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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?

Roman Matveev
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    Take care, I have not used it, but the ATMega64 datasheet says 4.5 to 5.5V power supply range (5V only) which will exceed input voltage ratings of the STM32F051 (even the 5V tolerant pins) when the STM32F051 is powered down (VDDIOx + 4.0V absolute maximum rating for 5V tolerant pins, and 4.0V max for other pins). The ATmega64L is rated for lower supply voltage, but I have not studied it's datasheet to provide a proper answer. – Tut Feb 12 '15 at 13:41

2 Answers2

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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.

Wouter van Ooijen
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Yup, i think you can assume that (all GPIO in Z-state). But of course you can force some state (0 or 1) by use pullup or pull down resistors.

MrMehow
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