For some time I wanted to add ability to provide RSSI from FrSky receiver to APM.

RSSI_PIN parameter works fine for that however some of receivers such as mine FrSky D8R-XP outputs 0 - 3.3 V. For that purpose I've added new param and called it RSSI_RANGE.
This was also issue requested before: https://github.com/diydrones/ardupilot/issues/648
After voltage rescaling it works. rxrssi is shown in scale 0-100.

And added ability to rescale in params:

Here few pictures:

I've used 4.7k resistor and 10uF capacitor to filter pulse signal coming from receiver's RSSI output.

Hope this helps anyone.

Lukasz