Check the data sheet, they have recommended some parts to go with it for serial port level drivers for lack of a better term.
Not available as samples yet tho, and only in 48 TQFN package.
http://datasheets.maxim-ic.com/en/ds/MAX14830.pdf 1-4MHz crystal. 1.8MHz only specific callout I saw.
The 128 word buffer per UART should make for smooth software operation I would think (vs sucking up a bunch of atmega RAM).