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Please clarify IO state in standby mode

AMacd.1
Senior

I would like to refer you to the following post from @JRuss.4 that was never satisfactorily answered:

STM32L4 Standby Mode GPIO State {Schmitt Trigger} Clarification from Reference Manual 

I also have the exact same question.   Here is my version of the question boiled down to a couple of sentences:

The STM32l4 series reference manual, RM0394 (Rev 5 - the latest available), on page 158, states, "In the Standby mode, the IO’s are by default in floating state. If the APC bit of PWR_CR3 register has been set, the
I/Os can be configured either with a pull-up (refer to PWR_PUCRx registers (x=A,B,C,D,E,F,G,H)), or with a pull-down (refer to PWR_PDCRx registers (x=A,B,C,D,E,F,G,H)), or can be kept in analog state if none of the PWR_PUCRx or PWR_PDCRx register has been set."

This is confusing.  The statement "In the Standby mode, the IO’s are by default in floating state" contradicts the statement, "or can be kept in analog state if none of the PWR_PUCRx or PWR_PDCRx register has been set".

So, what is it?  Are the IO "floating" or "analog"???

 

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Accepted Solutions
TDK
Super User

They are in analog mode. The schmitt triggers are disabled and the pins are floating. Analog/floating mean the same here.

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3 REPLIES 3
TDK
Super User

They are in analog mode. The schmitt triggers are disabled and the pins are floating. Analog/floating mean the same here.

If you feel a post has answered your question, please click "Accept as Solution".

Thanks @TDK.  Just to clarify further, in a scenario where there are pins left unconnected, these will "float" but actually will be in "analog" mode and, thus, these unconnected pins are in a safe lowest power state.  Is this correct?

However, if any of these pins are connected to an external device, each pin will merely follow the state of the external devices' pins.  So, if the pin on the external device happens to be a CMOS gate input, for example, and there is no external hard pullup or pulldown resistor, then the CMOS input will be floating, an undesirable condition.  Correct?

Therefore, it will be necessary to set the pullups/pulldowns for any pin that connects to an input that may need to be properly terminated for the lowest power operation.  Right?

 

Yes, yes and yes.

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