[you can skip the next part it just demonstrates the problem]
so I was tinkering around with some MOSFETs trying to have an arduino turned on by a button and turned off by itself. I need the Mosfet to control the GND line so I can have the gate voltage higher than the source. The Arduino then had Vcc connected to 5V and D4 connected to the gate of the MOSFET (with pulldown resistor), nothing else. However it always turns on even if the gate isnt pulled up.
I measured the voltage with my dvm and when only Vcc is connected to the arduino, all (yes, all off them) pins are at approx. Vcc - 0.2V. this is not just floating because with a 680 ohm pulldown resistor it can provide ~3mA.
Same when only GND is connected, all pins have a voltage of Vcc - 1V relative to 5V.
So my conclusion is, that without complete Power connected, the arduino behaves like a diode (?) between pins and GND/Vcc.
Did I make any mistakes? why does it happen? how do i prevent it?
I hope this isnt to specific for this section...
thanks in advance