Un-bricking a Parrot Ar.Drone 2.0 with a Xadow (arduino)

The Ar.Drone 2.0 creates a wifi access point using network 192.168.1.*, which conflicted with another network I was using on my laptop.  I telnetted in and changed bin/wifi_setup.sh to use 10.0.0. instead, but somehow when I rebooted, the Ar.Drone managed to assign my laptop the illegal address 0.0.0.2, which the rest of the TCP/IP stack refused to cooperate with.

I ordered a CP2102 USB adapter which people said they'd had success using to talk to the 1.8v TTL serial port on the Ar.Drone 2.0, but didn't want to wait for it to arrive.

So instead, I found an arduino board that runs on 3.3v and supports USB serial.  Some arduino variants won't do USB serial at all (even if they can be programmed over USB), and others tie the TX/RX pins to the USB serial.  The latter would have been fine, except that those boards tend to use 5v instead of 3.3v.

The xadow main + breakout boards fit the bill.  Unfortunately, the xadow board requires some tweaks to the arduino installation, but once I had that set up, I was able to use this sketch to link up the USB and TTL serial ports:

void setup() {
  Serial.begin(115200);
  Serial1.begin(115200);
}
void loop() {
  if (Serial.available()) {
    int r = Serial.read();
    Serial1.write(r);
  }
  if (Serial1.available()) {
    int r = Serial1.read();
    Serial.write(r);
  }
}

Then I connected the grounds together (pin3 on the header on the bottom of the Ar.Drone 2.0, accessible by removing a black rectangular sticker).  Pin5 went to rx on the xadow breakout board, then I connected tx to pin6 through a 3.3k resistor to limit the current that I think ends up flowing from the 3.3v xadow board through the protection diodes in the IC in the Ar.Drone that's receiving the data and running at 1.8v.  A proper level conversion would have been better, but the Ar.Drone doesn't seem to mind.

Once I got it connected, plugging in the battery on the Ar.Drone booted it up, and I saw the startup messages in the arduino serial monitor.  I exited out of the arduino IDE, then used 'screen /dev/ttyACM0' to connect to the USB serial port in a way that supports things like colors and arrow keys.

Unfortunately, I didn't look at ifconfig before I fixed the wifi_setup.sh script and rebooted.  The script looked fine, and later I was able to change BASE_ADDRESS back to 10.0.0. and it seemed to work fine.  So the original failure is a mystery to me.