TI’s first-generation HDC1x devices all share the same register map (as illustrated in Table 2-1). Therefore, the following explanation applies across the HDC1x family.

To begin, users must write a 16-bit value to the register in Table 2-2 (0x02) to define the measurement sequence – temperature, humidity or both sequentially.

In the example below, the HDC1x is configured to measure both temperature and humidity in sequence by writing 0x10 (MSB) and 0x00 (LSB) to the register. Figure 2-2 illustrates the register bits that are set.

Figure 2-2 Configuration Register Bits for Temperature and Humidity Acquisition

The code for setting the configuration is as follows:

Wire.beginTransmission(0x40); // initiate communication with HDC1x
Wire.write(0x02); // point to configuration register
Wire.write(0x10); // write 8-bit configuration to config register (MSB)
Wire.write(0x00); // write 8 0s to Reserved bits (LSB)
Wire.endTransmission();

Next, trigger the measurement process by writing to the device address (0x40).

Wire.beginTransmission(0x40); // initiate communication with HDC1x
Wire.write(0x00); // start measurements
Wire.endTransmission();
delay(20); // wait 20ms for conversion to complete.

Since temperature and humidity are measured in sequence, a 4-byte read from register 0x00 is required. The first two bytes correspond to the temperature while the next two bytes correspond to humidity data.

Wire.requestFrom(0x40, 4); // requesting 4 bytes from device

// once 4 bytes are received, store this in appropriate variables
if (Wire.available() == 4) {

    // stores raw temperature and humidity data
    // reads/stores first byte (MSB), then reads/stores second byte
    // combines each pair of bytes into a 16-bit integer

    uint16_t tempBytes = (Wire.read() << 8) | Wire.read();
    uint16_t humBytes = (Wire.read() << 8) | Wire.read();
}

Finally, apply the standard conversion formulas from the HDC1x data sheet:

// equation for converting temperature output in Celsius
temp = (tempBytes / 65536.0) * 165.0 - 40.0;

// equation for converting humidity output
hum = (humBytes / 65536.0) * 100.0;

This Arduino example demonstrates how to read and store temperature and humidity data from the HDC1x sensor when measurements are acquired sequentially. Once the raw data is stored, this can be converted to physical temperature and humidity values using the formulas provided in the HDC1x data sheet.

A complete working sample is available on the TI GitHub repository for environmental sensors here.