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Sensirion makes a line of very low power I²C temperature and humidity sensors that are almost trivially easy to interface to a mote using the On-chip SDK (OCSDK).  We only need to make a few changes to the 02-i2c sample to start making measurements with the SHT21. This exercise assumes you have already set up your IAR toolchain and can compile the examples found in the OCSDK.

Hardware Connection

Software

The first thing we need to do is select the correct slave address I2C_SLAVE_ADDR for the SHT21.  I²C uses 7-bit addresses, which are followed by a read/write bit.   The SHT21 data sheet gives the device address as '1000’000’ - it's tempting to interpret this as 0x80, but the OCSDK driver is expecting a right-justified 7-bit value.  The driver shifts the 7-bit word left and inserts the read/write bit automatically depending on the format of the dn_ioctl call. Thus the I2_SLAVE_ADDR is:

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The SHT21 uses 1-byte commands, so we need to change the contents of the buffer for the write.  The SHT21 datasheet defines the command set:

Normally the sensor holds the SCL line to force the master to pause (here during the measurement) until the slave is ready to answer. In the "no hold" versions of the command the sensor does not hold the line - this allows the master to move to another device in a multi-drop scenario.  We use the normal temperature measurement in the sample, so we set the i2cbuffer as follows:

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