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.

  • \$\begingroup\$ Would you please clarify a bit? You are using an STM32F which has both USB Hi-Speed and USB Full-Speed peripherals? You want to share a single USB socket between the two STM32F USB peripherals? The STM32F USB HS peripheral needs a phy. The two different STM32F USB peripherals will ever be active at the same time. You want to connect D+/D- (DP/DM) from the USB socket to both the USB HS phy, and the STM32F USB FS peripheral pins? \$\endgroup\$
    – gbulmer
    Sep 13, 2014 at 12:33
  • \$\begingroup\$ I'd be raising a support case with ST instead of asking here, particularly for a commercial product. They should actually know instead of just guessing. \$\endgroup\$
    – markt
    Sep 13, 2014 at 13:17
  • \$\begingroup\$ @markt I asked. They don't know. They asked me to try. \$\endgroup\$
    – Ktc
    Sep 13, 2014 at 15:43
  • \$\begingroup\$ @gbulmer to clarify I don't want to use them at the same time. I still want to be able to use FS dfu mode. But once the processor boots I will configure the FS pins as gpio input pins but don't do anything with them. I will configure ulpi interface as hi speed and stop. The only down side i see is that the Hs pins has additional trace. \$\endgroup\$
    – Ktc
    Sep 13, 2014 at 15:46
  • \$\begingroup\$ I apologise, the sentence should have read "The two different STM32F USB peripherals will never be active at the same time." \$\endgroup\$
    – gbulmer
    Sep 13, 2014 at 17:35

1 Answer 1


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 " 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?

  • \$\begingroup\$ Thanks. I have all the equipment but sounds like my suggested solution will be an adventure. Especially I feel that it will be problem with longer cables where there is more capacitance. I will still try though. \$\endgroup\$
    – Ktc
    Sep 14, 2014 at 6:27
  • \$\begingroup\$ @Ktc - It's a clever idea which I have never considered before. It seems well worth the attempt if you have the test equipment. If you are able, I'd very much like to know if it works. \$\endgroup\$
    – gbulmer
    Sep 14, 2014 at 11:19

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