Initially a USB device can only draw 100 mA from a host. If it needs more current it can negotiate with the host for more current, 5oo mA max for smart devices. Where is the current limiter circuit that limits a device current located on a motherboard?. Is it built into the Southbridge or is it a separate IC. Is it a separate IC for each port? Or do they have ICs that can handle multiple USB ports?
This question arises because somebody has three identical laptops and an iPod. He can use the iPod with two of the laptops but not the third, Using a USB cable modified to permit measuring voltages he found the Vbus voltage drop to 4.5 volts shorty after connecting the iPod. As an iPod is very temperamental about the magnitude of the VBus voltage it will not work when the Vbus voltage is this low.
If possible he would like to fix the problem. However, the biggest problem is finding information on the USB current limiting circuit. I have tried searching the web but haven't found any useful information. A link to a web page that explains how a USB current limiting circuit is implemented would really be appreciated.
Howard