Shared USB and Pseudo OTG Question

[johnfielden]

Hi, I have a Tower board and have been looking at the USB documentation deciding how to allow for OTG a special future use case.  I'm only using USB0

Questions:

1. If the Vybrid is not powered, is it ok for VBUS to come through to USB0_VBUS and VBS_DETECT?

2.  USB0_VBUS is an input, correct?

3. I don't see any clear evidence that the Vybrid has a boost supply built in to supply 5V to an external client, so if I add a 5V supply to supply 5V out, does that also tie into the VBUS_DETECT?  Or, will that cause an issue with detection of external clients?

4.  See the attached block diagram.  It shows that I have a USB mux for routing USB other places besides the Vybrid.   The load switch (blocking from both directions) is intended to keep an external VBUS from driving the Vybrid when it is off (see question 1 above).  The 5V supply is used to drive 5V out to a client when the Vybrid is on, and we sense USB_ID is low.

5.  Any reason I should use the USB_VBUS_EN to activate the external 5V supply rather than use a GPIO?

Thanks,

John

[naoumgitnik · Accepted solution]

Dear John,

1. If the Vybrid is not powered, is it OK for VBUS to come through to USB0_VBUS and VBUS_DETECT? – Yes, it is OK as per the Datasheet power sequencing table.
2. USB0_VBUS is an input, correct? – In this case, yes. If, however, you are thinking about using it strictly as per the USB spec, you have to follow it in the part of using the ID signal of J8.
3. I don't see any clear evidence that the Vybrid has a boost supply built in to supply 5V to an external client ( - It does not), so if I add a 5V supply to supply 5V out, does that also tie into the VBUS_DETECT?  Or, will that cause an issue with detection of external clients? – The VBUS_DETECT signal is a part of the OTG USB signaling (see details in the OTG USB spec, please), i.e. used for power voltage coming along the USB cable, not generated locally.
4. See the attached block diagram. It shows that I have a USB mux for routing USB other places besides the Vybrid.   The load switch (blocking from both directions) is intended to keep an external VBUS from driving the Vybrid when it is off (see question 1 above - see answer to question 1 above). The 5V supply is used to drive 5V out to a client when the Vybrid is on, and we sense USB_ID is low. – Up to you and OTG USB spec.
5. Any reason I should use the USB_VBUS_EN to activate the external 5V supply rather than use a GPIO? – This pin is a part of the USB block state machine hence acts automatically. Our idea was to make it easier from the software point of view. If, however, you desire to implement this function independently, no problem with that, as long as the USB spec timing is met.

Regards, Naoum Gitnik.