Hardcoded battery voltage scaler too low for Witespy boards

What steps will reproduce the problem?
1. Use r1.1 code with default voltage reference (1.1V)
2. Use a Witespy KV modded minimOSD board (22k/1k dividers)
3. Increase GUI calibration to max (255)
4. Voltage displayed on screen is still too low (example, shows 10.2V when it 
should read 12.4V).

I fixed the issue on my project by simply changing the hardcoded voltage 
scalers (mw_osd.ino, lines 588 & 598) from 0.0001 to 0.0003. My voltage display 
then calibrated properly using a GUI value of 104. Center of 255 calibration 
range is 122. So 104 is very close, and should allow proper calibration with 
all currently available Witespy OSD boards.

Using the 5V reference is not a very good option, because resolution is lost. 
Also, users may still need 1.1V for RSSI & current.

The current system checks out OK with basic theories and maths, but in practice 
we may have a problem low input impedance. The mega328 sheet does suggest 
output impedances <10kohm. I haven't measured linearity with the current 
hardware. I impedance is a problem, linearity should be verified. If 
significant non-linearity is found, then maybe the system should be changed to 
use lower value resistors in the future (like 10k & 450)?

Original issue reported on code.google.com by imel...@surewest.net on 3 Jun 2014 at 6:51

LabotteBanana / multiwii-osd

Hardcoded battery voltage scaler too low for Witespy boards #1