![]() You can purchase a charger that does outlet to USB at the store. You can get FTDI chips to handle this for you, which is nice, as you only have to treat the chip as a serial line. This is what you can expect from a computer. It shows that you can get right up to 500mA of power requested. ![]() If you look on this website they will show you all of the different pieces that can be set. Very specifically the configuration descriptors. Near the end of enumeration you setup device parameters. They discuss enumeration in one of their app notes. As you can see this is a long process, but a chip from a company like FTDI will handle the hard part for you. This is not a trivial process and can be seen in detail on Jan Axelson's site. When a device is connected it goes through enumeration. This can be frustrating, but you can always be guaranteed 100mA. ![]() Some computers that are cheaply built will use an bus-powered hub( all of your USB connections share the same 500mA source and the electronics acting as a hub use that source also) internally to increase the number of USB ports and to save a small amount of money. This is the most you can pull from a USB hub that does not have its own power supply, as they never offer more than 4 ports and keep a greedy 100mA for themselves. USB by default will deliver 100mA of current (it is 500mW power because we know it is 5v, right?) to a device.
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