USB CDC using USBX on F411

mahirmahota · ‎2025-07-28

I am just a bit confused about how the CDC examples using USBX work. In the sample I've seen it seems that the code sets up some sort of UART to USB bridge. I think I don't need the UART and wanted to send and process data like the classic library used to do. However, when I remove the UART code, the device enumerates but USBD_CDC_ACM_Activate is now never called. Is the UART essential to using USBX or did I miss something in refactoring the program?

FBL · ‎2025-08-11

Hi @mahirmahota

The UART in the USBX CDC example is just for bridging USB to a physical UART; it’s not required for USB CDC functionality.
If USBD_CDC_ACM_Activate is not called after removing UART code, it usually means the USB CDC interface is not fully or correctly initialized.
Check that your USB descriptors, endpoints, and USBX CDC class initialization remain correct and complete after removing UART. Make sure also, the CDC interface and endpoints (interrupt IN, bulk IN/OUT) are properly configured and enabled.

To give better visibility on the answered topics, please click on Accept as Solution on the reply which solved your issue or answered your question.

View solution in original post

FBL · ‎2025-08-11

Hi @mahirmahota

The UART in the USBX CDC example is just for bridging USB to a physical UART; it’s not required for USB CDC functionality.
If USBD_CDC_ACM_Activate is not called after removing UART code, it usually means the USB CDC interface is not fully or correctly initialized.
Check that your USB descriptors, endpoints, and USBX CDC class initialization remain correct and complete after removing UART. Make sure also, the CDC interface and endpoints (interrupt IN, bulk IN/OUT) are properly configured and enabled.

To give better visibility on the answered topics, please click on Accept as Solution on the reply which solved your issue or answered your question.