piclist 2008\07\04\070304a >
Thread: USB power in parallel
face picon face BY : email (remove spam text)(Olin Lathrop)

Herbert Graf wrote:
> It is absolutely a hack, and is not in spec, in the cases I've seen.
> The way USB works is you must ask permission to take more then a
> certain
> minimal amount of current. The "main" USB connection likely properly
> requests a full 500mA. In all the cases I've seen the second cable
> doesn't even connect to the USB data pins, but only takes power.

Yes, that would violate the spec.  And compliant hosts don't need to supply
much power until you ask for it and they feel like giving it to you.  You
are not guaranteed the full power.

However, a second cable by itself doesn't guarantee violation of the spec.
If separate communication is established over that cable, the extra power is
requested and granted, then there is nothing wrong.

Embed Inc, Littleton Massachusetts, http://www.embedinc.com/products
(978) 742-9014.  Gold level PIC consultants since 2000.
<011301c8ddc5$d9436e70$0300a8c0@main> 7bit

See also: www.piclist.com/techref/power.htm?key=power
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