General discussions about V-USB, our firmware-only implementation of a low speed USB device on Atmel's AVR microcontrollers
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I have a question that I haven't been able to figure out yet. I'm making a device that will require the full 500mA and I'm wondering what happens in v-usb and on the usb bus when the device tells the system that it will use 500mA and it's on a port (say an unpowered hub) and that port can only output 100mA? Will the host power-down the port, will there be a negotiation failure, will it just be purely current-limited?
The device starts out in low-power mode and communicates its desire for more power to the host through its descriptor. If the host determines that the device can operate in high power mode, it will let it know that it is authorized to. See 18.104.22.168 High-power Bus-powered Functions in the USB 1.1 specification.