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heating, it will return a NACK. Measurement data can only be received once and will be deleted

from the sensor’s register after the first acknowledged I2C read header.

Figure 8: I2C transfer types: First a write header is sent to the I2C slave, followed by a command,

for example “measure RH&T with highest precision”. After the measurement is finished the read

request directed to this I2C slave will be acknowledged and transmission of data will be started by

the slave.

4.2 Data type & length

I2C bus operates with 8-bit data packages. Information from the sensor to the master has a

checksum after every second 8-bit data package.

Humidity and temperature data will always be transmitted in the following way: The first value is

the temperature signal (2 * 8-bit data + 8-bit CRC), the second is the humidity signal (2 * 8-bit data

+ 8-bit CRC).

4.3 Checksum Calculation

For read transfers each 16-bit data is followed by a checksum with the following properties

Property

Value

Name

CRC-8

Message Length

16-bit

Polynomial

0x31 (x8 + x5 + x4 +1)

Initialization

0xFF

Reflect Input/Output

false/false

Final XOR

0x00

Examples

CRC(0xBEEF) = 0x92

Table 6 Data checksum properties.

The master may abort a read transfer after the 16-bit data, if it does not require a checksum.