Using USB Hub chip with Internal USB PHY on STM32H735

[Nicholas Yunker_2]

I would like to use the single USB internal PHY on the STM32H735 to connect to a 2 port hub chip (like the Microchip USB2422) so that I can connect to both a USB stick for firmware upgrades as well as a cellular module.  I started connecting everything up and then I came across a multiple sources saying that you can't connect USB Hubs to the internal PHY and you have to use the ULPI interface to connect to an external PHY, but none of the sources really explain why other than saying the internal PHY isn't capable of High Speed operation, and the USB software from ST doesn't allow for hubs.  I don't understand what this has to do with interfacing with a hub IC when I'm perfectly fine with operating at Full Speed.  So I have a few quesitons:

1) Can I use a USB Hub (like the USB2422) as long as I'm OK with it operating at Full Speed only?  Is it technically possible to connect these devices and get them to work?

2) Will the USB Host middleware provided through STM32CubeMX work with USB Hub ICs?  Do I have to use a 3rd party USB Host Middleware like Keil MDK's?

3) Assuming I can interface the hub chip how does the VBus pin on the STM32 and Hub's Upstream port work?  Do I just connect them together and also to the 5V power supply so that both the STM32 and the Hub see that they are connected and getting 5V?

[Nicholas Yunker_2]

Ok, since the OTG_xx_Vbus is an input to comparitors then I should connect it to 5V so it knows the voltage is there then I'm back to if I enable Vbus (which I have no problem doing) I can just connect it and the corresponding Vbus pin in the upstream port of the USB2422 directly to the 5V voltage that also powers the downstream power switches since the upstream port doesn't need a power switch.

Right?

[waclawek.jan]

You may want to test the software first with the Disco board.

JW

[FBL · Accepted solution]

Hi @Nicholas Yunker_2 

I guess you accidently hit your reply as accepted solution. As mentioned @waclawek.jan this errata/ limitation implies in certain conditions specified in the description : impacting the controller when interfacing low speed devices.
For the VBUS, there is no need for an additional power switch. No hardware changes are needed when interfacing a hub.

[Nicholas Yunker_2]

FBL,

Are you saying not to enable and connect VBUS to PA9 of the STM32H735?

[FBL]

Hi @Nicholas Yunker_2 

I meant no hardware changes are needed for Hub functionality. You can stick to the reference board STM32H735G-DK Discovery Kit. 

[Nicholas Yunker_2]

FBL,

Except this hub is fully internal and you said it doesn't require the power switch.  So...coming out of the MCU I have no need for the Enable or Overcurrent Flag signals, the DM and DP signals go into the upstream port on the USB2422 hub and that just leaves VBus both on the MCU and the Hub (the hub has a VBus input).  For VBus I was thinking since this is a fully internal hub and there is no power switch both of these pins on the MCU and Hub get connected to the 5V internal power supply.

For the downstream ports there is a duel power switch and the USB2422 handles the enable and overcurrent power switch signals on it's own.  There is no need for those to go into the MCU right?

[FBL]

Hi @Nicholas Yunker_2 

Thanks for the detailed description. Unfortunately, I cannot fully review or validate your complete hub hardware architecture. My support scope is focused on the STM32 MCU and its USB/OTG interface, not on the full HW design of external USB hub circuits.

For the STM32 side, I can help with:

• How the STM32H7 VBUS sensing and USB FS interface are intended to be used,
• Interpreting the STM32 MCU reference schematics and errata. 

For the hub implementation details (including whether an upstream power switch can be omitted in your specific internal hub use case), I recommend relying on:

• The Microchip USB2422 datasheet and reference designs, and
• Microchip’s technical support, who can provide a definitive answer on the hub’s power architecture.

I found something interesting for you: MB1605 ( using STM32MP25)

If you don't need to switch VBUS for compliance or power‑cycling, you can simplify it.

[Nicholas Yunker_2]

FBL,

Thank you for the detailed response and sharing the STM32 example with the USB2422.  I see the MCU in that design doesn't use VBus Sensing...but it also doesn't seem to have an option for it either.

I have spoken with Microchip support as well and they said in my situation the Vbus upstream input just gets pulled to 3.3V using a 100k resistor.  So the only part I'm not 100% confident in is disabling the VBus Sensing on the STM32H735.  I've disabled it for the moment because it appears to be part of the OTG protocol (which I'm not using), but if you can explain how it's intended to be used and why I would need it for Host Only applications like this I have no problem re-enabling it and connecting where it should be connected.