I'd like to make a device combined two controllers: ATMega64 I already familiar with and ARM Cortex-M0 STM32F051 I'd like to be familiar with.
I'd like to directly connect GPIOs of two controllers. I hope that I will be able to choose which controller is now operating just providing the power to the one I need.
So the question is: if one of the controllers is unpowered will it's GPIOs interfere with GPIOs of powered one? Can I assume that all GPIOs are in Z-state while controller has no power?
Answer
No, in general you can not assume anything about GPIO pins of an unpowered chip, unless this is explicitly stated in the datasheet.
What will likely happen is that the protection diodes in the chip will try to power the 'unpowered' chip from any pin that is high.
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