On Apr 7, 3:08*pm, Rob wrote:
Your new phone will have a USB cable supplied with it. Why not simply
charge your phone by connecting it via the cable to a USB port in a
computer or in a main/usb adapter?

Unfortunately its not quite as simple as it seems. The phone is not
allowed to draw very much current unless it has negotiated this with
the host computer. Some phones ignore the rules and take the power
anyway, but others do not. The negotiation will usually only succeed
if drivers specific to that phone manufacturer have been installed.

Even wall chargers have some surprising behaviours. Originally there
was no standard for USB chargers, so there had to be some way of
telling the phone using hardware alone that it was OK to draw the full
charging current from a simple wall charger. Apple went one way with
iPods and Motorola another. The Apple approach was to use a resistor
network to set voltages on the D+ and D- signals that would never
occur in normal USB operation. The Motorola approach (for Mini-USB)
used a 200k pullup resistor to V+ on the sense pin of the mini
connector which was originally intended for On-The-Go negotiation
(which was itself unaffected by this high value). It was a nightmare
trying to design chargers that would work with everything.

Then the Chinese banged the mobile phone manufacturers' heads together
and a few months later the USB Consortium proposed a new standard
which was very simple and quite elegant but incompatible with both the
main de-facto standards (probably so nobody would lose out
commercially). All new phones for sale in China would use the new
micro-USB connector in conjunction with a new way of sensing wall
chargers but still retaining the software negotiation capability for
use with PCs.

The wall charger has a link between D+ and D- and if there is no
attached cable it must use a USB-A (large flat) socket. Adapter
cables can be plugged into this to convert to mini- or micro-USB. If
there is an attached cable it must be mini- or micro-USB but for
phones in China or the EU only micro USB is allowed. The nice thing
about this is that old Apple or Motorola products and their clones can
use a modern wall charger by means of an adapter cable fitted with the
appropriate resistors.

Once all this was worked out, the EU came up with an identical
requirement to bring the smaller manufacturers on board. As already
mentioned in this thread, the large manufacturers had already decided
what they were going to do as they didn't want to be left out of
China.

This is a simplification - for more information read the standards.

John