working with USB Device, i discovered that, the most important manufactures of the motherboards, don't monitoring the USB current draw.
The +5V of the USB is directly connected to the +5V of the Power Supply (there is only a fuse/polyswitch) So, all motherboards aren't compliance to the chapter 7 of the USB Specifications!
Do You know some motherboards that control the current over the +5V USB supply and so can limit the current to 100mA? (in the enumeration process or in a possible overcurrent situations) What do You think about it?
Emanuele
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Yeah, that's not exactly a well-kept secret. :-) Given the intense price competition in motherboard manufacturing, being able to drop a current monitoring IC is something many manufacturers will vote for.
I'd suggest looking at, e.g., Intel brand motherboards -- they're much more likely to actually implement the USB specs (even when it "doesn't really matter that much") that the Taiwanese manufacturers. Are there any Fry's stores where you live? They usually have about 20 motherboards sitting out on display, and you could carefully look at each one and probably ascertain whether or not there's any active current limiting in place.
To tell you the truth, though, you're not going to find *any* motherboard that "hard limits" at 100mA like a bench power supply would. Most the USB current limiters out there have pretty wide tolerances, and have nominal trip points typically *twice* the USB spec limit (200mA and 1A). This is ostensibly to allow for surge current at start-up.
The flagrant abuse of the USB spec has actually created some useful outcomes, such as the USB connector becoming a standard for power adapters?
To tell You the truth, i saw the Intel's Motherboards, at the first, and they haven't the current limit IC. In my city (Rome) there are many hardware's store and i'll like to spy the MB..... perhaps next weekend.
Do You know some MBs that have the current limit IC? Can You post a picture?
Emanuele
P.S.
This is not the right way to go in heaven !! (electronic's heaven)
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