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Wrong USB line termination in official LPC11U24 mbed schematic (TGT D+ D- lines)

I've been prototyping new hardware based on the LPC11U24 and I believe I've found a bug on the official mbed LPC11U24, which worries me especially considering many people (such as myself) base our custom designs based on the implied best-practices of the mbed design.

The bug is related to the termination of the USB D+/D- lines connected to the LPC11U24 and (subsequently the broken-out pins D+/D- on the actual mbed).

In almost every other USB implementation I've been able to dig up from multiple vendors (including the LPC1768 mbed!), the resistors and low-pF value caps form a low-pass filter to help reduce interference. In the case of the mbed LPC11U24, it appears the resistors vs capacitors are accidentally reversed!

Have a look at my screenshot with an arrow pointing to where I believe the resistors should actually be. As I mentioned, the mbed LPC1768 does this correctly, as do the D+/D- lines on the mbed magic chips on BOTH mbeds!

BUT, I'm not 100% confident about the validity of my statements because 1) I'm a USB hardware novice and 2) I imagine the @mbed went through multiple levels of review compared to my very novice self. My take on it is that of course the USB features certainly still work most of the time, but the filter is basically reversed, and half as effective at stopping incoming interference (assuming the device at the other end of the connection has the proper termination). In harsh conditions, this *could* negatively affect reliability.

 


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