Table 1. HDC1008 ADDRESS

8.5.2.1 Serial Bus Address

To communicate with the HDC1008, the master must first address slave devices via a slave address byte. The slave address byte consists of seven address bits, and a direction bit that indicates the intent to execute a read or write operation.

8.5.2.2 Read and Write Operations

Access a particular register on the HDC1008 by writing the appropriate value to the Pointer Register. The pointer value is the first byte transferred after the slave address byte with the R/W bit low. Every write operation to the HDC1008 requires a value for the pointer register (refer to Figure 10).

When reading from the HDC1008, the last value stored in the pointer by a write operation is used to determine which register is read by a read operation. To change the pointer register for a read operation, a new value must be written to the pointer. This transaction is accomplished by issuing the slave address byte with the R/W bit low, followed by the pointer byte. No additional data is required (refer to Figure 11).

The master can then generate a START condition and send the slave address byte with the R/W bit high to initiate the read command. Note that register bytes are sent MSB first, followed by the LSB. A write operation in a read-only register such as (DEVICE ID, MANUFACTURER ID, SERIAL ID) returns a NACK after each data byte; read/write operation to unused address returns a NACK after the pointer; a read/write operation with incorrect I²C address returns a NACK after the I²C address.

Figure 10. Writing Frame (Configuration Register)

Figure 11. Reading Frame (Configuration Register)

8.5.2.3 Device Measurement Configuration

By default the HDC1008 will first perform a temperature measurement followed by a humidity measurement. On power-up, the HDC1008 enters a low power sleep mode and is not actively measuring. Use the following steps to perform a measurement of both temperature and humidity and then retrieve the results:

1. Configure the acquisition parameters in register address 0x02:
   1. Set the acquisition mode to measure both temperature and humidity by setting Bit[12] to 1.
   2. Set the desired temperature measurement resolution:
      • Set Bit[10] to 0 for 14 bit resolution.
      • Set Bit[10] to 1 for 11 bit resolution.
   3. Set the desired humidity measurement resolution:
      • Set Bit[9:8] to 00 for 14 bit resolution.
      • Set Bit[9:8] to 01 for 11 bit resolution.
      • Set Bit[9:8] to 10 for 8 bit resolution.
2. Trigger the measurements by executing a pointer write transaction with the address pointer set to 0x00. Refer to Figure 12.
3. Wait for the measurements to complete, based on the conversion time (refer to Electrical Characteristics for the conversion time). Alternatively, wait for the assertion of DRDYn.
4. Read the output data:

Read the temperature data from register address 0x00, followed by the humidity data from register address 0x01 in a single transaction as shown in Figure 14. A read operation will return a NACK if the contents of the registers have not been updated as shown in Figure 13.

To perform another acquisition with the same measurement configuration simply repeat steps 2 through 4.

If only a humidity or temperature measurement is desired, the following steps will perform a measurement and retrieve the result:

1. Configure the acquisition parameters in register address 0x02:
   1. Set the acquisition mode to independently measure temperature or humidity by setting Bit[12] to 0.
   2. For a temperature measurement, set the desired temperature measurement resolution:
      • Set Bit[10] to 0 for 14 bit resolution.
      • Set Bit[10] to 1 for 11 bit resolution.
   3. For a humidity measurement, set the desired humidity measurement resolution:
      • Set Bit[9:8] to 00 for 14 bit resolution.
      • Set Bit[9:8] to 01 for 11 bit resolution.
      • Set Bit[9:8] to 10 for 8 bit resolution.
2. Trigger the measurement by executing a pointer write transaction. Refer to Figure 12.
   • Set the address pointer to 0x00 for a temperature measurement.
   • Set the address pointer to 0x01 for a humidity measurement.
3. Wait for the measurement to complete, based on the conversion time (refer to Electrical Characteristics for the conversion time). Alternatively, wait for the assertion of DRDYn.
4. Read the output data:

Retrieve the completed measurement result from register address 0x00 or 0x01, as appropriate, as shown in Figure 10. A read operation will return a NACK if the measurement result is not yet available, as shown in Figure 13.

To perform another acquisition with the same measurement configuration repeat steps 2 through 4.

It is possible to read the output registers (addresses 0x00 and 0x01) during an Temperature or Relative Humidity measurement without affecting any ongoing measurement. Note that a write to address 0x00 or 0x01 while a measurement is ongoing will abort the ongoing measurement. If the newest acquired measurement is not read, DRDYn stays low until the next measurement is triggered.

Figure 12. Trigger Humidity/Temperature Measurement

Figure 13. Read Humidity/Temperature Measurement (Data Not Ready)

Figure 14. Read Humidity and Temperature Measurement (Data Ready)

Table 3. Temperature Register Description (0x00)

Table 4. Humidity Register Description (0x01)

Table 5. Configuration Register Description (0x02)

Table 6. Serial Number Register Description (0xFB)

Table 7. Serial Number Register Description (0xFC)

Table 8. Serial Number Register Description (0xFD)