[GreenKeys] New 60ma current loop driver board design

[Ralph Mowery]

This may help.  Seems the original usb 1 and 2 could deliver 500 ma.  The
newer connectors have more pins to negotiate how much power to send out.

http://www.extremetech.com/computing/115251-how-usb-charging-works-or-how-to
-avoid-blowing-up-your-smartphone




> -----Original Message-----
> From: GreenKeys [mailto:greenkeys-bounces at mailman.qth.net] On Behalf Of
> Rob Doyle
> >
> > So how is it then, that "dumb" USB-powered devices, such as these
> > little USB fans & lamps, can pull a current from the USB port without
> > sending any sort of request-to-enable signal? I was always under the
> > impression the the +5V was available at all times, on the port -
> 500mA
> > for 2.0, (less) for 1.X and (more) for newer 3.X specs?
> 
> I'm pretty sure that a computer will only deliver low power (100 mA)
> until the powered device requests, via the enumeration process, a high
> power (500 mA) setting. That's what the USB 2.0 spec says.
> 
> I have a USB desk lamp that definitely does not enumerate. I've always
> assumed that it draws less than 100 mA. Maybe I'll measure it.
> 
> > And if the USB port doesn't supply constant power, how does the
> > USB-powered device make that request without a power source?
> 
> The USB controller that John used has an internal EEPROM that controls
> the 'Max Power' parameter. He said "This is reprogrammed to talk 45
> baud, and to ask for 250mA of power from the USB port. Once the USB
> port has come up and the other end has agreed to supply 250mA, ENABLE
> goes high and U2 turns on power." In other words, his USB-powered
> device does not draw high power until after enumeration.
> 
> > This is genuinely confusing.
> 
>