Author Topic: What action does host take if self-powered device specifies max current?  (Read 1376 times)

jani12

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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?


Jan Axelson

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Re: What action does host take if self-powered device specifies max current?
« Reply #1 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 isnít in the Suspend state. This capability enables the deviceís USB interface to
function if a deviceís power supply is off and the host detects and enumerates the device. Otherwise, if a deviceís pull-up is bus powered and the rest of the interface is
self powered, the host will detect the device but wonít be able to communicate with it.