PORTS Forum

Ports and Interfaces => USB => Topic started by: jani12 on November 14, 2019, 12:04:28 pm

Title: What action does host take if self-powered device specifies max current?
Post by: jani12 on November 14, 2019, 12:04:28 pm
Please see attached configuration descriptor from a USB Device.  There seems to be contradiction?  Device is self powered and it is specifying maximum current of 100ma.
What action will host take?  Will the host supply power to the device or not? 

What bMaxPower value should self-powered devices specify?

Title: Re: What action does host take if self-powered device specifies max current?
Post by: Jan Axelson on November 14, 2019, 12:44:11 pm
From USB Complete, 5th Ed:

A self-powered USB 2.0 device may draw up to 100 mA from the bus any time the device isn’t in the Suspend state. This capability enables the device’s USB interface to
function if a device’s power supply is off and the host detects and enumerates the device. Otherwise, if a device’s pull-up is bus powered and the rest of the interface is
self powered, the host will detect the device but won’t be able to communicate with it.