UART Interface Description SVM41
Evaluation Board for SGP40 and SGP41 – Indoor Air Quality Sensor for VOC and NOx
Measurements

Table of Contents
1 General Considerations ............................................................................................................................................2
2 Physical Layer ..........................................................................................................................................................2
3 SHDLC Frame Layer ................................................................................................................................................3
4 UART Commands ....................................................................................................................................................6

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1 General Considerations
Settings as mentioned in Table 1 have to be used when communicating with SVM41 sensor module via UART interface.
Parameter

RatingBaud rate

115’200 bits s−1

Data bits

8

ParityNone

Stop bit

1

Table 1 UART settings.

2 Physical Layer
The SVM41 has separate RX and TX lines with unipolar logic levels. A transmitted byte looks as in Figure 1.
Bit Time(1/Baudrate)

Start
Bit

Bit 0

Bit 1

Bit 2

Bit 3

Bit 4

Bit 5

Bit 6

Bit 7

Stop
Bit

Figure 1 Transmitted byte.

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3 SHDLC Frame Layer
On top of the UART interface, the SVM41 uses the very powerful and easy-to-implement SHDLC 1 protocol. It is a serial
communication protocol based on a master/slave architecture. The SVM41 acts as the slave device. Data is transferred in logicalunits called frames. Every transfer is initiated by the master sending a MOSI2 frame. The slave will respond to the MOSI framewith a slave response, or MISO3 frame. The two types of frames are shown in Figure 2.
Frame Content

MOSI Frame

Start(0x7E)ADR
CMD
L(1 Byte) (1 Byte) (1 Byte)

TX Data
0...255 Bytes

CHK(1 Byte)Stop(0x7E)

Frame Content

MISO Frame

Start(0x7E)

ADR
CMD
State
L
(1 Byte) (1 Byte) (1 Byte) (1 Byte)

RX Data
0...255 Bytes

CHK(1 Byte)Stop(0x7E)

Figure 2 MOSI and MISO frames structure.

3.1 Start/Stop Byte and Byte-Stuffing
The 0x7E character is sent at the beginning and at the end of the frame to signalize frame start and stop. If this byte 0x7E occurs
anywhere else in the frame, it must be replaced by two other bytes (byte-stuffing). This also applies to the characters 0x7D,
0x11, and 0x13. Use Table 2 for byte-stuffing.
Original data byte

Transferred data bytes

0x7E

0x7D, 0x5E

0x7D

0x7D, 0x5D

0x11

0x7D, 0x31

0x13

0x7D, 0x33

Table 2 Reference table for byte-stuffing.

Example: Data to send = [0x43, 0x11, 0x7F] → Data transmitted = [0x43, 0x7D, 0x31, 0x7F].

3.2 Address
The slave device address is always 0.

3.3 Command
In the MOSI frame the command tells the device what to do with the transmitted data. In the MISO frame, the slave just returns
the received command.

3.4 Length
Length of the “TX Data” or “RX Data” field (before byte-stuffing).

3.5 State
The MISO frame contains a state byte, which allows the master to detect communication and execution errors.

1 Sensirion High-Level Data Link Control.
2 Master Out Slave In. Frame direction from master to slave.
3 Master In Slave Out. Frame direction from slave to master.

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b7
deviceerror flag

b6

b5

b4

b3

b2

b1

b0

execution error code

Figure 3 Status byte structure.

The first bit (b7) indicates that at least one of the error flags is set in the Device Status Register. The “Execution error code”
signalizes all errors which occur while processing the frame or executing the command. The following table shows the error
codes which can be reported from the device. Note that some of these errors are system internal errors which require additional
knowledge to be understood. In case of a problem, they will help Sensirion to localize and solve the issue.
Error code
Dec.

Hex.

Description

0

0x00

No error. Processing and execution of the command was
successful.

1

0x01

Wrong data length for this command (too much or little data)

2

0x02

Unknown command

3

0x03

No access right for command

4

0x04

Illegal command parameter or parameter out of allowed range

40

0x28

Internal function argument out of range

67

0x43

Command not allowed in current state

Table 3 Reference table for error codes.

3.6 Data
The data has a usable size of [0…255] bytes (original data before byte-stuffing). The meaning of the data content depends onthe command. The data in the frames is transmitted in big-endian order (MSB ﬁrst). If subcommands are used for the commandthe first byte will be used to define the subcommand.

3.7 Checksum
The checksum is built before byte-stuffing and checked after removing stuffed bytes from the frame. The checksum is defined
as follows:
1. Sum all bytes between start and stop (without start and stop bytes).
2. Take the least significant byte of the result and invert it. This will be the checksum.
For a MOSI frame use Address, Command, Length, and Data to calculate the checksum.
For a MISO frame use Address, Command, State, Length, and Data to calculate the checksum.
Example: start measurement (MOSI frame without start/stop and without byte-stuffing).
ADR

CMD

L

TX data

CHK

0x02

0x43

0x04

[0x64, 0xA0, 0x22, 0xFC]

0x94

The checksum is calculated as shown in Table 4.

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ParameterHex. code

ADR

0x02

CMD

0x43

L

0x04

Data 00x64

Data 10xA0

Data 20x22

Data 30xFC

Sum of all bytes

0x26B

LSB of sum

0x6B

Inverted (= checksum)

0x94

Table 4 Checksum calculation.

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4 UART Commands
Table 5 shows an overview of the available UART commands.
Command

Send command duringsvm41_start_measurement

idle mode

svm41_get_signals

measure mode

svm41_get_raw_signals

measure mode

svm41_stop_measurement

measure mode

svm41_get_temperature_offset

idle or measure mode

svm41_set_temperature_offset

idle mode

svm41_get_voc_parametersidle or measure mode

svm41_set_voc_parametersidle mode

svm41_get_nox_parametersidle or measure mode

svm41_set_nox_parametersidle mode

svm41_store_input_parametersidle or measure mode

svm41_get_voc_states

measure mode

svm41_set_voc_statesidle mode

svm41_get_device_version

idle or measure mode

svm41_reset_device

idle or measure mode

Table 5 UART commands available for SVM41.

4.1 Start Measurement
Commandsvm41_start_measurement

Description

This command triggers the operation mode of all sensors. It must be called once prior to thesvm41_get_signals or svm41_get_raw_signals commands, respectively. Per default the SVM41 starts in idle
mode after powering up.

Command hex. code

0x00

Max. response time

50 ms

Post processing time

0 ms

MOSI Data

Byte number

Description0

uint8 subcommand, must be set to 0x00

MISO Data

None

Table 6 Description of the UART start measurement command.

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Data

Hex. code

MOSI

0x7E 0x00 0x00 0x01 0x00 0xFE 0x7E

MISO

0x7E 0x00 0x00 0x00 0x00 0xFF 0x7E

Table 7 Example frames for UART start measurement command.

4.2 Get Signals
Commandsvm41_get_signals

Description

This command reads out relative humidity, temperature as well as VOC and NOx Index. It returns 8 bytes.

Command hex. code

0x03

Max. response time

50 ms

Post processing time

0 ms

MOSI Data

Byte number

Description0

uint8 subcommand, must be set to 0x10

Byte number

Description

0, 1

int16 provides the relative humidity (in % RH) compensated for the temperature offset with ascaling factor of 100, e.g., an output of +2’500 corresponds to +25.00 % RH.

2, 3

int16 provides the temperature (in °C) with a scaling factor of 200, e.g., an output of +5’000
corresponds to +25.00 °C.

4, 5

int16 provides the VOC Index (no unit) with a scaling factor of 10, e.g., an output of +250
corresponds to a VOC Index of +25.0.

6, 7

int16 provides the NOx Index (no unit) with a scaling factor of 10, e.g., an output of +250
corresponds to a VOC Index of +25.0.

MISO Data

Table 8 Description of the UART get signals command.

Data

Hex. code

MOSI

0x7E 0x00 0x03 0x01 0x10 0xEB 0x7E

MISO

0x7E 0x00 0x03 0x00 0x08 0x18 0x33 0x12 0x8D 0x01 0xC2 0x00 0x0A 0x3D 0x7E

Table 9 Example frames for UART get signals command.

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4.3 Get Raw Signals
Commandsvm41_get_raw_signals

Description

This command reads out relative humidity and temperature which are not compensated for temperature offset
and the VOC and NOx raw signals (proportional to the logarithm of the resistance of the MOX layer). It returns
16 bytes.

Command hex. code

0x03

Max. response time

50 ms

Post processing time

0 ms

MOSI Data

Byte number

Description0

uint8 subcommand, must be set to 0x0D

Byte number

Description

0, 1

int16 provides the uncompensated relative humidity (in % RH) with a scaling factor of 100,e.g., an output of +2’500 corresponds to +25.00 % RH.

2, 3

int16 provides the uncompensated temperature (in °C) with a scaling factor of 200, e.g., an
output of +5’000 corresponds to +25.00 °C.

4, 5

uint16 directly provides the VOC raw signal SRAW_VOC (in ticks) without scaling.

6, 7

uint16 directly provides the NOx raw signal SRAW_NOX (in ticks) without scaling.

MISO Data

Table 10 Description of the UART get raw signals command.

Data

Hex. code

MOSI

0x7E 0x00 0x03 0x01 0x0D 0xEE 0x7E

MISO

0x7E 0x00 0x03 0x00 0x08 0x17 0x2F 0x7D 0x33 0x62 0x79 0x78 0x48 0x98 0x68 0x7E

Table 11 Example frames for UART get signals command.

4.4 Stop Measurement
Commandsvm41_stop_measurement

Description

This command stops the operation mode of all sensors and returns the SVM41 to idle mode.

Command hex. code

0x01

Max. response time

50 ms

Post processing time

0 ms

MOSI Data

None

MISO Data

None

Table 12 Description of the UART stop measurement command.

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Data

Hex. code

MOSI

0x7E 0x00 0x01 0x00 0xFE 0x7E

MISO

0x7E 0x00 0x01 0x00 0x00 0xFE 0x7E

Table 13 Example frames for UART stop measurement command.

4.5 Get Temperature Offset for RHT Measurements
Commandsvm41_get_temperature_offset

Description

This command reads out the current temperature offset used for the compensation of RHT measurements byreturning 2 bytes.

Command hex. code

0x60

Max. response time

50 ms

Post processing time

0 ms

MOSI Data

Byte number

Description0

uint8 subcommand, must be set to 0x01

Byte number

Description

0, 1

int16 provides the temperature offset (in °C) with a scaling factor of 200, e.g., an output of+400 corresponds to +2.00 °C. Default is 0 °C.

MISO Data

Table 14 Description of the UART get temperature offset command.

Data

Hex. code

MOSI

0x7E 0x00 0x60 0x01 0x01 0x9D 0x7E

MISO

0x7E 0x00 0x60 0x00 0x02 0x00 0x00 0x9D 0x7E

Table 15 Example frames for UART get temperature offset command. Result based on default settings.

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4.6 Set Temperature Offset for RHT Measurements
Commandsvm41_set_temperature_offset

Description

This command sets the temperature offset used for the compensation of subsequent RHT measurements
when sent together with 2 bytes.

Command hex. code

0x60

Max. response time

50 ms

Post processing time

0 ms

MOSI Data

Byte number

Description0

uint8 subcommand, must be set to 0x81

1, 2

int16 provides the temperature offset (in °C) with a scaling factor of 200, e.g., an output of+400 corresponds to +2.00 °C. Default is 0 °C.

MISO Data

None

Table 16 Description of the UART set temperature offset command.

Data

Hex. code

MOSI

0x7E 0x00 0x60 0x03 0x81 0x00 0x00 0x1B 0x7E

MISO

0x7E 0x00 0x60 0x00 0x00 0x9F 0x7E

Table 17 Example frames for UART set temperature offset command based on default settings.

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4.7 Get Parameters of VOC Algorithm
Commandsvm41_get_voc_parameters

Description

This command reads out the current six parameters used for the VOC Algorithm returning 12 bytes.

Command hex. code

0x60

Max. response time

50 ms

Post processing time

0 ms

MOSI Data

Byte number

Description0

uint8 subcommand, must be set to 0x0D

Byte number

Description

0, 1

int16 directly provides VOC Index (no unit) value representing the average conditions.
Default is VOC Index = 100. Range is 1–250.

2, 3

int16 directly provides learning time (in h) which is used by the VOC Algorithm to estimate
its offset from the history. Events longer than approx. twice the learning time will be forgotten.
Default is 12 h. Range is 1–1’000 h.

4, 5

int16 directly provides learning time (in h) which is used by the VOC Algorithm to estimate
its gain from the history. Events longer than approx. twice the learning time will be forgotten.
Default is 12 h. Range is 1–1’000 h.

6, 7

int16 directly provides maximum gating duration (in min). During this period, the estimator of
the VOC Algorithm states is frozen when the VOC Index is very high. Default is 180 min. 0
disables this feature. Range is 0–3’000 min.

8, 9

int16 directly provides initial standard deviation (no unit) used during start-up of the sensor.
During start-up period, a lower value boosts VOC events while a higher value decreases
VOC events. Default is 50. Range is 10–5’000.

10, 11

int16 directly provides the gain factor to amplify or to attenuate the VOC Index output. Default
is 230. Range is 1–1’000.

MISO Data

Table 18 Description of the UART get VOC parameters command.

Data

Hex. code

MOSI

0x7E 0x00 0x60 0x01 0x0D 0x91 0x7E

MISO

0x7E 0x00 0x60 0x00 0x0C 0x00 0x64 0x00 0x0C 0x00 0x0C 0x00 0xB4 0x00 0x32 0x00 0xE6 0x4B 0x7E

Table 19 Example frames for UART get VOC parameters command returning default parameters.

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4.8 Set Parameters of VOC Algorithm
Commandsvm41_set_voc_parameters

Description

This command sets the six parameters used for the VOC Algorithm when sent together with 12 bytes.

Command hex. code

0x60

Max. response time

50 ms

Post processing time

0 ms

MOSI Data

Byte number

Description0

uint8 subcommand, must be set to 0x8D

1, 2

int16 directly provides VOC Index (no unit) value representing the average conditions.
Default is VOC Index = 100. Range is 1–250.

3, 4

int16 directly provides learning time (in h) which is used by the VOC Algorithm to estimate
its offset from the history. Events longer than approx. twice the learning time will be forgotten.
Default is 12 h. Range is 1–1’000 h.

5, 6

int16 directly provides learning time (in h) which is used by the VOC Algorithm to estimate
its gain from the history. Events longer than approx. twice the learning time will be forgotten.
Default is 12 h. Range is 1–1’000 h.

7, 8

int16 directly provides maximum gating duration (in min). During this period, the estimator of
the VOC Algorithm states is frozen when the VOC Index is very high. Default is 180 min. 0
disables this feature. Range is 0–3’000 min.

9, 10

int16 directly provides initial standard deviation (no unit) used during start-up of the sensor.
During start-up period, a lower value boosts VOC events while a higher value decreases
VOC events. Default is 50. Range is 10–5’000.

11, 12

int16 directly provides the gain factor to amplify or to attenuate the VOC Index output. Default
is 230. Range is 1–1’000.

MISO Data

None

Table 20 Description of the UART set VOC parameters command.

Data

Hex. code

MOSI

0x7E 0x00 0x60 0x0D 0x8D 0x00 0x64 0x00 0x0C 0x00 0x0C 0x00 0xB4 0x00 0x32 0x00 0xE6 0xBD 0x7E

MISO

0x7E 0x00 0x60 0x00 0x00 0x9F 0x7E

Table 21 Example frames for UART set VOC parameters command based on default parameters.

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4.9 Get Parameters of NOx Algorithm
Commandsvm41_get_nox_parameters

Description

This command reads out the current six parameters used for the NOx Algorithm returning 12 bytes.

Command hex. code

0x60

Max. response time

50 ms

Post processing time

0 ms

MOSI Data

Byte number

Description0

uint8 subcommand, must be set to 0x0E

Byte number

Description

0, 1

int16 directly provides VOC Index (no unit) value representing the average conditions.
Default is NOx Index = 1. Range is 1–250.

2, 3

int16 directly provides learning time (in h) which is used by the NOx Algorithm to estimate its
offset from the history. Events longer than approx. twice the learning time will be forgotten.
Default is 12 h. Range is 1–1’000 h.

4, 5

int16 directly provides learning time (in h) which would be used by the NOx Algorithm toestimate its gain from the history; however, it has no impact on the NOx Index output. Thisparameter is still in place for consistency reasons with the svm41_get/set_voc_parameters
commands. This parameter must always be set to 12 hours (0x00 0x0C).

6, 7

int16 directly provides maximum gating duration (in min). During this period, the estimator of
the NOx Algorithm states is frozen when the NOx Index is very high. Default is 720 min. 0
disables this feature. Range is 0–3’000 min.

8, 9

int16 directly provides initial standard deviation (no unit) which would be used during startup of the sensor; however, it has no impact on the NOx Index output. This parameter is stillin place for consistency reasons with the svm41_get/set_voc_parameters commands. Thisparameter must always be set to 50 (0x00 0x32).

10, 11

int16 directly provides the gain factor to amplify or to attenuate the NOx Index output. Default
is 230. Range is 1–1’000.

MISO Data

Table 22 Description of the UART get NOx parameters command.

Data

Hex. code

MOSI

0x7E 0x00 0x60 0x01 0x0E 0x90 0x7E

MISO

0x7E 0x00 0x60 0x00 0x0C 0x00 0x01 0x00 0x0C 0x00 0x0C 0x02 0xD0 0x00 0x32 0x00 0xE6 0x90 0x7E

Table 23 Example frames for UART get NOx parameters command returning default parameters.

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4.10 Set Parameters of NOx Algorithm
Commandsvm41_set_nox_parameters

Description

This command sets the six parameters used for the NOx Algorithm when sent together with 12 bytes.

Command hex. code

0x60

Max. response time

50 ms

Post processing time

0 ms

MOSI Data

Byte number

Description0

uint8 subcommand, must be set to 0x8E

1, 2

int16 directly provides VOC Index (no unit) value representing the average conditions.
Default is NOx Index = 1. Range is 1–250.

3, 4

int16 directly provides learning time (in h) which is used by the NOx Algorithm to estimate its
offset from the history. Events longer than approx. twice the learning time will be forgotten.
Default is 12 h. Range is 1–1’000 h.

5, 6

int16 directly provides learning time (in h) which would be used by the NOx Algorithm toestimate its gain from the history; however, it has no impact on the NOx Index output. Thisparameter is still in place for consistency reasons with the svm41_get/set_voc_parameters
commands. This parameter must always be set to 12 hours (0x00 0x0C).

7, 8

int16 directly provides maximum gating duration (in min). During this period, the estimator of
the NOx Algorithm states is frozen when the NOx Index is very high. Default is 720 min. 0
disables this feature. Range is 0–3’000 min.

9, 10

int16 directly provides initial standard deviation (no unit) which would be used during startup of the sensor; however, it has no impact on the NOx Index output. This parameter is stillin place for consistency reasons with the svm41_get/set_voc_parameters commands. Thisparameter must always be set to 50 (0x00 0x32).

11, 12

int16 directly provides the gain factor to amplify or to attenuate the NOx Index output. Default
is 230. Range is 1–1’000.

MISO Data

None

Table 24 Description of the UART set NOx parameters command.

Data

Hex. code

MOSI

0x7E 0x00 0x60 0x0D 0x8E 0x00 0x01 0x00 0x0C 0x00 0x0C 0x02 0xD0 0x00 0x32 0x00 0xE6 0x01 0x7E

MISO

0x7E 0x00 0x60 0x00 0x00 0x9F 0x7E

Table 25 Example frames for UART set NOx parameters command based on default parameters.

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4.11 Store Input Parameters to Non-Volatile Memory
Commandsvm41_store_input_parameters

Description

This command stores all parameters previously sent to the slave via the svm41_set_temperature_offset and/or
the svm41_set_voc_parameters and/or svm41_set_nox_parameters commands to the non-volatile memoryof SVM41. These parameters will not be erased during reset and will be used by the corresponding algorithms
after start-up. To reset the storage to factory settings the master has to set all parameters to the default values
followed by a subsequent call of the svm41_store_input_parameters command.

Command hex. code

0x60

Max. response time

500 msPost processing time

0 ms

MOSI Data

Byte number

Description0

uint8 subcommand, must be set to 0x80

MISO Data

None

Table 26 Description of the UART store input parameters command.

Data

Hex. code

MOSI

0x7E 0x00 0x60 0x01 0x80 0x1E 0x7E

MISO

0x7E 0x00 0x60 0x00 0x00 0x9F 0x7E

Table 27 Example frames for UART store input parameters command.

4.12 Get States of VOC Algorithm
Commandsvm41_get_voc_states

Description

This command reads out the states of VOC Algorithm by returning 8 bytes. These values can be used to set
the states (using the svm41_set_voc_states command) after resuming sensor operation, e.g., after a shortinterruption by skipping the initial learning phase of the VOC Algorithm.

Command hex. code

0x61

Max. response time

50 ms

Post processing time

0 ms

MOSI Data

Byte number

Description0

uint8 subcommand, must be set to 0x08

Byte number

Description

0…7

uint8[8] array of 8 bytes providing the states of the VOC Algorithm.

MISO Data

Table 28 Description of the UART get VOC states command.

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Data

Hex. code

MOSI

0x7E 0x00 0x61 0x01 0x08 0x95 0x7E

MISO

0x7E 0x00 0x61 0x00 0x08 0x00 0x00 0x00 0x00 0x00 0x32 0x00 0x00 0x64 0x7E

Table 29 Example frames for UART get VOC states command.

4.13 Set States of VOC Algorithm
Commandsvm41_set_voc_states

Description

This command sets the states of the VOC Algorithm when sent together with 8 bytes, which were retrieved by
the svm41_get_voc_states command before. This can be used when resuming sensor operation, e.g., after ashort interruption by skipping the initial learning phase of the VOC Algorithm.

Command hex. code

0x61

Max. response time

50 ms

Post processing time

0 ms

MOSI Data

Byte number

Description0

uint8 subcommand, must be set to 0x88

1…8

uint8[8] array of 8 bytes setting the states of the VOC Algorithm.

MISO Data

None

Table 30 Description of the UART set VOC states command.

Data

Hex. code

MOSI

0x7E 0x00 0x61 0x09 0x88 0x00 0x00 0x00 0x00 0x00 0x32 0x00 0x00 0xDB 0x7E

MISO

0x7E 0x00 0x61 0x00 0x00 0x9E 0x7E

Table 31 Example frames for UART set VOC states command.

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4.14 Get Version of Device
Commandsvm41_get_device_version

Description

This command returns information on the hardware, firmware, and protocol in 7 bytes.

Command hex. code

0xD1

Max. response time

50 ms

Post processing time

0 ms

MOSI Data

None

MISO Data

Byte number

Description0

uint8 provides the major version number of the firmware.

1

uint8 provides the minor version number of the firmware.

2bool provides the debug state of the firmware.

3

uint8 provides the major version number of the hardware.

4

uint8 provides the minor version number of the hardware.

5

uint8 provides the major version number of the protocol.

6

uint8 provides the minor version number of the protocol.

Table 32 Description of the UART get device version command.

Data

Hex. code

MOSI

0x7E 0x00 0xD1 0x00 0x2E 0x7E

MISO

0x7E 0x00 0xD1 0x00 0x07 0x03 0x01 0x00 0x03 0x00 0x01 0x00 0x1F 0x7E

Table 33 Example frames for UART get device version command.

4.15 Reset Device
Commandsvm41_reset_device

Description

This command performs a reset of the device and restarts the SVM41 in idle mode. Prior to executing thereset, the device will acknowledge the call. If the command is sent with broadcast, the reset is executed directly
after the reception of the command. All previously set parameters sent by svm41_set_temperature_offset,
svm41_set_voc_parameters, svm41_set_nox_parameters, and svm41_set_voc_states commands will be
lost. The temperature offset and the parameters of both VOC and NOx Algorithms can be stored to the nonvolatile memory of SVM41 by calling the svm41_store_input_parameters command.

Command hex. code

0xD3

Max. response time

50 ms

Post processing time

100 msMOSI Data

None

MISO Data

None

Table 34 Description of the UART reset device command.

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Data

Hex. code

MOSI

0x7E 0x00 0xD3 0x00 0x2C 0x7E

MISO

0x7E 0x00 0xD3 0x00 0x00 0x2C 0x7E

Table 35 Example frames for UART reset device command.

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version 1.1 – December 2021

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Revision History
Date

VersionPage(s)Changes

October, 2021

1.0

All

Initial release

December 2021

1.1

All

Editorial amendments

13

Descriptions of bytes 4, 5 and 8, 9 in Table 22 revised

14

Descriptions of bytes 5, 6 and 9, 10 in Table 24 revisedwww.sensirion.com

version 1.1 – December 2021

19/20

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