[GreenKeys] New 60ma current loop driver board design

[John Nagle]

On 10/27/2016 05:31 PM, COURYHOUSE at aol.com wrote:
> Absolutely great and amazing John !
> did  not know about option to get more current out of the usb that way!
> Keep up the great work!
>
> Ed#

      Now that everything runs on batteries, power management is
standard on almost everything.   When you plug a USB device into a host, 
or the host starts up, the host software goes through "enumeration" and 
finds all the devices on the USB port.  It adds up their power requests, 
and sends an "enable" signal to as many as it can power.

     Desktops can almost always deliver 500mA to each
USB-A port, but battery powered devices may have
lower limits.  USB devices are guaranteed 100mA; after that,
the host power source can say no.   If you use a USB hub to
plug multiple devices into one port, everybody on the hub has
to share the power quota for the host port. This is all handled
automatically; plug in enough USB devices and some of them will
get a "no" and will not power up at all.  This is better than
everybody getting a low voltage and not working.

			John Nagle