Electronic Design

1-Wire Technology Simplifies Humidity And Temperature Measurement

A new 1-Wire technology component, the DS2438 Smart Battery Monitor provides several functions. Developed by Dallas Semiconductor, this device offers a unique 64- bit serial number, an internal temperature sensor, and an A/D that measures both the supply voltage (VDD) and the sensor voltage (VAD). It also acts as an integrated current accumulator and an elapsed time meter, while providing 40 bytes of EEPROM memory for nonvolatile storage of data or calibration values.

In this application, the internal nonvolatile memory is used to store the humidity and temperature calibration values as well as the sensor location data (see the figure).

Using 1-Wire technology, the DS2438 transmits both power and bidirectional data over a single twistedpair cable. Therefore, no extra wiring or power is required. (For detailed protocol information for the 1-Wire technology, refer to the Dallas iButton handbook.)

The Honeywell HIH-3605 was selected as the humidity sensor. This device provides a linear voltage-versusrelative-humidity (RH) output. Although the sensor is powered by 5 V dc, it’s capable of operating over a 4- to 5.8-V range. When the supply voltage changes, the sensor’s RH output voltage varies proportionally.

Once the sensor’s RH output voltage and supply voltage have been acquired, the RH value can be calculated according to the following formula:

RH% = \[VAD − (0.157 × VDD)\]/(0.00616 × VDD)

The DS2438 reports its own temperature from an internal 13-bit sensor whose LSB represents an increment of 0.03125°. Both the supply voltage and the RH output voltage are part of the three-wire connection to the humidity sensor.

A Keil C compiler was used to implement the firmware for the 1-Wire communication between the DS2438 and the AT89C51 MCU. This program is necessary for creating the humidity and temperature measurement function. To view the listing, go to www.planetee.com and click on the “Ideas for Design” icon.

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