[SI-LIST] Re: USB drive Current Consumption

Does this have anything to do with SI?

> USB specifies two power classes: 500ma and 100mA. All devices have to
> power up at 100mA max. During enumeration the device makes its class
> known and the host can decide to enable or not. Usually, most PC host
> controllers are capable of 500mA, whereas many hubs have a 100mA
> limit. 

I'm not a USB wizard.  Corrections/tweaks welcome.

There is an interesting step in there.  The enumeration happens when the 
device is plugged in and/or host is powered up.  The host asks each device 
how much power it needs.  If the answer is 100 mA or below, no problem.  If a 
device wants more than 100 mA, things get interesting.

If the device is directly connected to a USB port on the host, the host can 
decide if it has enough current to supply the needs.   (Most hosts can 
provide lots of power.)

If the device connects through a hub, then the host has to add up the current 
requirements of all the devices connected through that hub and compare that 
to the power available from the host and the power available from a possible 
wall wart powering the hub.

Note that 100/500 mA are handy numbers relative to the typical 4 port hub.  
That's 100 mA for the hub and 100 mA for each device plugged into the hub.  
If the host can provide 500 mA, then the hub can support 4 (low power) 
devices without a wall wart.

There is another branch of the rules that says you can only have 5 5-meter 
cables in a chain.  (The max cable length is 5 meters so you have to have a 
hub/repeater every 5 meters.)  Again, if you use single port hubs (aka cable 
extenders), you get 100 mA for each of 4 hubs and 100 mA left for the device 
on the end of the chain.

Both cases work with no wall-warts as long as the devices (and hubs) need 100 
mA or less.




-- 
These are my opinions, not necessarily my employer's.  I hate spam.



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