Package dht22 implements a Raspberry Pi kernel module for the DHT22 temperature and humidity sensor. The implementation is intended to be an example of how a kernel module could be written. It has no ambitions to be production-grade. In particular, this is not an officially supported Google product.
To build the kernel module, you need to first install the following packages:
sudo apt install build-essential raspberrypi-kernel-headers
To learn more about Linux device drivers, see Linux Device Drivers, Third Edition. To learn more about Linux kernel modules in general, see The Linux Kernel Module Programming Guide. To learn more about the DHT22 sensor, see the data sheet.
To build, simply run make. You can switch debug logging on and off by
editing the Makefile. You can see these logs by running dmesg.
To load the module into the kernel, run:
sudo insmod dht22.ko
Once the module has loaded, you can read sensor data by running
cat /dev/dht22. To have the device accessible by regular users, one must add
the udev rules file 99-dht22.rules to the directory /etc/udev/rules.d/.
A succesul read from the /dev/dht22 device returns three comma-separated
numbers: the timestamp of the most recent successful sensor read, the relative
humidity as a percentage, and the temperature in degrees Celcius. If the
read fails, an error is returned.
*/10 * * * * cat /dev/dht22 >> /home/pi/data.csv
To unload the module from the kernel, run:
sudo rmmod dht22.ko
The module reads data from the sensor when the user opens the /dev/dht22
device. Since the sensor, as per the datasheet, can only be read once every
two seconds the kernel module returns -EBUSY if the user attempts to read the
sensor too soon after the previous read. If the sensor read fails for some
reason—usually due to the checksum validation not passing—the kernel module
returns -EIO.
The protocol for starting a sensor read is to first pull the GPIO pin low for at least 1ms and then pull the pin high and wait for the sensor to respond with an 80µs low pulse followed by an 80µs high pulse. After that initial response, the sensor sends 40 pulses whose widths encode the actual sensor data. The pulses are recorded by an interrupt handler; we simply wait for the read cycle to finish and then process the data that the interrupt handler gathered.
The interrupt handler triggers on a falling edge (high to low) on the GPIO pin. During a sensor read, there are in total 42 falling edges: two during the setup phase and then one for each transmitted bit of information. The transmission of each bit starts with the signal going low for 50 µs. Then signal goes high for 26 µs when 0 is transmitted; signal goes high for 70 µs when 1 is transmitted. We count the time between the falling edges and use 110µs as the boundary between reading a 0 and reading a 1. The threshold is derived from measurements: the long cycles are usually around 125µs while the short cycles vary between 70µs and 95µs.