Here's a curious tidbit you can store away for a rainy day.

I've had good success using the cheesy "capacitor + two resistor" method
for crudely measuring battery voltage with a digital pin. In a new design
I put it on a PORT D pin, one with a schmidt trigger. I knew it would
change the results, but I was getting very strange results. I finally put
a scope on the pin and found that the "on" threshold was not always the
same, it varied at regular times, by about a third of a volt, like a
switch flipping. I turned off as much other activity on PORT D as I could,
but it didn't seem to stop the wacky threshold switching. I finally gave
up and switched some pins around to get my battery pin onto a regular pin.

The chip in question was a 16C74A.

Cheers,
Bob