USB VBUS after cutting Trace

S

SamSam

Member
Hello,

This might be a really easy question but I can't figure it out so figured I'll ask. I know that if you want to connect external supply to Vin while using the USB connection, you need to cut the VUSB-Vin trace. Now, obviously, USB data transfer must still work or this wouldn't be advised. What I don't understand is how enumeration is completed if the VUSB line is disconnected. From what I can find, even if a device is self powered, it still needs the VUSB power for enumeration.

Thanks.

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USB enumeration by the PC host is using the data that is sent and received over the D+ and D- lines. Those lines are referenced from the USB GND line (not from VUSB).
The host will supply VUSB but it won't be used by the Teensy. Instead you supply this VUSB to the Teensy processor on pins USB1_VBUS & USB2_VBUS by externally supplying 5V.
So as long as you connect GND, D+ and D- to the host (and power the Teensy externally ofcourse), the enumeration will work.

Hope this clearifies,
Paul
 
Thank you Paul. This is how I figured it happened but the little I could find online regarding devices that have their own power made it seem almost as if the 100mA usually provided by the host had to come from the host during enumeration.
Thanks for clearing this up.
 
Yeah, a lot of the USB info is difficult to understand, partly because the USB specification is intentionally somewhat vague in many places to allow implementations a wide degree of freedom for design choices.

Common ground is the only real power requirement.

The host uses weak pulldown resistors to keep both data signals at zero volts while no device is connected. Devices are supposed to connect 1 of the signal pins to 3.3V through a ~1.5K resistor. Enumeration begins when the host (or hub) detects either D+ or D- signal has voltage due to that 1.5K pullup resistor.

Really doesn't matter where the device gets power for that 3.3V. Only needs to be 3.3V relative to the GND pin of the host.
 
Thanks for the additional info Paul. I ended up going down the rabbit hole of the specifications but didn't get too far down, thankfully.
 
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