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I want to implement STM32F746NG on my PCB - the one from the STM32F746-DISCO with display and SDRAM, Flash and other peripherals.

The MCU has a PDR_ON pin, which I have never met before. I searched for it and found different threads and STMicroelectronics documents, but ST documents are usually very cryptic when you read them for the first time and not familiar with topic yet. So I would want someone to make sure I understand stuff correctly and, if possible or necessary, "dumbify" it a bit.

What I understood about PDR_ON/Internal Power Supervisor
PDR_ON controls internal power supervisor, which is an internal device(if that word is applicable?) that basically makes sure the voltage on VDD is sufficient and stable, and only then it releases reset and lets MCU start. Tying PDR_ON to VDD turns it on, tying it to VSS turns if off. If the internal supervisor is off, then some external circuit should release reset with a little pause after the voltage is above minimal threshold.
Did I understand it correct?

About power supplies in my specific project
In my project the MCU will get a regulated 3.3V supply (via buck-boost from battery); Same supply goes to VDDUSB, VBAT (not expecting to use USB or RTC) and via ferrite bead is fed into AVDD and VREF+ (will use ADC/DAC). I will have a 10k from 3.3V to Reset pin (although I hear it's not mandatory due to internal pull-up, but it won't hurt anything anyway; I can always leave pads unpopulated) and the reset pin will have an external 0.1uF cap. All of this, I believe, will give sufficient pause for reset release.

PDR_ON state selection
Quote from the document attached above AN4938, page 18:

The supply ranges which never go below 1.71 V minimum should be better managed by the internal circuitry (no additional component needed, thanks to the fully embedded reset controller).

Does it mean I just connect PDR_ON to VDD and I'm done?

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If you do not wish to do anything special, just tie PDR_ON to Vdd and the MCU will work just like any other which does not have the PDR_ON pin.

See chapter 2.17.1 in the Datasheet:

On packages embedding the PDR_ON pin, the power supply supervisor is enabled by holding PDR_ON high. On the other packages, the power supply supervisor is always enabled.

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  • \$\begingroup\$ Yep, I read exactly that passage too, but it didn't actually mean anything useful to me. Do I even need to enable to supervisor or not? That is the question. To which you have given answer too, so thank you. I will let the post sit a little in case anyone else has anything to say. \$\endgroup\$
    – Ilya
    Dec 2, 2021 at 14:34
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    \$\begingroup\$ @Ilya Maybe you could get away with disabling the supervisor. But the supervisor does add an extra layer of protection against incorrect reset behavior. So unless you have any special requirements regarding low power consumption, why bother disabling it? \$\endgroup\$
    – Klas-Kenny
    Dec 2, 2021 at 14:48
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    \$\begingroup\$ because I wasn't sure about what it is and what it does and whether I need it :D ST documentation is very cryptic unless you're already familiar with the topic. Similar to mathematics on wikipedia. No chance to learn stuff from there, but very good at refreshing knowledge. \$\endgroup\$
    – Ilya
    Dec 2, 2021 at 14:51
  • \$\begingroup\$ @Ilya, yeah you're not wrong. Their doco is pretty useless on some topics. We have a board where PDR_ON is grounded; and we're not sure whether to just leave it be, or if it's worth the effort to re-route and reassemble all the boards.. \$\endgroup\$
    – NPras
    Feb 17, 2023 at 4:17

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