Mediatek LinkIt Smart 7688 with DHT22/11 sensor on custom LEDE/OpenWRT image

Examples will be with LEDE-Project latest sources from git.

check-humidity-7688 is a fork of Onion Omega2p checkHumidity which reads temperature values directly from memory.

We need LEDE Project / OpenWRT source configured for LinkIt Smart 7688 and compiled at least once. Kernel should be compiled with /dev/mem support as this is the way check-humidity-7688 works.

The checkHumidity binary should work on mediatek 7688 without problems but if you need to recompile it for different arch/platform here is how it is done:

getting the source and compiling it:

$ git clone https://github.com/devane/check-humidity-7688
$ cd check-humidity-7688
$ make clean
$ sh xCompile.sh -buildroot /home/user/lede

Cleaning...
 rm -f -r build bin
 /home/user/lede/staging_dir/toolchain-mipsel_24kc_gcc-5.4.0_musl/bin/mipsel-openwrt-linux-g++ -c -g  -I include -c -o build/fastgpioomega2.o src/fastgpioomega2.cpp
 /home/user/lede/staging_dir/toolchain-mipsel_24kc_gcc-5.4.0_musl/bin/mipsel-openwrt-linux-g++ -c -g  -I include -c -o build/main.o src/main.cpp
 /home/user/lede/staging_dir/toolchain-mipsel_24kc_gcc-5.4.0_musl/bin/mipsel-openwrt-linux-g++ -c -g  -I include -c -o build/module.o src/module.cpp
 /home/user/lede/staging_dir/toolchain-mipsel_24kc_gcc-5.4.0_musl/bin/mipsel-openwrt-linux-g++ -c -g  -I include -c -o build/dht_read.o src/dht_read.cpp
 /home/user/lede/staging_dir/toolchain-mipsel_24kc_gcc-5.4.0_musl/bin/mipsel-openwrt-linux-g++ -c -g  -I include -c -o build/common_dht_read.o src/common_dht_read.cpp
 Linking...
 /home/user/lede/staging_dir/toolchain-mipsel_24kc_gcc-5.4.0_musl/bin/mipsel-openwrt-linux-g++ build/fastgpioomega2.o build/main.o build/module.o build/dht_read.o build/common_dht_read.o -o bin/checkHumidity

We can check if it is compiled for the right architecture:

$ file bin/checkHumidity
bin/checkHumidity: ELF 32-bit LSB executable, MIPS, MIPS32 rel2 version 1 (SYSV), dynamically linked, interpreter /lib/ld-musl-mipsel-sf.so.1, with debug_info, not stripped

Now just copy the binary on your mt7688 and it should work just fine.

Notes:

xCompile.sh is a shell script (included in source) from MediaTek documentation page on how to cross-compile your own program. You should only point where is the source of the LEDE-Project / OpenWRT with -buildroot option.

Wiring diagram connecting DHT11 sensor to LinkIt Smart 7688, signal (data) wire is connected to pin 26 - GPIO19.

DHT11 Pinout

Example output of checkHumidity:

root@LEDE:~# ./checkHumidity 19 DHT11
33.000000
26.000000

LEDE Project/OpenWRT DHT11/22 humidity and temperature sensor on Raspberry Pi 3

Using latest developement version of LEDE Project from git

Kernel config rpi3: lede-project-kernel-config-rpi3.txt (rename it to .config and put it in lede main directory)

Differences between DHT11 and DHT22 are in accuracy of returned values. DHT11 works with interger values (example: 23 C) and DHT22 is more precise (23.6 C)

Pins connected (DHT22):
DHT22 - RPI3
=========
Data - GPIO4
Vcc - Vcc
Gnd - Gnd

Boot rpi3 image and then edit /boot/config.txt adding the following line:

dtoverlay=dht11,gpiopin=4

Save and exit and then reboot the rpi3

Under /sys/devices/platform/dht11@0/iio:device0/ you will find two files

in_temp_input
in_humidityrelative_input

By reading them you will get current values of the dht11/22 sensor.