USB personally do not like it much, especially since I’m in one subject at CTU try to implement a USB interface chip processor. Yet it is one of the most popular computer interface now so we can only watch as progressively pushed out version 2.0 and version 3.0 FireWire themotorcyclers displaces. One of the advantages of USB, which apparently caused its massive expansion, the supply voltage of 5 V. associated with it a lot of myths, half-truths and misconceptions. Unfortunately, some behave accordingly, and although they do not jeopardize our and their safety, but then needlessly surprised that something does not work. Two such complementarily associated myths I would like to refute.
Myth One: The USB port can offer you the 500 mA
Untruth. The value of 500 mA is a maximum load current of one USB 2.0 port. The third version of the maximum increase at 900 mA. However, the device can not just start taking the maximum current value. Just after connection, before the enumeration will be allowed to draw a maximum of 100 mA (for USB 3.0 is 150 mA). Only when the computer reads descriptors from the device requires more current and will meet the requirement, the equipment has permitted to remove the maximum current value.
As stated, for example, Wikipedia, current consumption of the device is expressed in one to five units, each unit represents a 100 mA USB 2.0, 150 mA USB 3.0. If you connect to a computer’s USB hub without external power supply (bus powered), a computer orders from all five current units, but also connected device already offers a maximum of one current unit. That is why a lot of USB devices requires the involvement of directly into the computer, or externally powered hub (self-powered).