Programming Arduino Getting Started with Sketches

Figure 10-3
Finding an unused IP address
Figure 10-4
A Simple Arduino server example
The listing for sketch 10-01 is as follows:

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As with the LCD library discussed in 
Chapter 9
, a standard Arduino library takes care
of interfacing with the Ethernet shield.
The 
setup
function initializes the Ethernet library using the 
mac
and IP addresses that
you set earlier.
The 
loop
function is responsible for servicing any requests that come to the web server
from a browser. If a request is waiting for a response, then calling 
server.available
will

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return a client. A client is an object; you will learn a bit more about what this means in
Chapter 11
. But for now, all that you need to know is that whether a client exists (tested by
the first 
if
statement); then you can then determine whether it is connected to the web
server by calling 
client.connected
.
The next three lines of code print out a return header. This just tells the browser what
type of content to display. In this case, the browser is to display HTML content.
Once the header has been written, all that remains is to write the remaining HTML back
to the browser. This must include the usual 

and 

tags, and also includes a

header tag and two 


tags that will display the value on the analog input A0 and
the value returned by the 
millis
function; that value is the number of milliseconds since
the Arduino was last reset.
Finally, 
client.stop
tells the browser that the message is complete. The browser then
displays the page.