AdHoc-Mode sounded create for my use case and works really flawlessly with any computer. Unfortunately Android does not support joining AdHoc networks by default (there are some hacks that involve rooting the phone). Therefore this mode can’t be used here. Ok, so let’s try the other way round – open a HotSpot on your Android phone and let the RN-XV WiFly connect to it. This actually works really good. There’s one limitation here – you can’t open the HotSpot programmatically (not supported by the API). This means that users of your app would need to do that manually (which is not too big of a problem as this is DIY space anyway :-). So here’s how to do it.

First, open the HotSpot on your phone choosing an SSID and a password. Also, you might want to disable that the HotSpot switches itself off if there’s no activity for a certain amount of time. On my HTC One X it looks like this:

Second, configure the RN-XV WiFly module. Do this by putting the module in AdHoc Mode (connect pin 8 to 3.3V) and connect a computer to the network. Then, telnet to 169.254.1.1, port 2000. I used these commands for configuration:

$$$     // Enter command mode

set sys printlvl 0     // Disables debug output
set uart baud 115200     // Set baudrate on UART interface to Arduino
set wlan auth 3
set wlan phrase YOURPASSPHRASE
set wlan ssid YOURSSID
set wlan rate 8      // 6 Mbit/s - according to docs lowering the data rate increases range
save
reboot

Third, put a test program to the Arduino. I used the MultiSerial example from the Arduino site, slightly modified:

void setup() {
  // initialize both serial ports:
  Serial.begin(115200);
  Serial3.begin(115200);
}

void loop() {
  // read from port 1, send to port 0:
  if (Serial3.available()) {
    int inByte = Serial3.read();
    Serial.write(inByte);
  }

  // read from port 0, send to port 1:
  if (Serial.available()) {
    int inByte = Serial.read();
    Serial3.write(inByte);
  }
}

Fourth, test the connection with your computer. Open the serial monitor from the Arduino IDE alongside your telnet connection to the module (still running in AdHoc). You should be able to type something in the telnet session and see it on the serial monitor and vice versa.

Last but not least we need an Android app. I will not go into the details of creating an Android app here (Google provides good docs on it). Here’s the source code for connecting a socket to the first (and hopefully the only) client of your HotSpot and send some chars to it.

InetAddress in;
try {
 List<ClientScanResult> clients = getClientList(true, 500);
 for (ClientScanResult client : clients) {
  Log.d(TAG, "Found client " + client.getHWAddr() + " " + client.getDevice() + " " + client.getIpAddr());
 }

 in = InetAddress.getByName(clients.get(0).getIpAddr());
 if (in.isReachable(500)) {
  Log.d(TAG, "Arduino reachable");
  socket = new Socket(in, 2000);
  for (int i = 0; i < 7; i++) {
   // Read *HELLO*
   socket.getInputStream().read();
  }
 } else {
  Log.d(TAG, "Arduino not reachable");
 }
} catch (UnknownHostException e) {
 Log.e(TAG, "Could not connect: " + e.getMessage(), e);
} catch (IOException e) {
 Log.e(TAG, "Could not connect: " + e.getMessage(), e);
}

Note that the getClientList() method (called in line 3) and the ClientScanResult class is from Whitebyte. Thanks for this!

And that’s it. You now have a wireless connection between Arduino and Android that you can use just like a serial link.