On STM32L433, is PA9 really dedicated to the VBUS sensing input?

[EThom.3]

I am designing a PCB for a self-powered industrial device with a USB port, and therefore need to use VBUS sensing. In the generic USB hardware guideline (application note 4879), it is stated that "Pin PA9, a five V-tolerant pin, is natively dedicated to VBUS sensing".

What confuses me is that the STM32L433 datasheet doesn't mention this. In fact, nether the datasheet or the manual mentions VBUS at all, and PA9 doesn't seem to have anything to do with the USB port at all.

So my question is this: Does anyone here have experience with VBUS sensing on the STM32L433? Does PA9 really work as VBUS sensing on these devices, even though it isn't mentioned in the manual or the datasheet?

I had a look at the datasheet for the STM32L475, where the additional function for PA9 is listed as OTG_FS_VBUS. Is PA9 dedicated to VBUS sending only in OTG capable devices?

[Piranha · Accepted solution]

There is no dedicated VBUS sensing pin on L43x devices. Also take a note that the VBUS sensing is not an absolute requirement. (read Jan's comment) Generally it's necessary for energy saving or some interactive functionality related to USB connection.

[EThom.3]

@Piranha​ Thanks a lot. I really appreciate your answer. :thumbs_up:

I think that I'll connect the VBUS to an input through a voltage divider, and then (if practical or necessary) I can sense VBUS manually in the code. If I decide not to use it, it will just cost me two resistors. No biggie.

[waclawek.jan]

> Also take a note that the VBUS sensing is not an absolute requirement.

This requirement stems from this requirement put in USB2.0 7.1.5.1 as

The voltage source on the pull-up resistor must be derived from or controlled by the power supplied on the USB cable such that when V BUS is removed, the pull-up resistor does not supply current on the data line to which it is attached

and then reiterated in 7.2.1 as

No device shall supply (source) current on V BUS at its upstream facing port at any time. From V BUS on its upstream facing port, a device may only draw (sink) current. They may not provide power to the pull-up resistor on D+/D- unless V BUS is present (see Section 7.1.5). When V BUS is removed, the device must remove power from the D+/D- pull-up resistor within 10 seconds.

(I deliberately copied also the sentences forbidding to source VBUS).

I don't know the rationale for this requirement.

Some speculate that this is to prevent damage to the connected hub port if the hub is unpowered. I am yet to see a digital IC which gets damaged if 2mA gets injected to it.

JW

[EThom.3]

@Community member​ Thank you for the added details.

I have seen low-power circuits act strangely when they were supposed to be off, but were semi-powered through some logic inputs. This might be what the writers of the standard want to avoid.

I agree with. Data signals into an unpowered USB port should not damage anything.

[Piranha]

I stand corrected and I updated my comment. Thanks!