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Version 1.0 – July 2020 – D1

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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 EF) = 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 SGP40 are listed in Table 8.

Command

Command hex.

code

Parameter length

including CRC

[bytes]

Response length

including CRC

[bytes]

Measurement duration

[ms]

Typ.

Max.

sgp40_measure_raw

0x26 0F

6

3

25

30

sgp40_measure_test

0x28 0E

–

3

240

250

sgp40_heater_off

0x36 15

–

–

0.1

1