Total power from USB

[Sameer Verma]

On Sat, Apr 24, 2010 at 4:55 PM, Tiago Marques <tiagomnm at gmail.com> wrote:
> Hi all,
> I've ran into something on the 1.5 B2 I have here, which I'm in doubt if may
> or may not be relevant for OLPC's hardware people(it seems not), so I'm
> reporting it anyway.
> All three USB ports can deliver enough power to an USB 2.5" HDD, no problem.
> They cannot in the following conditions:
>  - An extra SD card is plugged in
>  - Another USB device is plugged in one port, even one of lower power like a
> flash drive. (sometimes it only takes one, sometimes it takes two, SD card
> power also seems to be an issue)
> When I try to plug in the 2.5" HDD, it doesn't have enough power to spindle
> at enough speed and sits there trying to start itself.
> When plugging in an USB device or SD card after the 2.5" HDD is already
> working, it suffers a loss of power and enters the restarting state. This is
> true if the SD card is plugged in instead(with another USB device present),
> which suggests me that the power is shared.
> No other plugged devices seem to be rendered temporarily unaccessible due to
> the power failure on the high power device.
> Best regards,
> Tiago
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>

I see a similar issue with a Lacie drive on my Dell desktop. This
drive actually comes with a cable that has two USB cables to draw
power from two ports. With two ports, the platter spins without any
problem. With one port, the disk clicks on the Dell.

On a XO-1 G1G1, XO-1.5 B2 and XO-1.5 C2 I am able to power the same
drive via one port only both with a SD card and without a SD card. The
issue may very well be the USB drive power draw. I am using a LACIE
Design by FA Porsche http://bit.ly/9HD5uI

This will be important for XS-on-XO scenarios, where we would
piggyback the /library partition onto an external drive.

cheers,
Sameer

-- 
Dr. Sameer Verma, Ph.D.
Associate Professor, Information Systems
Director, Campus Business Solutions
San Francisco State University
http://verma.sfsu.edu/
http://cbs.sfsu.edu/
http://is.sfsu.edu/