ESP8266 node firmware update

I made some progress on my ESP8266 node firmware. My goal is to build single, robust, configurable and reliable piece of code for these little IoT devices, to avoid redundant implementations and unnecessary bugs. You can download it from my GitHub. Users should be able to configure it in kconfig interface and create firmware their own IoT device without need to write single line of code or even modifying any header file.

List of esp8266-node updates:

• Added service topics - I added various service MQTT topics which node sends into network when it connects. This is useful for statistics purposes. debugging or for searching nodes with obsolete firmware or hardware versions.
• Device class - I divided nodes into two basic classes:
  • Sensor - Sensor device sends data to network.
  • Reactor - Reactor devices reacts to data from sensors and take some actions. I have already implemented some prototype code, but this part is in the main development.
• Asynchronous sensors - I implemented abstraction layer for synchronous and asynchronous sensors. Synchronous sensor periodically sends data. Asynchronous ones waits for some event and sends data only when event happens (with retain bit set). This feature takes advantage of MQTT to save bandwith and improve response time for a other devices. I'm building a door sensor which reports every time when door status changes (open / closed).
• Initial data publish - Nature of asynchronous sensor it that it sends data only when event happens. I implemented logic which sends sensor state when node becomes online, if the state is relevant.
• Various bug fixes - I fixed some bugs in the code and hopefully improved device reliability.

I have lot of ideas for another improvements, stay tuned :) http://mqopen.org

MQTT node software

In last article I described a hardware aspects of my IoT endpoint device. Lets look at it software now.

Software must handle DHT22 sensor for measurements, implement enc28j60 chip driver, TCP/IP stack for it and MQTT client for sending data to network. My first requirement is to use plain C instead of using any Arduino software. Reason for this is it gives me lot more possibilities, smaller code footprint (I assume) and more convenient way to fixing a bugs. Also once a software is done, it can be very easily reused in another projects.

TCP/IP stack

Most challenging part of node software is TCP/IP stack. Fortunately, there is already quite good implementation called avr-uip, which is uIP project ported to AVR platform.

uIP is minimalistic TCP/IP stack written by Adam Dunkels designed for very small embedded systems. It is designed as event-based framework. It requires some learning, but it's not too hard.

My friend from our local hackerspace already made very similar project which uses this TCP/IP stack. So his work can be used as good reference point.

Only downside is that avr-uip is very outdated, so I had to make some changes to it and fix some bugs. But it works now.

Get it work

With working TCP/IP stack is very easy to get data from the AVR over the network. I found some another projects which implement enc28j60 driver, MQTT client and DHT22 driver, forked them and merged them into my software. I had to create man state machine and after some refactoring and bug fixes my node started to sending MQTT data.

You can find the code on the projects GitHub page. If you want to build your own MQTT node, please read attached readme file for configuration instructions.

Future features

I want to create more user-friendly device, which doesn't need to be manually configured for each build. Main features should be some DHCP client code to dynamically assign node IP address.

Another major feature should be DNS client to resolve local broker IP address from DNS server.

This could allow to develop very cheap plug and play device which doesn't require any configuration. One challenging aspect is very limited resources of used MCU, which has only 2kB of RAM.

Simple MQTT node

In the previous article I described a basic idea of building my network of interconnected devices. In next few articles I'll describe building of very simple sensor with network interface, which can send data using MQTT protocol.

Node requirements:

• Use well known Arduino hardware
• Software written in plain C
• Cheap components
• Scalable design
• Easy to use

MCU

Because I don't want to create my own board, I have to use some existing one. For that reason I decided to use well known Arduino Nano board with atmega328p MCU. I have lot of these in my shelf. This MCU has 32KBytes of flash storage for code, 1KByte of EEPROM as non-volatile storage and 2KBytes of RAM.

I have plan to use Arduino Pro Mini for final product. It costs about 2.50 USD on ebay.

Another advantage of using Arduino hardware is lot of existing code for it. I can easily connect some components, upload existing sketch and check, if my wiring is correct. After verifying that hardware is connected properly, I could start writing my own software. This safes me lot of work by finding bugs in both hardware and software.

Software

Although I'm using Arduino hardware, I'm not using any Arduino software. I decided to implement all software logic in plain C instead.

I thing that Arduino is really great for learning of programming microcomputers or to make quick proof of concept sketch. But its code is quite messy, not efficient and it limits many hardware possibilities. So for making some serious project it is insufficient.

Use of plain C has many advantages. You have complete control about memory usage and its optimization, interrupts, timers, power consumption and other things. You have to also create Makefile, which brings another control in code configuration, compilation and optimization.

Ethernet controller

There are two common Ethernet controller chips. WIZnet w5100 and Microchip enc28j60. W5100 is chip used in official Arduino Ethernet Shield. It has its own TCP/IP stack implemented in hardware and it is well supported by community. But it is twice as expensive as the enc28j60 chip. Because I want to make many of these devices, price is crucial. I had already some enc28j60 modules in my shelf.

Using enc28j60 chip requires more work in software. It requires to implement own TCP/IP stack in software layer. But it seems that this work is already done by someone else.

Sensors

For development purposes I decided to make a node only with DHT-22 (also known as AM2302) sensor. This sensor can measure relative humidity and temperature in 2s time period. DHT-22 is more expensive variant of DHT-11, which has very limited accuracy and measurement range.

I used this sensor mainly for development. Final software should be capable to use any other sensor or device.

Wiring

Finally, wiring of my prototype looks like this.

Next time I'll describe node software and building MQTT brokers.