Fehler „WiFi-Schild nicht vorhanden“.

Ich habe mir gerade das Arduino Wifi Shield gekauft. Ich habe die Setup-Anweisungen befolgt und ihren Code hochgeladen, aber ich erhalte die Fehlermeldung: "WiFi-Schild nicht vorhanden". Auf dem Schild leuchten keine LEDs. Ich verwende den Arduino Uno 2009. Nicht die neueste R3-Version, also habe ich den Anschluss für die 3,3 V gemacht. Warum funktioniert der Code nicht?

Der Code

/*

 This example  prints the Wifi shield's MAC address, and
 scans for available Wifi networks using the Wifi shield.
 Every ten seconds, it scans again. It doesn't actually 
 connect to any network, so no encryption scheme is specified.

 Circuit:
 * WiFi shield attached

 created 13 July 2010
 by dlf (Metodo2 srl)
 modified 21 Junn 2012
 by Tom Igoe and Jaymes Dec
 */


#include <SPI.h>
#include <WiFi.h>

void setup() {
  //Initialize serial and wait for port to open:
  Serial.begin(9600); 
  while (!Serial) {
    ; // wait for serial port to connect. Needed for Leonardo only
  }

  // check for the presence of the shield:
  if (WiFi.status() == WL_NO_SHIELD) {
    Serial.println("WiFi shield not present"); 
    // don't continue:
    while(true);
  } 

  // Print WiFi MAC address:
  printMacAddress();

  // scan for existing networks:
  Serial.println("Scanning available networks...");
  listNetworks();
}

void loop() {
  delay(10000);
  // scan for existing networks:
  Serial.println("Scanning available networks...");
  listNetworks();
}

void printMacAddress() {
  // the MAC address of your Wifi shield
  byte mac[6];                     

  // print your MAC address:
  WiFi.macAddress(mac);
  Serial.print("MAC: ");
  Serial.print(mac[5],HEX);
  Serial.print(":");
  Serial.print(mac[4],HEX);
  Serial.print(":");
  Serial.print(mac[3],HEX);
  Serial.print(":");
  Serial.print(mac[2],HEX);
  Serial.print(":");
  Serial.print(mac[1],HEX);
  Serial.print(":");
  Serial.println(mac[0],HEX);
}

void listNetworks() {
  // scan for nearby networks:
  Serial.println("** Scan Networks **");
  int numSsid = WiFi.scanNetworks();
  if (numSsid == -1)
  { 
    Serial.println("Couldn't get a wifi connection");
    while(true);
  } 

  // print the list of networks seen:
  Serial.print("number of available networks:");
  Serial.println(numSsid);

  // print the network number and name for each network found:
  for (int thisNet = 0; thisNet<numSsid; thisNet++) {
    Serial.print(thisNet);
    Serial.print(") ");
    Serial.print(WiFi.SSID(thisNet));
    Serial.print("\tSignal: ");
    Serial.print(WiFi.RSSI(thisNet));
    Serial.print(" dBm");
    Serial.print("\tEncryption: ");
    printEncryptionType(WiFi.encryptionType(thisNet));
  }
}

void printEncryptionType(int thisType) {
  // read the encryption type and print out the name:
  switch (thisType) {
  case ENC_TYPE_WEP:
    Serial.println("WEP");
    break;
  case ENC_TYPE_TKIP:
    Serial.println("WPA");
    break;
  case ENC_TYPE_CCMP:
    Serial.println("WPA2");
    break;
  case ENC_TYPE_NONE:
    Serial.println("None");
    break;
  case ENC_TYPE_AUTO:
    Serial.println("Auto");
    break;
  } 
}