XCore Exchange

First post here. I am very new to this device. There does not seem to be a wealth of information in the data sheet regarding this, so I thought I would ask here. My system is to be powered from an external source - not USB powered. It is unclear what must be done with the USB_VBUS pin in this situation.

Thanks in advance,
Dan

Wire it directly to USB vbus (add protection if you desire)

Ross wrote:Wire it directly to USB vbus (add protection if you desire)

What protection would you suggest in this case?

Also, do we need to worry about VBUS being present when the board is powered off? E.g. when the host PC is turned on with the USB cable connected, but the XMOS board powered off?

Ross,
Thanks for the reply. I do not intend on powering the part via the USB connector, so I am wondering if I still need to make this connection at all, and if I don't, what do I do with the pin? Vcc? Gnd?

Thanks again

The current incarnation of the device requires it to be wired up I'm afraid. An internal VBUS comparator enables some of the phy logic when VBUS is present.

What would keep me from connecting the pin to my external power supply? How would the pin know the difference between being sourced by the USB connector vs another source?

Thanks again for the reply!

Dan

It would fail a USB compliance test, in practise its likely to work most of the time, but you might get some issues.

From USB spec:

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 VBUS is removed, the pull-up resistor does not supply current on the data line to which it is attached.

The xcore, in a self powered device, therefore needs a VBUS connection in order to know when the pull-up can should be enabled.

Thank you, Ross. I greatly appreciate the help!

Dan

np