c58da9fbc3
IMPORTANT: this change has a docs/UPDATING entry! This rename is unfortunately necessary because NetBSD has decided to create its own service(8) utility, and we will want to import theirs as well. The two can obviously not coexist. Also move ours from /bin to /sbin, as it is a superuser-only utility. Change-Id: Ic6e46ffb3a84b4747d2fdcb0d74e62dbea065039
68 lines
2.3 KiB
Plaintext
68 lines
2.3 KiB
Plaintext
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 minix-service command, but it doesn't seem too useful at
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this time. See the data sheet for the trade-off between faster conversion time
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and 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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/sbin/minix-service up /service/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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/sbin/minix-service down sht21.3.40
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