I've just noticed that the K20 (I'm using the 48-pin QFN version) has the USB signals (USB0_DP and USB0_DM) on pins 3 and 4, respectively. However, most normal Micro-USB and Mini-USB connectors provide the same signals in opposite order, e.g D- and then D+. Unless one gets a (rare) reversed connector, traces need to be crossed. Which one is not supposed to do with a differential USB pair.
That seems strange: I would have thought the pinout would be designed to fit common connector types placed right next to the chip on the same side of the board. This seems to be how Atmel does it, for example: on an ATMEGA32U4 D- comes first on pin 3, then D+ on pin 4.
What am I missing? And how do people deal with this? Ignore the recommendations and cross the traces? Use reversed connectors only? Mount connectors on the other side and use vias?