Posts: 40
Joined: Fri Aug 12, 2011 4:09 pm

LapDock/RP USB power issues.

Tue Jun 05, 2012 3:59 pm

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
(data only)?
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
recommended power?

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)

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Location: Susanville CA.

Re: LapDock/RP USB power issues.

Thu Jun 07, 2012 8:18 pm

The lapdock will supply the standard 500ma per port from the internal battery via the internal power supply, for the 2USB ports on the Lapdock.

You have to build an adapter cable that supplies 5v via the micro USB power connector on the RasPi and sends data to one of the two USB ports on the RasPi. There is a micro USB on the back of the lapdock. It has power and data.

The micro USB on the lapdock (this is the one that is on the extension on the back of the Lapdock) should supply more than 500ma as it was meant to supply the Atrix cellphone during full power usage as a laptop. I have not seen what the output specs are for this connector but would guess it at up to 2amps but surely over 1amp.

The "Y" connectors do not need anything inside. They both are at the same voltage. They are wired in parallel so that you can draw double the amperage, 500ma plus 500ma equals 1000ma or 1amp at 5v.

I am sure someone will show some wiring diagrams of the adapters that need to be built. If they do not I will. But I do not want to trespass on their project. I hope there will be a list of components needed to complete one of these as a truly mobile solution for the Raspberry Pi. For right now they are still experimenting. I have ordered a Lapdock with a couple of short cables and a HDMI adapter. My RasPi is supposed to be on its way here. So hopefully I can join the discussion as an owner/experimenter very soon.
512MB version 2.0 as WordPress Server
Motorola Lapdock with Pi2B
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Posts: 40
Joined: Fri Aug 12, 2011 4:09 pm

Re: LapDock/RP USB power issues.

Sat Jun 09, 2012 6:43 pm


Thanks for the reply. Yeah, after looking closer to this thread, I noticed the modified microusb cable. I was looking at the other thread where the connections where made differently. The Y cable is the way to go and the safest. Makes total sense that the power out the phone docking port would supply more that 500ma or 1a. I purchased the Droid version of the Lapdock and discovered I could attach the RP microusb connection directly to the Droid microusb dock port (bottom side of board facing front of Lapdock). Given this one could cut the end off one end of a usb cable and solder the two data leads to the unused data pins on the RP's microusb power connector. The only problem there is not much room to make the HDMI connection between the RP and the microhdmi port on the Droid Lapdock. Since no USB power is supplied to the microusb or standard usb rear ports unless the HDMI is connected between the LapDock and an HDMI source, maybe an HDMI jumper cable could be built so the LapDock could be used as a generic 5V power source (ie only use the Lapdock internal 5V battery to power any 5V device)

Still waiting patiently for my cable/adapter to arrive from China.
Regards, Ron

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Joined: Thu Aug 16, 2012 2:08 am

Re: LapDock/RP USB power issues.

Thu Aug 16, 2012 2:13 am

Hey. Did you ever get the raspberry pi to work with the lapdock. I can't get my lapdock to do anything. I ordered an atrix lapdock and followed the directions here: rpidock.blogspot.com . I had the same thing happen as you, I connected everything up, opened the lapdock, then...nothing. I saw the post about hdmi powering it so I guess it's possible my F-F mini hdmi adapter or mini hdmi to hdmi cable came broken. It's hard to know where the problem lies. Let me know if you figured anything out

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Location: Rotherham, South Yorkshire, UK.

Re: LapDock/RP USB power issues.

Tue Aug 21, 2012 7:11 pm

I've been using my RasPi with my lapdock for a little over a week and have not had any real apreciable power issues the whole time (I did have a mechanical issue with Micro USB connector on lapdock, but removing the "I've got a case on my phone" plate and filing down edge of my connector works perfectly).

For the record, I made a Y connector (Micro USB female to Micro USB male for power & USB A-type for data male) as to previous posts with the addition of a on-off switch at the join (in a 35mm film can). And my HDMI connection is a Micro-HDMI extension into a Micro-HDMI female to HDMI male. Makes the Lapdock connections slightly less of a squeeze.

I have attached my Miltimeter to check on power and it is a little low at just over 4.5V (between TP1 & TP2). When I tested the current, it appeared to be a little on the low side (drawing about 300mA at first and rising to about 500mA) which does suggest that the port may be current limited to 500mA and it's struggling to keep up, hence voltage drop.

There is however a major issue with my multimeter, it does tend to give off some really unrealistic readings at times. But I can't afford a really good one right now. For instance, my Kindle adapter (which is what Ebon clams to use) puts out only slightly higher and still below the suggested Voltage range of 4.75V - 5.25V. I need to go meter shopping!

It would really be better to make an Rpi Laptop using a Model-A Raspberry Pi (if it ever gets released). It's stated power consumption of max 300mA is pretty appealing. Plus, you can use the in-built hub for a WiFi card and not blow a poli-fuse. :D
Model-B (rev.1 & rev.2)
Motorola Atrix Lapdock
Pibow (beta version)
Adafruit Cobbler
Humble Pi
7dayshop.com WiFi (RTL8188CUS)
7dayshop.com 8GB Class 10 SDHC
Arduino Uno (rev.3) via homebrew level shifter

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