I have an AVRtiny84A in a really space sensitive application where it needs to digitally detect a signal that is either 0V (to detect as 0) or anything between 10V and 20V (to detect as 1). The generic way would be to feed the signal through a resistor to a port pin and add a 4V7 zener (for 5V power) to GND. Since I do not have the space for the Zener (the PCB is 10.1mm x 23.2mm x 1.8mm right now and it cannot be much larger), I would like to do without the zener. This should be possible as I believe pretty much all modern controllers have a zener on port pins anyway, but it requires a careful choice of resistor value. The $100000 question is: "How much current can it sink?" It seems to me Atmel (Microchip) does not list the information in the datasheet?
So if it could sink 1mA, for instance, I'd choose half for safety, and use a (20V-5V)/0.5mA=30Ohms. But Atmel seems to only specify maximum overvoltage, which really isn't enough to settle this question. An input port is really high Z, so if for instance there are no zeners, then even 1MegOhm would drive the input voltage beyond anything the chip can tolerate. For signal integrity I would rather not use such high resistor values unless they are necessary.
Anybody in the know?