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Version 7.1 – March 2025

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4

Sensor Operation

4.1 I2C Communication

I2C communication is based on NXP’s I2C-bus specification and user manual UM10204 [3]. Supported I2C

modes are standard, fast mode, and fast mode plus. Data is transferred in multiples of 16-bit words. In order

to increase reliability of data transfer, I2C glitch protection is offered in form of 8-bit checksum (cyclic

redundancy check = CRC, see section 4.4). 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 Figure 14. 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 14. 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 [2]. Please

follow mandatory capacitor and resistor requirements given in Table 4.

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

0x31 (x8 + x5 + x4 +1)

Initialization

0xFF

Reflect Input/Output

false/false

Final XOR

0x00

Examples

CRC(0xBEEF) = 0x92

Table 7. Data checksum properties

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