What's the status of this chip? Announced last november, it's apparently shown up only as the eval board. That's fairly typical, but in this case:
1) There's no 328PB chip in include/avr/iom*.h, or selectable from Atmel Studio.
2) The datasheet is a mess, mentioning a second UART and TWI interface in several places ("Features", table 6-1 "Port Function Multiplexing", Sec 15.11 "Power reduction register") but not in other places (UART or TWI sections, "register summary.")