I use Stm32 and recently upgrading my design. Current product uses built in USB full speed and I do everything via this port. (Fw upgrade, service, other usual use cases).
In the new design marketing asked for Hs USB and I will add a ulpi phy to make stm32 Hs capable but there is a catch. The Hs operation doesn't allow dfu etc.
So, can I connect the dp/Dm from ha USB to FS port and use them together. The only time the FS port will be used is during dfu where the ulpi won't be working. As soon as the CPU boots I will make FS port high impedance so that it doesn't interfere with the Hs operation.
Best Answer
My reading of the STM32F205xx/207xx datasheet, section 5.3.16 "I/O port characteristics", Table 41. "I/O static characteristics" is an I/O pin has a typical Cio (I/O pin capacitance) of 5pF.
All I can find in the usb_2.pf spec "7.1.6.2 High-speed Input Characteristics", subsection "Lumped capacitance guideline for the transceiver component" is:
"Capacitance to Ground on each line: CHSLOAD ≤ 10 pF"
So I interpret that to mean the STM32F USB FS peripheral pins, when set to GP I/O high impedance, might account for 1/2 the capacitance guideline limit.
There are also a bunch of tighter timing specs for USB HS. Have you got equipment able to measure those accurately enough, or can you hire test equipment?
Do you have any other loading on the USB D+/D- signals, for example ESD protection which might also contribute signal loading?