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Another question – are the U8’s extra pins wired to any pads or vias?
@joby – we added that, thank you.
One potential issue is that there is a worldwide shortage of Atmel USB chips. Word from their disty is that all their fab capacity is being taken up by a large cell phone customer (not Apple). It’s possible that this may affect the Uno’s availability depending on the stockpile of 8U2’s they have.
So if the ATmega8U2 include UBOOT and a LUFA CDC implementation, is there still room for other stuff as well? Or would becoming a mouse necessitate getting rid of the serial code?
For driverless operation, you’d have to get rid of the serial code. Otherwise you’d require a mixed USB class driver if you wanted your device to run CDC class drivers as well as HID (mouse). This would be a major pain because you’d need to write drivers for all platforms so I doubt this would get done.
I just noticed a curious part of the schematic where the USB port is connected to the mega8U2.
Doesn’t the addition of a “BLM21” ferrite bead (L1) as a connection between the USB connector case shielding and board ground render the TVS protection useless?
The path from the USB data lines via the TVS is low-impedant to the USB case, but it is a high-impedance path to the board ground via the ferrite bead, at least for transients, so the mega8U2 can be damaged this way. Even worse, since the USB case is intended to make contact first, a transient impulse can propagate from the case via the TVS into the mega8U2…
Except if this ferrite bead is of a low-inductance and low-resistive design, but that isn’t stated.
Furthermore, what is the purpose of the 27 Ohms resistor R3 in series with the mega8U2 quartz crystal? Is this because of EMI reasons? But then, isn’t XTAL1 the high impedance
/input/ pin for the mega8U2’s internal pierce oscillator? If so, an EMI series-resistor would only make sense on XTAL2. Plus, isn’t the 1Meg across the quar(t)z already internally integrated, too?