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

Compiling and using PyNaCl on Windows 7

Step-by-step how to compile PyNaCl Python package on win32.

Requirements:
Python 2.7.x for Windows - download it from here.

libsodium-1.0.2-msvc - download precompiled binaries from here.
Microsoft Visual C++ Compiler for Python 2.7 - you can download MS C++ compiler for Pyhton from here.

This guide is based on https://github.com/pyca/pynacl/issues/100 and and is a focused reiteration of it.

Python for Windows is compiled with MSVC and because of that it is not possible to compile extensions with MinGW/MSYS. The main problem is with CFFI (Common Foreign Function Interface) and most probably if you succeed with compiling, the lib will not work (will hang if trying to use its functions)

1. Download and install Python for Windows

2. Download and install Microsoft Visual C++ Compiler for Python 2.7 

3. Install python setuptools (needed for MSVC++ Python) with pip:

C:> pip install setuptools

4. Download libsodium latest release with -msvc at the end. Unzip it in C:\work\libsodium-1.0.2-msvc

5. Go to C:\work\libsodium-1.0.2-msvc\Win32\Release\v120\dynamic and rename libsodium.lib to sodium.lib. On 64bit Windows use this dir: C:\work\libsodium-1.0.2-msvc\x64\Release\v120\dynamic

6. Download PyNaCl source and unzip it to C:\work\PyNaCl-0.3.0

7. Start the MSVC++ Python shell: Start -> All Programs -> Microsoft Visual C++ Compiler Package for Python 2.7 -> Visual C++ 2008 32-bit Command Prompt

8. Set these 3 variables in cmd prompt:

set INCLUDE=%INCLUDE%C:\work\libsodium-1.0.2-msvc\include
set LIB=%LIB%C:\work\libsodium-1.0.2-msvc\Win32\Release\v120\dynamic
set SODIUM_INSTALL=system

on 64bit Windows change:
set LIB=%LIB%C:\work\libsodium-1.0.2-msvc\Win32\Release\v120\dynamic
to:
set LIB=%LIB%C:\work\libsodium-1.0.2-msvc\x64\Release\v120\dynamic

9. Probably you will need these two files when compiling:

http://msinttypes.googlecode.com/svn/trunk/stdint.h
http://msinttypes.googlecode.com/svn/trunk/inttypes.h 

Download and copy them to: C:\work\libsodium-1.0.2-msvc\include

10. Start the building process:

C:\>cd C:\work\PyNaCl-0.3.0
C:\work\PyNaCl-0.3.0>python setup.py build

11. If everything is ok, then install it:

C:\work\PyNaCl-0.3.0>python setup.py install

12. Finally, copy the original libsodium.dll in PyNaCl install dir:

C:>copy C:\work\libsodium-1.0.2-msvc\Win32\Release\v120\dynamic\libsodium.dll C:\Python27\Lib\site-packages\PyNaCl-0.3.0-py2.7-win32.egg\nacl\_lib

Here is a test program (from doc examples https://pynacl.readthedocs.org/en/latest/public/) slightly modified:

import nacl.utils
from nacl.public import PrivateKey, Box

skbob = PrivateKey.generate()
pkbob = skbob.public_key  

skalice = PrivateKey.generate()

pkalice = skalice.public_key 

bob_box = Box(skbob, pkalice)
message = b"Kill all humans"
nonce = nacl.utils.random(Box.NONCE_SIZE)

encrypted = bob_box.encrypt(message, nonce)
print "Encrypted Message:", encrypted
alice_box = Box(skalice, pkbob)

plaintext = alice_box.decrypt(encrypted)
print "Plaintext Message:", plaintext

If it works, you should see something like:

C:\work>python nacltest.py
Encrypted Message: ₧╫fαIé├l(α▀W¬½♥↔≈‼╟  üRδD≈é☻'^∞v√oòΣls╣8,ƒ   ↓ü↓╓+ô╓è╣=§╣
Plaintext Message: Kill all humans
C:\work>