5 May
2012
5 May
'12
4:49 p.m.
On 05/05/2012 23:58, Alexander Schreiber wrote:
On Sat, May 05, 2012 at 06:52:21PM +0100, Pete
Turnbull wrote:
OK, "after negotiation" -- but if the iPhone doesn't get any negotiation
how does it know what it can draw? Should it assume the full 500mA, or
might it be a low-power charger that won't even do that? Some laptops
will give 5V but only at a few mA when asleep, some chargers give
substantially less than 500mA. Hence the resistor network. In fact an
iPhone /will/ negotiate, but if the negotiation fails it relies on the
cruder simpler hardware.
I'm not suggesting it's perfect -- I've found plenty of situations where
it fails (like with some docking units) -- but I can see some logic to it.
--
Pete Peter Turnbull
Network Manager
University of York
If the voltage on each is within a specific
(fairly
narrow) range the iPhone knows it can pull at least an amp. A different
range tells it 500mA. Else nothing.
Which is still wrong since you can pull up to 500 mA from a standard USB
port (after negotiation, but you probably can get away with trying to pull
500 mA regardless, especially if the other end is a stupid charger and not
a full USB host implementation - which shouldn't be too hard to figure out).