Before sending the reinit
command, the stop measurement command must be issued. If the reinit command does not trigger the desired re-initialization,
a power-cycle should be applied to the SCD42.
Table 25: reinit I2C sequence description
Write
(hexadecimal)
Input parameter: -
Response parameter: -
Max. command
duration [ms]
length [bytes]
signal conversion
length [bytes]
signal conversion
0x3646
-
-
-
-
20
Example: reinit
Write
0x3646
(hexadecimal)
Command re-initialization

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3.9 Checksum Calculation
The 8-bit CRC checksum transmitted after each data word is generated by a CRC algorithm. Its properties are displayed in
Table 26. The CRC covers the contents of the two previously transmitted data bytes. To calculate the checksum only these two
previously transmitted data bytes are used. Note that command words are not followed by CRC.
Property
Value
Example code (C/C++)
Name
CRC-8
#define CRC8_POLYNOMIAL 0x31
#define CRC8_INIT 0xFF
uint8_t sensirion_common_generate_crc(const uint8_t*data,uint16_t count) {
uint16_t current_byte;
uint8_t crc =CRC8_INIT;
uint8_t crc_bit;
/* calculates 8-Bit checksum with given polynomial */
for (current_byte =0;current_byte <count; ++current_byte) {
crc ^= (data[current_byte]);
for (crc_bit =8;crc_bit >0; --crc_bit) {
if (crc &0x80)
crc =(crc << 1) ^ CRC8_POLYNOMIAL;
else
crc = (crc << 1);
}
}
return crc;
}
Width
8 bit
Protected Data
read and/or write data
Polynomial
0x31 (x8 + x5 + x4 + 1)
Initialization
0xFF
Reflect input
False
Reflect output
False
Final XOR
0x00
Examples
CRC (0xBEEF) = 0x92
Table 26 I2C CRC properties.

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4Mechanical specifications
4.1 Package Outline
Figure 3 schematically displays the package outline. The notched corner of the protection membrane serves as a polarity
mark to indicate pin 1 location. Nominal dimensions and tolerances are listed in Table 27.
Figure 3: Packaging outline drawing of the SCD42: (left) top view and (right) side view. Nominal
dimensions and tolerances are listed in millimeters.
Dimension
a
b
c
d
e
f
Nominal [mm]
10.1
10.1
8.5
7.8
5.5
0.8
Tolerance [mm]
± 0.3
± 0.3
± 0.2
± 0.2
± 0.3
± 0.2
Table 27: Nominal dimensions and tolerances SCD42 (all in mm). The weight of the sensor is approx.
0.6 g.
Note that the white protection membrane on top of the sensor must not be removed or tampered with to ensure proper sensor
operation.
4.2 Land Pattern
Recommended land pattern, solder paste and solder mask are shown in Figure 4. These are recommendations only and not
specifications. The exact mask geometries, distances and stencil thicknesses must be adapted to the customer soldering
processes.
Figure 4: SCD42 footprint (top view): landing pads (a), solder mask (b) and solder paste (c).

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4.3 Tape & Reel Package
Figure 5: Technical drawing of the packaging tape with sensor orientation in tape. Header tape is to
the right and trailer tape to the left on this drawing. Dimensions are given in millimeters.
4.4 Moisture Sensitivity Level
Sensirion SCD42 sensors shall be treated according to Moisture Sensitivity Level 1 (MSL1) as described in IPC/JEDEC J-STD-
033B1. Exposure to moisture levels or temperatures before sensor assembly, which exceed the limits as stated in this document,
can result in yield loss and sensor performance degradation.

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4.5 Soldering Instructions
For soldering, standard reflow soldering ovens may be used. The sensors are designed to withstand soldering profile based on
IPC/JEDEC J-STD-020 with a maximum peak temperature of 235°C during up to 30 sec for Pb-free assembly in IR/Convection
reflow ovens.
Note that due to the comparably large size of the SCD42 sensor significant temperature differences across the sensor element
can occur during reflow soldering. Specifically, the temperature within the sensor cap can be higher than the temperature
measured at the pad using usual temperature monitoring methods. Care must be taken that a temperature of 235° C is not
exceeded at any time in any part of the sensor.
Do not apply any board wash process step subsequently to the reflow soldering. Note that the dust cover on top of the cap must
not be removed or wetted with any liquid. Finally, the SCD42 is not compatible with vapor phase reflow soldering.
Average ramp-up rate
< 3 °C / second
Liquid phase
▪TL
▪tL
< 220 °C
< 60 seconds
Peak temperature
▪TP
▪tP
≤235 °C
< 30 seconds
Ramp-down rate
< 4 °C / seconds for
temperature > TL
Table 28 Soldering profile parameter
4.6 Traceability
All SCD42 sensors have a distinct electronic serial number for identification and traceability (see chapter 3.8.2). The serial
number can be decoded by Sensirion only and allows for tracking through production, calibration, and testing.
5Ordering Information
Use the part names and product numbers shown in the following table when ordering the SCD42 CO2sensor. For
the latest product information and local distributors, visit http://www.sensirion.com/.
Part Name
Description
Ordering quantity (pcs)
Product Number
SCD42-D-R2
SCD42 CO2 sensor SMD component as reel, I2C
600 sensors per reel
3.000.634
SEK-SCD41-Sensor
SEK-SCD41-Sensor set; SCD41 on development
board with cables
1
3.000.455
SEK-SensorBridge
Sensor Bridge to connect SEK-SCD41-Sensor to
computer
1
3.000.124
Table 29 SCD42 ordering options
6Revision History
Date
Version
Page(s)
Changes
September 2021
0.9
all
Preliminary version
September 2021
0.91
1, 20
Minor revisions (page 1), addition of product number (page 20)
October 2021
0.92
4
Moisture sensitivity level (MSL) changed to 1