Rod Whitby wrote: > I have two GTA01's that are dead due to 12V being applied to the power > supply of an upstream USB hub instead of 5V ...
That's an interesting scenario because it shows the effect of abuse of an inherently less reliable interface being propagated to a supposedly more reliable interface. E.g., this discussion is about whether it's okay to feed USB's VBUS to an input that has an "absolute maximum rating" of 5.5V without any further protection. The USB 2.0 spec says that the maximum VBUS is 5.25V, so the question was "how likely is it that someone is violating the spec ?" My gut feeling would be about the same as hopping on the swivel chair in my office to change a light bulb. Probably nothing bad will happen, but it's not safe. (So yeah, I do it anyway.) Then Joerg found that the USB charger spec allows transients of up to 6V. I didn't expect that. So then the question became "how serious are NXP about this 5.5V limit ?" So that's more like running a red light with your car. Chances are that you still get away with it, particularly if you're "careful", i.e., you watch for conflicting traffic, cameras, or the police, but you're probably aware that you're doing something very wrong. Now the hub scenario extends this: anything connecting to USB is almost certainly another USB device, so we can be reasonably confident that this device will make a honest effort of playing by the rules. However, whether the power supply that you plug into that barrel connector of your hub outputs the 5V the hub expects, 12V, -12V, or something meandering around the general vicinity of 5V, the plug will fit just as well. So now the safety of the Neo depends on either nothing ever going wrong at that fragile interface, or the hub's own protection circuits preventing further disaster. Now, if we're too cheap to care about adequate protection for our USD 399 device, how likely is it that the makers of as-cheap-as-possible no-name generic USB hubs aren't ? (And your example shows that the honest effort we assumed above doesn't seem to go that far.) So this is more like building your chalet in a known avalanche zone and counting on everybody else being prudent enough to not set off an avalanche. I can see a trend in the risks we discover in this process, and it seems that the initial gut feeling that this isn't right was quite correct. - Werner _______________________________________________ hardware mailing list [email protected] http://lists.openmoko.org/mailman/listinfo/hardware
