STM32F469 VDDUSB when using external USB ULPI PHY
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
