Unknown Alternate Function for USB on STM32 NUCLEO-F303RE

[JoeyWaters]

Hi,

I'm looking at implementing USB on my the NUCLEO-64 STM32F303RE EVAL BRD, with of course the STM32F303RE microcontroller. I'm having issues understanding what alternate function is needed for the USB-DM and DP pins. In the datasheet https://www.st.com/resource/en/datasheet/stm32f303re.pdf Table 13 shows that the USB pins are "Additional Functions" on PA11/PA12. I'm not sure what that means? Additionally I'm used to going to Table 14 the alternate function table to see what AF is needed on those 2 pins for USB. However in Table 14 the USB_DM and USB_DP are no where to be found. 

Your help would be appreciated. 

Thanks

[STOne-32]

Dear @JoeyWaters ,

Welcome in our STCommunity and happy to help on this question:

The USB_DM and USB_DP ( PA11 and PA12) pins are not listed in the alternate function table for the STM32F3 MCUs series datasheet because they are categorized as "Additional Functions" rather than "Alternate Functions."

This means that the USB data signals are directly mapped to the USB PHY when the USB Peripheral is enabled, and there is no need to configure these pins as GPIO alternate functions. From a software point of view, configuring the USB pins in GPIO mode is unnecessary.   In our Recent devices after STM32F3,  we moved them to be Alternate and that mux is mondatory from firmware point of view to configure such as STM32L4 here : Datasheet - STM32L476xx - Ultra-low-power Arm<Sup>® </Sup>Cortex<Sup>®</Sup>-M4 32-bit MCU+FPU, 100DMIPS, up to 1MB flash, 128 KB SRAM, USB OTG FS, LCD, ext. SMPS  see table 16 and Table 17.

Hope it helps you

STOne-32.