+ Voltage values are reported as raw ADC values, so you
+ need to know the voltage divider in front of the ADC
+ as well as the ADC range. Boards with a 15 volt range
+ use 100k/27k divider, boards with a 30 volt range use
+ 100k/12k. All Mega boards have a 12 bit ADC so the
+ values range from 0 to 4095, and they all use a 3.3V
+ reference, so an ADC value of 0 represents 0V and an
+ ADC value of 4095 represents 3.3V. In sum, to convert
+ a raw ADC value to a voltage, take the value, divide
+ by 4095, multiply by 3.3 then multiply by (adc_hi +
+ adc_lo)/adc_lo, where adc_hi is 100k for both ranges
+ and adc_lo is 27k for 15V boards and 12k for 30V
+ boards.
+