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structured in data words, where one word consists of two bytes of data (most significant bit first) followed by a one-byte CRC

checksum. Each byte must be acknowledged by the master with an ACK condition for the sensor to continue sending data. If

the sensor does not receive an ACK from the master after any byte of data, it will not continue sending data.

master can abort the read transfer with a NACK followed by a STOP condition after any data byte if it is not interested in

subsequent data, e.g., the CRC byte or following data bytes, in order to save time. Note that the data cannot be read more than

once, and access to data beyond the specified amount will return a pattern of high bits.

4.6 Checksum Calculation

The 8-bit CRC checksum transmitted after each data word is generated by the CRC algorithm according to the properties as

stated in Table 7. The CRC covers the contents of the two previously transmitted data bytes.

Property

Value

Example code

Name

CRC-8

uint8_t CalcCrc(uint8_t data[2]) {

uint8_t crc = 0xFF;

for(int i = 0; i < 2; i++) {

crc ^= data[i];

for(uint8_t bit = 8; bit > 0; --bit) {

if(crc & 0x80) {

crc = (crc << 1) ^ 0x31u;

} 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 (0xBE 0xEF) = 0x92

Table 7 Checksums are used for the 2-byte data packets only. The command codes themselves already contain a 3-bit CRC and therefore,

a checksum must not be appended.

The available measurement commands of the SGP41 are listed in Table 8.

Command

Command hex.

code

Parameter length

including CRC

[bytes]

Response length

including CRC

[bytes]

Measurement duration

[ms]

Typ.

Max.

sgp41_execute_conditioning

0x26 0x12

6

3

45

50

sgp41_measure_raw_signals

0x26 0x19

6

6

45

50

sgp41_execute_self_test

0x28 0x0E

–

3

300

320

sgp4x_turn_heater_off

0x36 0x15

–

–

0.1

1

sgp4x_get_serial_number

0x36 0x82

–

9

0.1

1