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Content:
By Guido Socher (homepage)

 

Using the AVR microcontroller based web server

[Illustration]

Abstract:

There are two related articles which describe how to build the AVR web server discussed here:

  1. An AVR microcontroller based Ethernet device

  2. HTTP/TCP with an atmega88 microcontroller (AVR web server)

The first article describes the hardware and the second introduced the web server software.

In this article we will discuss now some ideas on how to use this hardware and the web server software. The web server can be used towards the open internet if some additional safety measures are taken. My suggestion is to use a cgi-proxy script. The cgi-script can be downloaded at the end of this article.

The components for building this avr web server can be ordered from http://shop.tuxgraphics.org
_________________ _________________ _________________

 

Switching on and off a printer

I like my xerox laser printer because it is reliable and prints in good quality. However normally I print a couple of pages in a row and then for a long time nothing. The printer will go into sleep mode after 15 minutes but until then it has a loud fan an consumes a lot of power. I could walk to the printer, switch it on, go back to the computer, print, walk back to the printer and switch it off but it would be much nicer if one could switch on/off the printer remotely.

Remote control is especially convenient when I am sitting on a computer in a different room or with a laptop in the garden. This is how I use the avr web server:
[avr web server, printer power saving]

 

Preparing fresh coffee for the break while you are giving a presentation

This is an idea which a customer had who bought the kit at shop.tuxgraphics.org.

You attach a coffee machine to the avr web server and fill the coffee machine with water, coffee powder,... Now you can give a nice and inspiring presentation to your customers. Shortly before the end of your presentation you can switch on the coffee machine directly from your computer and without leaving the room. .... and you can serve fresh coffee for the break.
[avr web server, coffee machine control]

Note: Be careful and use proper insulation in a possibly "wet" environment near the coffee machine.  

Control your hardware over the public internet

With a network enabled microcontroller such as our avr web server you can of course control anything from anywhere in the world if you connect this device to the internet.

Mobile phones have these days integrated web browsers. In other words you don't even need a computer to switch on/off a coffee machine, control the heating in your house,...

The software for the avr web server (eth_rem_dev_tcp-2.X.tar.gz) is however optimized for size and is not made to handle attacks or mal-formed packets. It works well in a friendly LAN environment but on the open internet you have to be ready for other dangers.

All DSL routers include these days already a firewall and something called NAT (network address translation). This NAT feature allows you to use the internet from several computers at the same time. Inside you network you use a private address range (e.g 192.168.0.1 up to 192.168.0.254) while your internet service provider has actually given you only a single public address e.g 216.109.112.2. The translation (NAT) between private and public addresses is possible because udp/tcp uses also two (client and server) 16 bit numbers called port numbers for each connection.

A web server runs normally at port 80 but you can run it at any port. The avr web server offers also a port number configuration (look at main.c of eth_rem_dev_tcp-2.X.tar.gz). Typing http://your.domain:80/ is the the same as http://your.domain/. In order to make a web server in your local network available through the NAT firewall of your DSL router you need to tell the router to forward a given external port to an internal IP address and port number. Not all DSL routers can do that. Here is an example of a netgear router which provides this forwarding functionality.

[netgear port forwarding]


It will forward with those settings a request to the external IP address and port 80 to the internal IP address 192.168.0.2 and port 80. E.g like this: 
http://216.109.112.2:80/  --->  192.168.0.2:80
We could now run the avr web server at 192.168.0.2:80 and it would work:
[how it looks like on a mobile phone]
First tests with a mobile phone.


However I don't recommend to connect the avr web server directly to the internet except for some initial testing purposes. It is better to put an apache web server at 192.168.0.2 and let it forward the request via a cgi script to the actual avr web server (e.g at 192.168.0.5). This has several advantages. The setup looks now like this:
[harware control through the internet]


Here is a screen shot of how the web page generated by the cgi-script looks like.

[harware control through the internet, cgi-script]

The file which you can download at the end of the article contains two cgi scripts written in perl. One uses udp for the requests towards the avr ethernet device and the other http. A README file is included which explains further details.  

How much should I be concerned about security?

I have to tell you about the risk and what to do about it. However it is also a fact that many others who sell embedded web servers don't tell you about this risk. That does not mean that other solutions are more secure.

If you don't publish the link on your homepage and don't tell people that this is an embedded web server then the risk very minimal. I use it also over the public internet without the extra apache server.  

Dynamic DNS

Most ISPs will give you a dynamic IP address. That is: your external IP address changes every time you switch on/off your router or even worse most telecom companies will break the connection every 24 house or every 12 hours or every 3 hours... It depends on the fine print in your contract. If you can get a static IP address from your ISP/DSL provider then go for this solution. It makes things much easier. If you can not get it the you can use a service like http://www.dyndns.com/services/dns/dyndns/. Some DSL routers do even have build-in support for dyndns.com. A list of supported hardware is available from their web site. dyndns.com works basically such that a small "update-client" software runs on the server in your local LAN and reports the current IP address to dyndns.com. This IP address is then aliased to a DNS name.

If your not sure how often your external IP address changes or if you even have a static IP then check your external IP address once in a while using this page: http://checkip.dyndns.org/. It tells you your current external IP address. You can use this also during first tests to find your external IP address.

Finally a word on reliability: My personal experience with the local telecom company which provides DSL connection here is that their service is pretty unreliable. It happens quite often that DSL is down for 2-3 hours per month especially at night. In some cases you even have to call them to do a DSL port reset on their equipment because it died again. I know from discussions with friends that it is not better in other countries. In other words don't use such a setup when 100% reliability is important.  

More than just switching on/off

All the above ideas and examples where about switching on or off a single device. This is because the current example software (eth_rem_dev_tcp-2.X.tar.gz) has this functionality. The hardware has however a number of free IO-ports and ADC input lines and could switch multiple devices and even measure voltages.

The next extension to this circuit will add two temperature sensors and an air pressure sensor. This will allow you to build a little weather station. You can have the data from the sensors then instantly available in your whole network readable with any web browser.
[ethernet weather station]

 

Analog data presented on the public internet

The same data can of course also be made available over the public internet using a DSL connection similar to the example above. All this can be done with just minor modifications to the cgi-script.
[ethernet weather station, over the internet]


The apache web server is again there for security reasons. It's kind of a firewall for avr web server. You can however use it for more. You can store historic data and make that available too.  

Automation and scripting

With a simple web interface we have not only a user interface for humans. It is also possible to use it as a machine interface. It is so easy that this can even be donw from a unix shell script. All we need for this is a command line ASCII only web browser: On top of all those web browsers you have also the option to modify the cgi perl script (see download at the end of this article).

Any of the following 4 commands will e.g switch the hardware connected to the avr web server on: 
w3m -dump http://10.0.0.24/secret/1
or
lynx -dump http://10.0.0.24/secret/1
or
wget -q -O - http://10.0.0.24/secret/1
or
elinks -dump 1 http://10.0.0.24/secret/1

 (please change IP address and password as needed. This example
  refers to the switch on/off function as provided by the
  eth_rem_dev_tcp-2.X.tar.gz avr web server software).

To query the status you can e.g write a UNIX shell script like this one: 
#!/bin/sh
# Query the status of the relay connected to the avr web server.
# Needs the eth_rem_dev_tcp-2.X.tar.gz software on the avr web server.
#
# the following just retuns the html line where is says "Output is:" on or off
htmlcode=`w3m -dump http://10.0.0.24/secret/| grep "Output is"`
#
if  echo "$htmlcode" | grep -i "on" >/dev/null ; then
    echo "The output is on"
elif echo "$htmlcode" | grep -i "off" >/dev/null ; then
    echo "The output is off"
else
    echo "Error: no output state, avr web server may be disconnected"
fi
#
You can save this in a text file called getavrdevstatus, make it executable (chmod 755 getavrdevstatus) and you have a command which report the status.

This is of course a very simple example as such but it can be modified for different things. E.g give water to your flowers every day. Just connect a little pump.... The beauty of a simple web interface is that you can use it as a machine interface and a interface for humans at the same time to give e.g you plants some extra water ;-).  

Conclusion

I find it really fascinating what one can do with a small web server. Suddenly the microcontroller is not anymore a standalone piece of electronics. It becomes part of a network.

It is however not only a web sever providing web pages. It is an interface to other hardware. This makes the number of applications almost endless and it is completely different from a large web server running on a PC.  

Download and links


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© Guido Socher, tuxgraphics.org

2007-07-14, generated by tuxgrparser version 2.55