Re: [avr-gcc-list] measuring uC supply voltage

[Ned Konz]

On Friday 29 October 2004 8:43 am, I wrote:
> So if you keep your 5V Vref, and then make an 3R/R resistive voltage
> divider (you can go pretty high with these resistor values; 3.3Meg/1Meg
> would be fine), you can read the voltage at the resistor junction with one
> of your A/D inputs. Using a 3.3Meg/1Meg, the voltage at 5V VREF would be
> 1.16V (3.3Meg/1.2Meg gives 1.33V)

Note that the higher you go on the resistor values, the longer it'll take 
between choosing the MUX input and being able to get a stable reading, due to 
the capacitors inside the chip that have to charge up.

But unless you drive the voltage divider from an output pin (and so can turn 
it off at will), the little bit of extra power required to wait for the cap 
to charge will be much less than the constant drain of a voltage divider with 
small-value resistors. And this isn't something that needs to be measured 
often, typically.

-- 
Ned Konz
http://bike-nomad.com