USB ustream power specification

[jituelect]

USB ustream power

hi,

Can someone give the explanation of this sentence in usb 2.0 specification,
line 45, page 171

"No device shall supply (source) current on VBUS at its upstream facing port at any
time"

or

"No USB device, whether bus powered or self powered can drive the VBUS on
its upstream facing port "

TIA

 

[pkuhar]

Re: USB ustream power

This means that the supply current should not be flowing from device to host(computer).

This is only applicable for self powered devices.

 

[daviddlc]

Re: USB ustream power

You also need the Host know that if you are a powered or non-powered device. This is on the initialization protocol.

 

[FvM]

Re: USB ustream power

You also need the Host know that if you are a powered or non-powered device.

Although this information is included in the device descriptor, it doesn't produce different host action to my opinion. Particularly it's not related to VBUS requirements.
The said specification for VBUS, that the device must not drive out to this line is supplemented by the requirement to deactivate D+/D- drivers depending on the VBUS state. The conclusions for a self-powered device are: VBUS must be monitored, e. g. by a uP input pin and it must be used to control the USB interface state by hard- or software means.