Using a pre-build LKM (Loadable Kernel Module) to read temperature and humidity from the SHT21 sensor (I2C).
The same functionality could be achieved by programmatically accessing the i2c device (see i2c directory).
Here's the source code for the kernel module: https://github.com/torvalds/linux/blob/master/drivers/hwmon/sht21.c
The official documentation: https://docs.kernel.org/hwmon/sht21.html
First, power up the BBB and ssh into it.
Then connect the +3.3V and GND pins to the external circuit.
Finally:
# gain root privileges, or use `sudo` each time you're asked
sudu su
# verify that the sht21 kernel module exists
find "/lib/modules/$(uname -r)/kernel/drivers/hwmon" -name sht21.ko.xz
# load the module
modprobe sht21
# let the kernel know to which bus the sensor is connected
echo sht21 0x40 > /sys/bus/i2c/devices/i2c-2/new_device
# the sensor values should now be exposed at /sys/class/hwmon/hwmon0 (or hwmon<some other number>)
cd /sys/class/hwmon/hwmon0
# read temperature, humidity, and eic (electronic identification code) through sysfs
cat temp1_input
cat humidity1_input
cat eicLinux provides a set of tools, named i2c-tools, for interfacing to I2C bus devices.
To install it:
sudo apt install i2c-toolsAmong those tools, i2cdetect is supposed to detect which devices are present on the bus.
Turns out it's not able to detect the SHT21:
i2cdetect -y -r 2Output:
0 1 2 3 4 5 6 7 8 9 a b c d e f
00: -- -- -- -- -- -- -- -- -- -- -- -- --
10: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- --
20: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- --
30: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- --
40: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- --
50: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- --
60: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- --
70: -- -- -- -- -- -- -- --
Check this post for a more detailed description: https://forum.beagleboard.org/t/bbb-debian-10-3-sht21-sensor-not-detected-i2c/32946