Raspberry Pi kernel module for DHT22 (AM2302) sensors

The most reliable dht22 sensor reader!

This project implements a Raspberry Pi kernel module for the DHT22 temperature and humidity sensor. The goal of the driver is to read multiple sensors so that the GPIOs can be dynamically set without reloading the kernel module. The sensor data can easely read from /dev/dht22mX character devices. This kernel module uses hardware interrupts to collect sensor data, which results in much more reliable results (fewer crc errors) than user-mode readers that constantly monitor the status of the gpio.

Introduction & Backgrounds

I was created a project which handle multiple DHT22 sensors. My code collected statistics on the number of successful and failed readings, and I found that some cases approximately 20 percent of the readings were erroneous. It is very big amount of error!

Initially, I attempted to improve the user-space reading code, but the DHT22 sensor transmits data using so very short pulses (20, 50, and 70 microseconds) which makes very hard to read without loosing a bit. Accurately capturing these signals with conventional GPIO polling proved to be unreliable. Due to the operation system task switching, interrupts, and system load, the number of failed readings remained high.

To achieve more precise timing and improve reading accuracy, I turned to kernel module development. By handling GPIO signal processing at the kernel level, the module ensures much finer timing control, significantly reducing read errors and enhancing overall reliability.

Configure sensor GPIOs to read data

You can send your dht22m sensor GPIOs numbers to as sysfs file: /sys/class/dht22m/gpiolist The GPIO numbers are separated by spaces. After If success the /dev/dht22mX devices will be created in same order as the GPIOs listed in the sysfs file.

echo "2 3 22" > /sys/class/dht22m/gpiolist

You can always check the GPIOs settings of your dht22 devices

cat /sys/class/dht22m/gpiolist
2 3 22

ls /dev/dht22*
/dev/dht22m0 /dev/dht22m1 /dev/dht22m2

Note: The kernel module is written in such a way that if a configuration request is received matches the running configuration, recognizes the matching and does nothing. So the user programs can safely send the necessary settings at startup, it will not cause kernel overhead.

Read the values

You can read temperature and humidity values by simple read from /dev/dht22mX device files.

cat /dev/dht22m*
Ok;19.7;38.2
Ok;20.1;42.1
Ok;11.7;26.0

The device files could send the following strings

Received string	Possible events / meaning
Ok;TEMPERATURE;HUMIDITY	Successful read, with the read values.
ChecksumError	Checksum error during the read. Wait 2 sec and read again!
ReadTooSoon	Read the sensor too quickly. Read later!
IOError	GPIO/Sensor/Other error
NotRead	Data not read

Pre-requisites to build the kernel module

To build the kernel module, you need to first install the following packages:

sudo apt install build-essential raspberrypi-kernel-headers

On some raspbian versions, the sources for the running kernel may not be installed from a package. In this case, check the https://github.com/RPi-Distro/rpi-source tool which downloads the current running raspbery pi kernel for you. If you did so, run the following commands to configure soruces as your running kernel:

cd /lib/modules/$(uname -r)/build
zcat /proc/config.gz > .config
make olddefconfig
make modules_prepare

Building and install the kernel module

Clone dht22m sources from github. To compile module enter the dht22m directory and build & install it.

git clone https://github.com/hyper-prog/dht22m
cd dht22m
sudo make
sudo make install

If success, you can load the module:

sudo modprobe dht22m

In case you would like to make the dht22m module loads after reboot run:

sudo echo "dht22m" > /etc/modules-load.d/dht22m.conf

Wiring of DHT22 & Hardware connections

The hardware connection of the dht22 sensor is the same as the method described in user mode reading, which can be found here https://github.com/hyper-prog/DHT22-Raspberry-Pi

References

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.

Author

• Written by Péter Deák (C) [email protected], License GPLv2
• Thanks to the Google and Lars Engebretsen for sensor handling tips and codes!
• The author wrote this project entirely as a hobby. Any help is welcome!

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