STM32H7 PLLFRACN Support

[MDS]

Posted on April 19, 2018 at 21:26

Are there plans to fully support the STM32H7 PLL fractional multiplier (PLLFRACN) for all 3 PLLs in STM32CubeMX? The RCC Configuration allows the PLL1 fractional value to be set, but not for PLL2 and PLL3. Setting the PLL1 fractional value is not reflected in the Clock Configuration tab. It would be great if the fractional value for each PLL could be entered in the Clock Configuration tab and that value automatically entered in the Configuration tab. Actually any additional STM32CubeMX support for PLLFRACN would be welcomed.

#pll-fractional-multiplier

[Nawres GHARBI]

Posted on April 23, 2018 at 15:32

Hi Merle, 

All PLL Frac (1, 2 and 3) are supported bu CubeMX. 

In fact you have to set something using to PLL to enable the related configuration.

If the PLL is not used no configuration parameter will be shown in the RCC configuration tab

[MDS]

Posted on April 23, 2018 at 16:40

Once I got PLL3 to be enabled, I did get the Fractional value displayed. My problem was that I assigned PLL3Q to be the USB clock, but when I clicked on the PLL3 DIVN3 field in Clock Configuration tab, I got a message stating something like 'PLL3 requires LPUART or LPTIMER to be enable'. Why won't PLL3Q assigned to USB also enable PLL3? I enabled LPUART just to get PLL3 to enable. This may be a bug.

Another bigger problem is that the fractional value appears not to be included in the Clock Configuration tab calculations. As I am using the fractional multiplier on PLL1, all the system related clocks in the Clock Configuration tab are wrong, but the calculated SystemCoreClock in the code is correct. Maybe the clocks are  just being rounded/truncated Clock Configuration tab. It would be convenient if one could hover over a clock in the Clock Configuration tab and see the actual frequency. Currently if one hovers over a clock in the Clock Configuration tab the label to the right of the clock name is displayed, which is of no help.

[Amel NASRI]

Posted on April 25, 2018 at 18:43

Follow-up in

https://community.st.com/0D50X00009XkYNUSA3

[Amel NASRI]

Posted on April 26, 2018 at 12:47

PS: I checked the particular case for PLL3Q used as clock source for USB and I don't see a message for DIVQ3. You need just to put correct values to get the 48MHz output for USB.

I am using CubeMX 4.25 and following configuration gives me the 48MHz output on PLL3Q:

-Amel