MCBSTM32F200: USB disconnect detection fails

Information in this knowledgebase article applies to:

• MDK v5.x
• Keil MCBSTM32F200 and MCBSTM32F400 board

SYMPTOM

When a Keil MCBSTM32F200/400 board is used as a self-powered USB-Device, it will not be possible to detect a USB-disconnect from a USB-Host.

CAUSE

The Keil MCBSTM32F200/400 board has ESD protection devices (IC5 and IC8 USBCL6-4) connected to both USB interfaces. The VBUS pins of both USB interfaces are connected to the corresponding inputs of the STM32F207/407 (PA9) and the external USB PHY (IC6). The microcontroller and the external USB PHY can connect an internal 1,5K pull-up resistor to the D+ USB signal to signal a connect condition. This resistor is powered by the on-board power supply which is independent of USB VBUS.

When the board is disconnected from a USB host, the VUSB voltage usually drops to LOW and the microcontroller or external USB PHY can detect this condition. Unfortunately, this does not work on a MCBSTM32F200/400 board because the diodes of the ESD protection devices (IC5 and IC8) prevent that the VUSB voltage drops below the D+ voltage.

RESOLUTION

The ESD protection devices IC5 and IC8 can be removed or exchanged with a WE-TVS 824014 (Würth Elektronik) to avoid this issue.

When using Keil MDK middleware, the define USBD_VBUS_DETECT must be set in the file USBD_FS_STM32F2xx.c (USB full speed) or USBD_HS_STM32F2xx.c (USB high speed) (names for STM32F4xx accordingly) in order to enable VBUS sensing for self powered USB devices.

MORE INFORMATION

• Refer to Schematics in the MCBSTM32F200/400 User's Guide.
• Refer to USB Interface in the MCBSTM32F200/400 User's Guide.

Last Reviewed: Friday, January 8, 2021