To use freeIMU processing visualization tool, I have implemented float number transmission over uart (default /dev/ttyS2, 115200)
But this not tested yet. I should.
Test flight has been performed with nonlinear SO(3) attitude estimator.
Here are few observations:
 - When the system initialized, roll angle is initially reversed.
   As filter converged, it becomes normal.
 - I put a negative sign on roll, yaw. It should naturally has right
   sign, but I do not know why for now. Let me investigate again.
 - Gain : I do not know what gain is good for quadrotor flight.
   Let me take a look Ardupilot gain in the later.

Anyway, you can fly with this attitude estimator.