On Tue, 10 Jun 2014, Grant wrote: > >> >> Can I disable VBUS while keeping the rest of USB functional for a > >> >> device that does not require bus power? > >> > > >> > unfortunately not, your device would see a disconnection. The reason is > >> > that even though you don't really put any load on the bus, the PHY still > >> > samples VBUS levels to know when the session is valid (VBUS > 4.4V). > >> > >> What if I use a USB cable with a separate DC input for the VBUS? > > > > What would be the point? I mean, if you've got a separate DC input for > > Vbus then you aren't really saving any power or disabling anything. > > The point would be to minimize the amount of electrical noise the USB > device is exposed to by using a low-noise supply for this such as a > battery with a linear regulator. Okay. Is Vbus noise really a problem for you? > > If you did this and then turned off the separate DC power, your device > > wouldn't work afterward. As Felipe pointed out, the device would think > > the cable had been disconnected. > > Understood. Can you tell me how to disable VBUS on the USB > hub/controller? I'll be attempting this on a Beaglebone. I don't know what controller the Beaglebone uses. In general, however, it isn't possible to disable Vbus on a USB host port and still use the port. To do what you want, you would have to physically cut the Vbus wire in the USB cable and splice the device side of that wire to the external power supply. Alan Stern -- To unsubscribe from this list: send the line "unsubscribe linux-usb" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
