Looking around, I see some applications of micros and USB with
22 to 27 ohm resistors in series with D+ and D-. So far though I've
not found why some constructors choose to use them and some
don't. The only reason I can think of is that they would be protection
againsgt a short to ground, yet a short to ground somewhere in the
middle of a PCB seems the least likely place. And why 22-27 and
not some higher value ? I ask because I'm drawing up a circuit for
the 18F4550. I can, of course, and will, put pads for esistors but
would like to know if and/or why they're needed

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