is anybody familiar with STM32F446RE and external clock ?

[MNapi]

I tried to sort through the manual. When I choose external clock do i actually connect anything to the PH0 and PH1 pins and what kind of oscillator.

I see that there is already what looks like 8 MHz crystal is this one I am connecting when using external crystal

[S.Ma]

It's either a crystal (on 2 pins to oscillate) or a digital clock feed on OSCIN.

Clock feed on Nucleo can be the 8MHz coming from the STLink.

Why? Clock generators have some tolerances which depends on voltage, temperature and process.

This might have impact such as RS232 at high baudrate.

STM32 has a safety fallback to the 16MHz internal RC oscillator if the external one fail to operate properly.

Look at the reference manual and datasheet.

[Tesla DeLorean]

Depends on the board, the DISCO/NUCLEO boards typically feed a 8 MHZ signal derived from their own clock and output via the PA8 MCO pin of the F103 part and into the HSE/OSC_IN pin. On the F4 side you'd ideally be using HSE BYPASS mode.

The DISCO/NUCLEO schematics should show the option for crystal usage, and there is a whole Application Note on crystal/load capacitor selection.

[MNapi]

as usual STM documenation lacks good examples, I see some drawing, there is some resistor between pin and crystal. And it says the resistor value depends on crystal.

and how am I supposed to find this out. they do not give you part number, values like whoever wrote the manual had no clue what to do.

[Tesla DeLorean]

Only matched with your inability to find things

https://www.st.com/content/ccc/resource/technical/document/application_note/c6/eb/5e/11/e3/69/43/eb/CD00221665.pdf/files/CD00221665.pdf/jcr:content/translations/en.CD00221665.pdf

Many of the boards actual have BOM documentation you can download

https://www.st.com/resource/en/bill_of_materials/nucleo_64pins_bom.zip