I have a low power circuit with an almel 2560 chip in it, and it works as an arduino 2560. I need to set some digital ios to high impedance state (by using pinMode(x, input) I guess). But I would like to have these IOs stay in this state after the chips is off, so that the current consumed by these IOs is as low as possible even then the mcu is off. Those ios are connected to some 3V source, and my expectation is that they consumes as low as uA. It this possible?
So far all the method I can think of to fix it is to add diodes to avoid undesired current.
How high does the impedance have to be? Try a series resistor... (If the resistance is too high the input will be sensitive to noise.)
The chip has small (1) internal protection diodes for static discharge and over-voltage protection. These create a "short" to the power supply when the input exceeds the power supply voltage or goes negative. See section 13.1 of the ATmega Datasheet.
When the power is off you are basically trying to power everything through the I/O pin.
(1) By "small" I mean low current. I think they are rated for 1mA so if you apply excess voltage directly from a power supply or high-current source they won't do any good.