nucleo board STM32CubeMX clock configuration nonsense

[SCott.1]

Hi,

I'm trying to set up the HSI (16MHz) on a nucleo-f446ze board. The clock configuration GUI is kinda misleading I guess:

• default PLLMUX configuration is selecting HSE, but HSE oscillator is not present by default on the nucleos
• selecting HSI on PLLMUX highlight an error that goes like "PLLMUX should have an HSE input"....is this just a warning (wording is "should") ? if not and this is a proper error and is not possible to select HSI why is there a MUX at all?

with that said, what is the correct way (if any) to use the HSI with PLL?

hope someone can shed some light.

cheers

Stef

[TDK]

An HSE crystal is not present, however there is a clock signal coming from the ST-Link chip (at 8 MHz). This is being used in HSE bypass mode.

To use HSI with PLL, select HSI as the PLL Source Mux just as you've done.

The warning is there because you're using USB which requires a very precise clock. The HSI cannot meet the timing requirements of USB, so CubeMX is flagging a warning.

[SCott.1]

Hi @TDK​ thanks for the heads up. So, to test my project where a 8MHz crystal will be there (as well as USB so timing is crucial), I should leave everything as it is now on the nucleo and later disable HSE bypass as I move to real board.

thanks for the help

stef

[Pavel A.]

What is interesting in these Nucleos - the default 8 MHz HSE input that STM32 receives from on-board ST-LINK is actually HSI output of the ST-LINK:

So it is not obvious how good it is. The other option is HSE of the ST-LINK but then the frequency is 8.33... (25/3) MHz.

[TDK]

> What is interesting in these Nucleos - the default 8 MHz HSE input that STM32 receives from on-board ST-LINK is actually HSI output of the ST-LINK:

On most boards, the ST-Link crystal is 8 MHz. Only on some (possibly newer/faster?) boards is the HSE 25 MHz.

On the board in question, it's an 8 MHz crystal and there's no option to change it within the ST-Link firmware upgrade. Presumably, it is using the actual HSE as the clock output.