Programming the Arduino ethernet shield

The Arduino is an open-source electronics prototyping platform based on flexible, easy-to-use hardware and software. It features an IDE that allows you to program the Arduino using a standard USB cable. Several shields exist that extend the functionality of the standard Arduino platform. In the picture above the Arduino is shown on the left and the ethernet shield is shown on the right.

The ethernet shield adds IP communication, acting as a client or as a server. The most well-known protocols are supported right out of the box, including TCP, UDP and ICMP. Several enthousiasts have added support for even more protocols, such as DHCP and NTP.

Creating programs for the Arduino with the ethernet shield is simple:

#include 

byte mac[] = { 0xDE, 0xAD, 0xBE, 0xEF, 0xFE, 0xED };
byte ip[] = { 10, 0, 0, 177 };
byte server[] = { 64, 233, 187, 99 }; // Google

Client client(server, 80);

void setup()
{
   Ethernet.begin(mac, ip);
   Serial.begin(9600);

   delay(1000);

   Serial.println("connecting...");

   if (client.connect()) {
      Serial.println("connected");
      client.println("GET /search?q=arduino HTTP/1.0");
      client.println();
   } else {
      Serial.println("connection failed");
   }
}

void loop()
{
   if (client.available()) {
      char c = client.read();
      Serial.print(c);
   }

   if (!client.connected()) {
      Serial.println();
      Serial.println("disconnecting.");
      client.stop();
      for(;;)
         ;
   }
}

Every Arduino program has at least two parts: a setup() function and a loop() function. The Arduino IDE creates an actual proper C program around these two functions before it uploads the complete program to the Arduino.

The Arduino IDE contains a serial monitor, which allows you to send strings to the Arduino program or receive strings from it. The example program above shows how the Serial class works. In the setup() function, the serial communication is started at 9600 baud. In the loop() function, data is sent over the serial line with the method Serial.print(). If the serial line is actually connected, the program on the Arduino will halt until the serial monitor is opened. If the serial line is not connected, e.g. when you are done debugging, the program will run normally.

The ethernet library contains the Ethernet class. In the setup() function, the ethernet stack is started with Ethernet.begin(mac, ip). The mac and ip parameters are both byte arrays. If only these two parameters are present and the IP address has the form a.b.c.d, the gateway is assumed to be a.b.c.1 and the subnetmask is assumed to be 255.255.255.0. There are other constructors that take one or two extra parameters for people with different network setups.

After ethernet initialization, the class Client is used to create a TCP connection to the supplied server IP address and port number. The Client.connect() method tries to connect to the server and a call to Client.println() sends data over the TCP connection to the server. The Client.available() method tells if the server has sent data over the TCP connection to our client. If it has, a call to Client.read() gives these bytes.