Archive for category Android

Motorola’s USB charging scheme

As I mentioned in the previous post, I have a Motorola Xyboard 8.2 Android tablet [1].

Motorola Xyboard 8.2 (Xoom 2 ME) Android tablet

Motorola Xyboard 8.2 (Xoom 2 ME) Android tablet

Among other things, I use it as a GPS (with live Google Maps) in the car – it’s a WiFi-only tablet, but I tether it to my phone via WiFi or Bluetooth, so the tablet can talk to Google in the car. This works great.

But after a day of driving, the tablet always runs out of power, even though I had it plugged into a car charger the whole time.

I tried a bunch of different car chargers, including ones rated for tablets and iPads, claiming to source anywhere up to 3.1 amps, but they all did the same thing.

Finally, I hooked up the USB charging port to some meters and an oscilloscope to see what is going on.

About USB charging

USB charging is an unreasonably complex subject. You can read lots about the details here and here and here (and of course here), but I’ll summarize:

  • Per the USB 2.0 spec, all USB devices can draw 100 mA (at 5v) without negotiation.
  • Also per the spec, USB devices can draw up to 500 mA (again at 5v) after negotiating with the USB host

In practice, lots of devices draw 500 mA without negotiating. They get away with it because most PC USB ports just supply 500 mA to all the ports, regardless of whether or not the device negotiates.

Smartphones usually want more, and tablets, much more. My Nexus 4 draws up to about 750 mA when charging. And the Xyboard tablet can draw up to 1.5 A. Most phones and tablets are smart enough to reduce the amount of current they draw if the USB port can’t supply what they want – the result is they charge slower. In the case of my Xyboard tablet, 500 mA isn’t enough to keep it from discharging – it just drains the battery slower. That’s why I run out of power after a day of driving.

It would be expensive for vendors to make a dumb charger that’s smart enough to do the USB power negotiation, and even if they did, 500 mA isn’t really enough. So they came up with their own schemes:

  • Apple has a complicated and slightly-secret scheme where on their dumb chargers they put resistors on the D+ and D- USB data lines. The effect of the resistors is to put different voltages on these lines, which indicate to the device how much current the charger can supply.
  • In 2007, the Chinese set a standard that if the D+ and D- lines are shorted together, that indicates that the charger can supply more than 500 mA (at least 1000 mA; often more).
  • In 2009, the EU set a standard almost identical to the Chinese one, except they say to put 200 ohms between D+ and D-. Happily, this is close enough to the Chinese standard that they work with each other.

So as of 2013 (as I write this), virtually all non-Apple high-current chargers do the Chinese thing and short D+ to D- to indicate their capability.

But the result of this mess is that Apple chargers can’t charge Android devices (very fast), and vice-versa.

What Motorola does

But that still doesn’t explain why I couldn’t charge the Xyboard (well enough to keep it from draining), even with a high-current Android-type car charger.

It turns out that, at least on the Xyboard 8.2/Xoom 2 ME (and possibly on other Motorola devices), they don’t follow either standard.

I started by looking at the various car chargers I’d been trying. Some were Apple type, some were Android type. I even put power resistors on the output and measured how much current they could supply without the voltage dropping – lots (2.6 to 3.1 amps, depending on which charger).

But none would charge the Xyboard at faster than 640 mA.

Motorola uses their own scheme. When you plug in the USB charging cable, it gradually loads the charger over a period of about 2 seconds. If you watch the voltage on an oscilloscope, you can see it gradually dropping as the tablet pulls more current.

If at the full current draw of about 1.5 amps the voltage drops below about 5.3 volts, then the tablet decides the charger can’t supply that much, and drops the charge rate to about 640 mA. (The threshold was somewhere between 5.28 and 5.33 volts when I tested.)

Since USB ports are supposed to supply 5 volts DC, +/- 5%, 5.3 volts is slightly more than the maximum 5.25 volts allowed. So any conformant USB charger will be interpreted by the tablet as “low current”, even if D+ is shorted to D-, and no matter how much current it can actually supply.

I’m guessing only Motorola’s own chargers supply 5.3+ volts.

It’s a shame that Motorola’s devices can’t charge from standard Android-type USB chargers. I have one of the Motorola car chargers on order; I hope that works.

And I hope this info is useful to somebody.

[1] I’ve tried 7″ and 10.1″ tablets, and I really think the 8.2″ size is the sweet spot, at least for me.

I like the Xyboard 8.2 – Motorola priced it way too high so it didn’t sell.

As it ought to be for the price they asked, it’s very well built (they claim it’s waterproof), and the ICS implementation is good (except for the minor Bluetooth problem I mentioned in my last post). It’s getting old now – if you can find one at a cheap price, it’s a nice unit.

But be aware that it’s orphaned – ICS is the last version of Android you can put on it; it sold so few units that there aren’t any custom ROMs available (although you can root it). It’s called the “Xoom 2 ME” outside of North America.

Pairing a Bluetooth keyboard with a Motorola Xyboard (Xoom 2) tablet

I bought this rather nice Bluetooth keyboard for use with my Motorola Xyboard (Xoom 2) tablet.


It was about $18, shipped, on eBay. It’s small, designed for Android (it has Android-specific keys instead of iPad keys), and runs on two AAA batteries, so I don’t have to worry about keeping it charged. (It uses about 2 mA – I measured – so should be good for about 500 hours of run time on a fresh set of alkaline AAAs.)

Unfortunately, I had trouble pairing it. I don’t think it’s the keyboard’s fault, since it paired perfectly with my Nexus 4.

After much fiddling, I got it to pair – here’s the trick:

  1. Before pairing, make sure the tablet does not already know about the keyboard. (That will happen if you had a previous unsuccessful attempt at pairing.) To clear the keyboard from the tablet, do a “scan for devices” on the tablet, with the keyboard powered off.
  2. In Settings>Input, select “SwiftKey Tablet X”.
  3. Then pair.

Then it works. At least it did for me, on Motorola’s stock Android 4.0.4 build for the Xyboard 8.2 (Xoom 2 ME).  I’d guess this might apply to other Motorola tablets, too.

I think this is Motorola’s fault, but given that this is an “orphan” tablet at this point, I don’t think they’re going to fix it.

Dropbox is still better than Google Drive

[Update 2014: This is obsolete. Google Keep solves the problem better now.]

In July I switched from an iPhone to a Galaxy Nexus running Android Jellybean.

I was fed up with Apple’s sue-happy arrogance and with being locked to AT&T (I travel; AT&T’s international roaming rates are highway robbery).

Plus Jellybean just looked like a more advanced platform. Google’s price of $350 unlocked and usable anywhere in the world was a great deal. When Apple got an injunction against the Galaxy Nexus, I pulled the trigger and snapped one up before it got yanked off the market.

Back in 2009 I went thru a painful transition from the Palm Treo to the iPhone. One of the big issues then was getting my memos onto the iPhone in a form that was:

  • Synchronized with the PC (so I can edit on either device)
  • Reasonably secure (encrypted sync)
  • Editable offline (on either the PC or the phone)

My solution was to store memos in the “notes” field of address book entries. This synced securely with Google Contacts and I could edit my memos offline. It worked great.

Unfortunately the Android version of Google Contacts has a limitation on the size of the “notes” field. Neither the iPhone or the web version of Google Contacts have this problem, but I had plenty of memos that were too long to read on Android.

So I decided to move the memos into Google Drive (ex-Google Docs) files, thinking I could edit those on the phone or PC.

It turns out you can’t edit Google Drive documents when you’re offline. Also, the Google editor is complex enough to take an annoyingly long time to start on the phone (when I just want to check my shopping list).

But Dropbox works great:

  • Store the memos in Dropbox as plain text.
  • Put a shortcut to the memos folder on your “links” toolbar in Windows
  • Put the Dropbox widget (direct link to the memos folder) in the Android dock
  • Mark the memo files with a star (“favorite” them), so Dropbox will cache them locally in the phone for offline access.

That’s it. You can edit either on the PC or phone, and it syncs securely. It’s even smart enough to create a “conflict” copy if you make conflicting changes on the PC and phone.

I’m disappointed with Google Drive – this should be easy. And I don’t see why they put the size limit on Android address book notes (when there isn’t one on the PC or iPhone).

I’ll say this – the transition to Android was easy; nothing like the nightmare moving to the iPhone.

I think Apple has started a long, slow descent into irrelevance. They must think so too – winners compete, losers sue.