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FX3(S) without VBUS? | Cypress Semiconductor

FX3(S) without VBUS?

Summary: 1 Reply, Latest post by SRMS on 09 Jan 2014 06:47 AM PST
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mstx's picture
31 posts


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...


srms's picture
Cypress Employee
34 posts


1. There won't be any problem as long as the supply input to the device is 5 V. It will work fine. But, you should have a VBUS sense, or you will miss the connect and disconnect events.

2. You can use "CyU3PConnectState(CyFalse, CyFalse)" to disable the USB PHYs on the FX3 device. Re-enumeration can be achieved by disabling and then enabling the USB connection.

CyU3PConnectState(CyFalse, CyFalse)>> Disconnect

CyU3PConnectState(CyTrue, CyTrue)>> Reconnect as Super speed device


CyU3PConnectState(CyTrue, CyFalse)>> Reconnect as HS/FS device



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