What they are talking about there is voltage drop across the F3 polyfuse. This is a self-resetting fuse that runs between the PSU and the rest of the circuit. A polyfuse is a bit like a "classic" fuse, except that when it blows, it resets itself afterwards. Sounds great, right? It is, except that polyfuses have a non-negligible resistance to start with (so you will see a certain amount of voltage drop across them even in a normal situation), and once they have blown once, they have a tendency to increase in resistance (and thus voltage drop) even in the normal case. They also have a non-negligible "reset time", if you blow the fuse once, and then retry straight away, you're liable to make things worse. It is possible (and appears to have been the case in the thread you linked to) that excessive polyfuse resistance can cause voltages seen by USB devices to be well below the spec, and cause problems at the USB device end (anything less than 4.75 volts at the device, like the 4.5 volts measured in the linked thread, is out of range).
There's a few issues that can cause "out of range" voltages on the Pi.
The first is the PSU. If it's too weedy, then you will see voltage drops as the processor or any pi-powered peripheral "wakes up" and pulls more juice. Obviously, the problem here is the PSU, and the solution is obvious. As a hint, you should generally look at your PSU, take the rated supply current, and divide it by the number of outlets
, because manufacturers are twats; if you have a PSU that claims to supply 2A, but it has 4 USB outlets, chances are it will supply 500mA maximum per port, and that's not
going to be enough for a Pi with SD card, keyboard and mouse. The only really safe option is to take a brand name PSU that's intended for something like an iPad, and not some $0.99 no-name chinese wall wart from eBay.
Secondly, there's the F3 polyfuse, which might be out of range (either manufacturing flaw, or due to having been tripped heavily in the past). This is measurable using the method described in the thread you linked to - if it's OOR then you have a decent chance at getting your Pi replaced - it's defective.
Thirdly, on "first batch" Pis, there were also F1 and F2 polyfuses on the USB ports themselves (other side of the board to F3, next to the USB ports). These have been removed / replaced with 0Ohm resistors on later batches. If you have them (they should be green SMDs like F3, probably marked "14", the replacement resistors will probably be black and will definitely be marked "0"), the measurement process is exactly the same as that for F3. Again, OOR -> defective Pi.
How do polyfuses blow? By putting devices on the USB port that try to draw too much current. Anything more than 100mA is too much. A good example would be a USB-powered hard drive. Putting one of these on your Pi will probably fry the polyfuses straight away. The Pi has /nothing/ to stop a device trying to pull track-melting currents from USB apart from the polyfuses. Many, if not all, USB WiFi dongles will pull more than 100mA...
GT-Force wrote:I have neither time, nor will to exchange boards until I hit a good one. From my perspective, This is a huge opportunity wasted with a bad design. I wish it was as advertised, but looks like it just isn't.
I can sympathise with this, but it should probably be pointed out that the Pi was never advertised as a magical $35 media centre.
FWIW, Gordon, I just pulled out the multimeter and re-measured my Pi. Powered off the USB charger, 0.02V drop over F3 and 0.04 / 0.03V over F1 and F2 respectively. Worst case, I'm seeing 4.92V at the device (My Matias is currently gutted, and I have access to the incoming USB lines...)