STM32F469 VDDUSB when using external USB ULPI PHY

[eamonn2]

Hi,

We're using an STM32F469NI in combination with an external ULPI PHY for HS-USB. All is working good but I have a question about VDDUSB. We connected it to 3.3V as we thought we needed to do when using any USB functionality but we didn't realise that this would mean that pins PA11, PA12, PB14 and PB15 would be working at 3.3V. This is a problem for us because our system is designed to be configurable (via a jumper) to work at VDD of 1.8V or 3.3V. When we're running with a VDD of 1.8V then PA11, PA12, PB14 and PB15 are still at 3.3V. This isn't viable long-term so need to either we come up with a way to get these pins to operate at 1.8V or 3.3V like all our other I/O pins or we need to stop using them (not preferred).

Looking at the F469 reference manual it says "The VDDUSB is an independent USB power supply for full speed transceivers (USB OTG FS and USB OTG HS in FS mode)." Does this mean that, because we're not using either the FS or HS transceivers, we can connect VDDUSB to VDD (1.8V or 3.3V) therefore allowing PA11, PA12, PB14 and PB15 to operate at the same I/O voltage as all our other I/O pins?

Assuming we are ok to connect the VDDUSB pin to 1.8V or 3.3V, there is further commentary in the reference manual about VDDUSB being "the last supply to be provided and the first to disappear". It is something we're doing at the moment but I'm not sure if this is actually relevant in our use case i.e. not using the integrated transceivers. It would determine if we can literally connect VDDUSB to the VDD supply or if we need a separate 1.8V/3.3V supply that is always enabled after VDD.

Thanks,

Eamonn

[waclawek.jan]

I'm not ST, but if a second opinion based on reading the same documentation provides you a bit of assurance... 🙂

> Does this mean that, because we're not using either the FS or HS transceivers, we can connect VDDUSB to VDD (1.8V or 3.3V) therefore allowing PA11, PA12, PB14 and PB15 to operate at the same I/O voltage as all our other I/O pins?

Yes.

> "the last supply to be provided and the first to disappear"

Connecting VDDUSB to VDD is OK.

JW

[eamonn2]

Thanks Jan,

We're going to try to test this today but it would be nice to get official confirmation 😋

Eamonn

[waclawek.jan]

This is a primarily user-driven forum, with casual ST presence.

For anything more official you'd need to contact ST directly.

JW

[eamonn2]

Just to confirm, I tested this on my system and everything worked perfectly. Also, ST have confirmed that if you're not using the USB transceivers then you're free to connect VDDUSB to VDD. So all good!

[Khouloud GARSI]

Hello,

1 - The VDDUSB is an independent USB power supply for full speed transceivers (USB OTG FS and USB OTG HS in FS mode). 

Since you're using an external HS PHY:

* you don't need to use the VDDUSB to supply the FS transceivers

* you may connect the VDDUSB to VDD in order to use the PA11, PA12, PB14 and PB15 pins (not for USB communication!)

-> Thus those pins will operate at the same I/O voltage as all the other I/O pins (VDD voltage).

2- "When the VDDUSB is connected to a separated power supply, it is independent from VDD or VDDA but it must be the last supply to be provided and the first to disappear."

-> Since you will not use an external power supply, no issue will be occurring.

-> In case of using an external power supply, the condition above should be respected, otherwise the MCU behavior will not be guaranteed and it could even be damaged.

Khouloud.