Please refer to these links below as they relate to powering the Raspberry Pi
via the Motorola Lapdock.
Post title "I made a Raspberry PI Laptop"
http://www.raspberrypi.org/phpBB3/viewt ... 63&t=6747
And excellent Blog related to using the RP with the LapDock.
http://rpidock.blogspot.com/2012/05/ras ... dock.html
The intent of this post is to spur dialog regarding the best way of powering
the RP via USB and specifically using the Motorola LapDock to do so. The LapDock is a
display device so that is why I posted it in this forum; however, posting in another forum might be more appropriate (moderators move if appropriate) or cross posting (if allowed?)
Please feel free to correct me on any statements I make in this post as the
intent is to discuss the best way to properly/safely power the RP.
After reviewing the excellent blog link above for power connectivity, this is my
understanding of the connectivity and how power is being applied.
The micro-usb connection between the LapDock and RP USB host port issues:
1.) Is supplying the data connection between the USB hubs of the two devices.
It is also probably supplying back door power to the RP HOST mode USB hub
which I believe is NOT recommended. The RP USB ports are designed to source
a maximum of either 100ma or 140ma? total for both ports? or for each port?
Side note: this might not be enough power to operate some USB wireless
Anything connected to RP USB hub should ideally not sink any power
How much power is being supplied by the LapDock and whether there is any
USB power negotiation involved is currently an unknown? or could this over
time potentially damage the RP USB host ports polyfuse protection?
The USB cable connected between one of the two LapDock USB ports and the
micro-USB power port on the RP:
1.) This connection should be the only power source to the RP? How much power
the LapDock is supplying is currently unknown? If host mode USB power
negotiation is applicable to the USB ports on the LapDock, it might be
supplying no power? 100ma? 500ma? XXXma? The schematic for the RP micro-usb
power connection does NOT have anything connected to data lines only
power; therefore, USB power negotiation between the devices would not be
Assuming the LapDock USB port were supplying 500ma and no other power
source was involved (ie.. power sinked into RP USB Hub port) , this is
probably marginal operational power? Note: 750ma @ 5V is minimum
Possible use of USB Y Cable to supply power using both USB ports on LapDock:
1.) One might consider using a Y splitter like is commonly used to power
external spinning hard drives. This could supply more power using both USB
ports on the LapDock.
I believe data and power is used on the main connection of the Y cable and
power only is wired into other host side USB Y connector? I am not sure
how the two hosts ports handle having the power connected in parallel via
the Y cable? is a diode or something similar used to isolate? Since the
power only leg does not have any data lines, how is USB power negotiation
handle on the that port (assuming the host requires negotiation)?
Sorry for the long post but I hope fellow forum members can help me understand
these issues and ultimately get some of the information in the Raspberry Pi
FAQ (assuming it is not already there)