why USB is not recognized when I reduce the clock speed? and How to keep it working at low clock speeds

[HAlzo]

I am using STM32h743 , code is generated using STM32CubeMx.

I am using USB_OTG_FS in device only mode, with USB_DEVICE middleware configered as CDC class .

Here's my clock configuration

The usb is clocked from RC48

I could reduce the clock from 480MHz to 120 MHz by changing RCC_OscInitStruct.PLL.PLLP from 2 to 8 .

But when I try to change clock to 96MHz or 80 MHz by changig RCC_OscInitStruct.PLL.PLLP to 10 or 12, the USB is not recognized by windows!!!

I am using windows 10 .

How to overcome this problem?

[waclawek.jan]

JW

[HAlzo]

When I change RCC_OscInitStruct.PLL.PLLP fro 2 to 8 the AHB Clock is reduced from 240MHz to 60MHz,

when I set RCC_OscInitStruct.PLL.PLLP to 10 the AHB clock is 48MHz

when I set RCC_OscInitStruct.PLL.PLLP to 12 the AHB clock is 40MHz

So the AHB Freuquanc is still greater than 30MHz

[waclawek.jan · Accepted solution]

OK so try with decreasing the clock change the AHB prescaler so that AHB frequency stays at or above 60MHz.

Did that change anything?

JW

[HAlzo]

yes, it's working

reducing main CPU clock while keeping AHB clock high solves the problem

[HAlzo]

Sometimes the USB device is mapped to different VCOMPort even that I am using the same serial number string descriptor

and sometimes The device is not recognized by Windows as shown in the attached pics

[waclawek.jan]

Sorry, I don't have a magical answer to all the bugs which come with Cube, or may be introduced by yourself, or Windows, or the used hardware...

JW