avr python serial communication all data \xff
Possibility 1

Your avr-to-bluetooth connection does not have flow control and receiving side does not verify the stop bit.

There should be a compounding problem of wrong baud rate or just broken transmission.

In this case you'll see bursts of broken data when you expect data and nothing otherwise.

Possibility 2

You are missing a pull-up or pull-down (depending on ttl/rs232 signal), thus every little bit of noise triggers reception of a character.

If this is the case, you will receive a lot more data than you expect, in fact you'll probably receive continuous stream of data. Check number of characters received per second against baud rate / 10 (start, 8 data, stop)