I have a question about the STM32H743...

[damon kutten]

If this device is powered-down (i..e voltage is 0 on all rails), what impedance will be presented by the Port pins? If a pin from a powered-down STM32H743 is connected to a pin on another (fully powered) device, what current drain will occur?

[TDK]

Definitely a situation you should avoid.

The protection diodes are going to try to sink as much current as possible. They're either going to sink enough to bring the voltage down to 0.3V, or the diode is going to blow out. Or possibly the device will get backpowered. Not a test I want to run.

Wrong, see below.

[prain · Accepted solution]

when VDD is zero you may damage protection diodes:

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[damon kutten]

Thank you. That answers my question.

[Uwe Bonnes]

Look in the datasheet. FT pins are high impedance between 0 and 4 Volt .

[TDK]

Yeah, I was totally wrong on this. Apologies. Thanks for the correction @Uwe Bonnes.

I tested it on a board as well. PA0 sits happily at 3.3V while VDD=GND=0 with no ill effects. I didn't measure the drain, but I had it hooked up with a 10 kOhm resistor in series and it had a negligible voltage drop, so drain is much less than 1mA.

[damon kutten]

Thanks for the clarification and the additional work you both have done on this issue.

Do you have the location in the STM32H743 datasheet that indicates high impedance?

[Uwe Bonnes]

go to st.com, search STM32H7 and follow the links

[waclawek.jan]

Zero current at any voltage (within given range) means high impedance.

JW

[damon kutten]

This is question for TDK:

You did your test with a 10Kohm resistor and found a "negligible voltage drop".

Is it possible for you to repeat this test with a 100Kohm resistor, or a 1Mohm resistor?