freerunner: charging and usb-host mutually exclusive?
andy at openmoko.com
Tue Mar 11 18:15:36 CET 2008
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Somebody in the thread at some point said:
>> Currently the pump that provides 5V for the external device is enabled
>> whenever we are in host mode. I don't know what that thing will do if
>> we provide 5V externally too, probably it will be okay but it needs testing.
> You wrote "currently". Does that mean with a software change we might
> turn off the pump and reenable charging while in host mode?
The situation is that there is only one bit that controls if we are
physically in host mode (this bit comes out of a PMU GPIO and can be
controlled by the CPU).
That same level is used to both enable the pump and to enable 15K
pulldowns on USB D+ and D-. So the fate of the 5V generation and these
pulldowns is one and the same, both active or neither active.
A nice solution would be if the pump reacted well to getting external 5V
shoved on it, I will try this later tonight. If it is okay you can just
give it 5V externally and the pump will just idle. If it isn't okay,
That Would Be Bad (TM).
Otherwise, the job of the 15K pulldowns in host mode is to detect if you
have a low speed or high speed device insertion. I don't know how we
can detect insertions in a good way if we turned off the pulldowns and
pump, despite we did provide 5V for the USB device without the pulldowns
it won't "see" the insertion. I guess you could add 15K pulldowns
permanently if you knew you would only use it in externally-powered-host
mode but it is a bit sucky.
So we must do the test to find out where we are.
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