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Version 3

– April 2022

10/18

4 Sensor Operation

4.1 I2C communication

I2C communication is based on NXP’s I2C-bus specification and user manual UM10204, Rev.6, 4

April 2014. Supported I2C modes are standard, fast mode, and fast mode plus. Data is transferred

in multiples of 16-bit words and 8-bit checksum (cyclic redundancy check = CRC). All transfers

must begin with a start condition (S) and terminate with a stop condition (P). To finish a read

transfer, send not acknowledge (NACK) and stop condition (P). Addressing a specific slave device

is done by sending its 7-bit I2C address followed by an eighth bit, denoting the communication

direction: “zero” indicates transmission to the slave, i.e. “write”, a “one” indicates a “read” request.

Schematics of the I2C transfer types are sketched in Error! Reference source not found.. The

sensor does not support clock-stretching. In case the sensor receives a read header and is still

busy with e.g. measurement or 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 11: 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 I2C Communication Timing

All details on the timing are following the interface specification of NXP’s user manual UM10204,

Rev.6, 4 April 2014. Please follow mandatory capacitor and resistor requirements given in Table

3.

4.3 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).

4.4 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

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.