AN4899 un-used pins procedures

[BobaJFET]

IN AN4899, figure 18, it states:

"To reduce power consumption, set
the pin as a push-pull output and
drive it in the same direction as the
internal pull-up / pull-down resistor.
Alternately, set pin as analog input."

I'm a little confused here. If there is already an internal pull-up, for example, then why would you drive it high again in push-pull mode? Why not use open drain with a pull-up/pull-down or simply output low in push-pull configuration? 

STM32 microcontroller GPIO hardware settings and low-power consumption - Application note

I am using STM32H743 

[RomainR. · Accepted solution]

Hello @BobaJFET 

I agree with you and this text in the Pin floating box needs to be clarified in this AN4899.
What should remain a general rule concerning the pins is indicated in the first step of the flowchart.

• No floating pins.
• No pins driving in opposite direction of the internal pull-up / pull-down resistor

The idea behind the text is that if the pin output is set to the same level as the external components are driving it, there is no current, therefore no power consumption.
But in the case of the pin is physically disconnected, the risk is ESD and it also depends on the package.
The universal rule in this case (the least bad) is to configure the unconnected pins as analog input.
We plan to make some corrections in this application note. I cannot yet tell you when the new revision will be available. but it's in our plans

Best regards,

Romain,

[BobaJFET]

Thanks for clarifying. I'll configure them as analog inputs.