[PLUG-TALK] micro usb for charging

[Galen Seitz]

On 03/16/13 21:06, Denis Heidtmann wrote:
> I wonder about charging a cell phone from USB.  The phone in question has a
> micro usb socket (as I expect most do).  The dedicated wall charger comes
> with a wart and wire--no plug in between.  The micro usb has 5 pins.  As
> best I can determine the pin next to the ground pin is the extra one,
> having a function called ID.  I presume this is to tell the device, my
> phone, whether the thing plugged into it is a charger or a computer port.
>   The Web says "ID pin permits distinction of host connection from slave
> connection: host: connected to Signal ground, slave: not connected.  What
> this means for a charger I cannot discern.  Wiki says "A portable device
> can recognize the type of USB port from the way the D+ and D- pins are
> connected. For example, on a dedicated charging port, the D+ and D- pins
> are shorted."  My dedicated charger does not have D+ and D- pins shorted,
> at least when it is not powered.  The ID pin on my dedicated charger
> measures 0 ohms, or nearly so, to ground.
>
> If I use a USB A to micro B cable, what is done with this extra pin in the
> cable?  I intend to use such a cable to use a car USB charger.  Will the
> phone get confused?  Is there some special cable for this use?

The 5th pin is used in on-the-go(OTG) situations.  When USB was 
originally designed, USB devices like scanners, printers, cameras, etc. 
were considered to always be slave devices(B) which must connect to a 
USB host/master(A).  Later it became desirable to connect two slaves 
together, thus OTG was created.  For example, normally a phone would 
connect to a computer as a slave(B), but lets say you want to connect a 
USB keyboard to your phone.  In this situation you have two slave 
devices, which under normal circumstances cannot be interconnected. 
However, if your phone supports USB OTG, with the proper cable your 
phone could act as a host(A) device and connect to the keyboard 
slave(B).  In an OTG cable, the 5th pin is grounded on one end and open 
on the other.  The grounded end becomes the host, and the open one the 
slave.

AFAIK, the 5th pin is irrelevant in charging situations.  In a USB A to 
micro B cable, the 5th pin will be open.

http://electronics.stackexchange.com/questions/35462/why-does-micro-usb-2-0-have-5-pins-when-the-a-type-only-has-4

Regarding charging, with a charger and device that conform to the USB 
Battery Charging Specification, if D+ and D- are shorted, the device 
knows that there is no data connection and that it may draw more current 
for charging.

In your situation with the A to micro B cable, I think it will work 
fine, but your phone will be limited to 500 mA when charging.


galen
-- 
Galen Seitz
galens at seitzassoc.com