68 lines
2.2 KiB
Text
68 lines
2.2 KiB
Text
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SHT21 Driver (Relative Humidity and Temperature Sensor)
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=======================================================
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Overview
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--------
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This is the driver for the relative humidity and temperature sensor commonly
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found on the WeatherCape expansion board for the BeagleBone.
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Interface
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---------
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This driver implements the character device interface. It supports reading
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through /dev/sht21b{1,3}s40. When read from, it returns a string containing
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a data label, a colon, and the sensor value.
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Example output of `cat /dev/sht21b3s40`:
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TEMPERATURE : 35.014
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HUMIDITY : 25.181
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Temperature is expressed in Celsius (a.k.a. centigrade). Valid values are
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-40.000 to 125.000.
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Humidity is expressed as a percentage. Valid values are 0.000 to 100.000.
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Limitations
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-----------
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Intense activity causes the chip to heat up, affecting the temperature reading.
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In order to prevent the chip from self-heating more than 0.1C, the sensor
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values will only be read once per second. Subsequent reads within the same
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second will return cached temperature and humidity values.
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The measurement resolution is configurable in the chip, but this driver just
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uses the default maximum resolutions (12-bit for Humidity, 14-bit for
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temperature). It could probably be implemented with an ioctl() or by passing
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an argument via the service command, but it doesn't seem too useful at this
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time. See the data sheet for the trade-off between faster conversion time and
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lower resolution.
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In testing, the temperature sensor reported a value several degrees higher
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than an indoor thermometer placed nearby. It doesn't appear to be a bug in the
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driver as the Linux driver reports similar temperature. Additionally, the
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BMP085 temperature sensor on the same cape reports a temperature about 2
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degrees lower than the SHT21. This could be due to heat produced by the
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BeagleBone heating the cape slightly or maybe just a bad chip on the test
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board.
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Testing the Code
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----------------
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The driver should have been started by a script in /etc/rc.capes/ If not,
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this is how you start up an instance:
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cd /dev && MAKEDEV sht21b3s40
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/bin/service up /usr/sbin/sht21 -label sht21.3.40 -dev /dev/sht21b3s40 \
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-args 'bus=3 address=0x40'
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Getting the sensor value:
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cat /dev/sht21b3s40
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Killing an instance:
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/bin/service down sht21.3.40
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