I'm running an Xbee 868 MHz attached directly to the Arduino and all comms work fine, except when the XBee is not receiving any RF data, it's outputting garbage - an endless stream of 0xFFs.
The datasheet says that the line idles high, but that shouldn't cause the UART to receive data (start bits are low).
I'm using it in transparent mode, i.e. no API frames.
Could it be because I'm not using a level shifter?