FX3(S) without VBUS? | Cypress Semiconductor
FX3(S) without VBUS?
In our current USB device design (FX2), I have no VBUS line available at all (my 4 USB lines are D+, D-, GND, shield). The system is powered over a 12V supply, coming over the same (custom) 10-wire cable. Assumed is that VBUS is present at the host when 12V supply is present at the device. No issue was encountered so far.
Things get worse in lab: 12V is coming from a Lab supply, the 4-wire-USB is a legacy USB cable with a custom connector that does not connect VBUS. But even there, so far I've encountered no enumeration issues I assign to this fact.
Now I'm planning a similar design with FX3(S). Again, the 5V-VBUS is not wanted, as 12V-power is available in the cable and the cable shall be as lean as possible, so no second power line. Again I assume VBUS available at the HOST when 12V-power is available at the Device.
Here my questions concerning that setting:
1. If I just add the locally (from 12V) generated 5V-supply to FX3.VBUS (therefore, D+/D- pullups may be operated even if VBUS is not present at the host in case the 12V-supply is from lab supply and independent of VBUS), may there be any destruction of any part in the system? What would be the impact? Can I live with that for lab (=development) environment? Does it work at all?
2. In lab environment I can plug in a standard micro-B plug and have VBUS available. However as a HW change between the lab and the product is unwanted, I'd have to implement a separate VBUS sense going to a GPIO (the way others did in FX2), leaving FX3.VBUS input still on the local 5V0. Is there a possibility to shut off the USB device D+/D- pullups through API, depending on a GPIO based VBUS-sense input and FX3.VBUS-pin permanently powered from local 5V? Any literature, application not or maybe even an example available? I'd be happy for every hint or support in this topic...