STM32U5A5 USB Device Only Question

[calebsmythpac]

Hi, I'm seeking some advice on using a STM32U5A5 in USB device only via a USBC connection.

Any previous USB "device only" designs I've worked on only required connections to D+, D- and ground to communicate. These designs have been USB2x.

The U5A5 has lots of cool features regarding the USB module etc but we have assigned other features to the pins that are associated with the more the more advanced features like CCx and VBUS SENSE etc. As we plan to use the part in device only mode am I correct in assuming that reallocating the unused pins OK to do and that D+, D- and GND are the only connections required?

We are using the USBC port for a communications link to a PC using CDC, and developing with the associated NUCLEO-U5A5ZJ-Q board. However our product schematic will look like the one in the attached schematic snippet.

Thanks in advance

Caleb

[TDK · Accepted solution]

For a device-only connection, yes, only D- and D+ need hooked up to pins on the chip. CC1/CC2 need a pulldown but don't require a pin connection.

If your device is bus powered, you do not need to monitor VBUS. If your device is self-powered, you need to monitor VBUS which can be done on any pin and turn USB off when VBUS is not present.

[calebsmythpac]

Thanks TDK, really appreciate your reply!

We have a 5K1 to GND connected on both the CC1 and CC2 pins connector side to free those pins up on the MCU.

We are having issues getting the CDC com port working when we set it up in its basic form with the other USB pins allocated to other peripherals on the STM32. Sounds like we need to revisit the firmware as on the NUCLEO it does work but uses all the other pins CCx, VBUS SENSE etc. even tho its configured as "Device Only".

If anyone else had any pointers as to why this would be the case Id love to hear! :) 

Thanks again TDK

[FBL]

Hi @calebsmythpac 

You can reassign the unused pins (e.g., CCx, VBUS sense) to other functions in your design. However, ensure that:

• D+, D-, VBUS and GND are properly connected.
• CC pins have proper pull-down resistors 5.1 kΩ when left unused from connector side.
• DBn could be grounded as well if used.
• VBUS sensing pin is mandatory in self powered devices. Check if your device is bus powered.

[calebsmythpac]

Hi FBL, thanks for you reply. When you say VBUS sensing pin to you mean the pin OTG_HS_VUSB (pin101 LQFP) to be connected to 3V3?: we have not got this pin connected in on our actual product design. We do however have VDDUSB (pin106 LQFP) connected to 3V3.

Thanks for help so far :)

[FBL]

Hi @calebsmythpac 

VBUS sensing is a different IO meant to monitor VBUS, to detect whether the USB cable is connected and whether the USB host is supplying power.