kristoiv / multi-am2301

Linux kernel module that supports multiple AM2301 (DHT21) sensors simultaneously connected to Raspberry Pi.

multi-am2301

This repository contains Linux kernel module reading temperature and relative humidity data from multiple AM2301/AM2023 (DHT11/DHT21/DHT22) sensors simultaneously connected to Raspberry Pi via GPIO pins.

This kernel module is an evolution of a module presented on Blackwire Embedded blog. Compilation issues against proc_fs were fixed and multiple sensors support added. It works stable with kernels: Linux raspberrypi 3.12.22+ #691 PREEMPT Wed Jun 18 18:29:58 BST 2014 armv6l GNU/Linux, Linux raspberrypi 4.1.19+ #858 Tue Mar 15 15:52:03 GMT 2016 armv6l GNU/Linux, Linux raspberrypi 4.4.26+ #915 Thu Oct 20 17:02:14 BST 2016 armv6l GNU/Linux. It should work with other kernel versions as well unless some major API changes were done in the kernel.

Latest measurement data from connected sensors is available under /proc/multi-am2301 and can be displayed anytime using e.g. following command: cat /proc/multi-am2301. For each connected sensor it contains parameters like:

• temp_curr - value of last temperature measurement in Celsius degrees
• temp_1m - average value of temperature for last 1 minute
• RH_curr - value of last relative humidity measurement
• RH_1m - average value of relative humidity for last 1 minute
• date - formatted measurement date and time
• timestamp - measurement timestamp (millis from Epoch)
• QUAL - how many sensor read attempts were there and how many of them were successfull. Note: idling Raspberry Pi has QUAL more than 90% for each correctly connected sensor, however when CPU is fully loaded or lots of interrputs occure (e.g. when reading lots of data from serial communication etc.) QUAL will drop down.

Repository layout

Kernel module sits inside module directory. Example commandline script which logs temperature and humidity into sqlite database sits inside scripts directory. This script can be invoked by system cron e.g. every 3 minutes.

Installation

• clone this repository
• cd module
• in source file decoder.c find static int _pins[] = {23, 24, 25}; array initialization and correct GPIO logical pin numbers according to your needs (don't use board physical pin numbers!)
• please check /lib/modules/VERSION/build directory in Makefile against your OS and correct if needed. Note: you need kernel headers present in your system to be able to build kernel modules. You may want to check project rpi-source
• execute make
• sudo cp multi-am2301.ko /lib/modules/{VERSION}/kernel/drivers/
• sudo depmod
• to load kernel module: sudo modprobe multi-am2301
• to unload kernel module: sudo modprobe -r multi-am2301

Additionally to load this kernel module on Raspberry Pi startup add multi-am2301 to /etc/modules configuration file.

Have fun :)