For inputs from 5V logic to the Cygnal part you have to do nothing. The inputs are 5V tolerant.

For outputs to 5V logic you have 2 choices. If the 5V logic has a Vih less than 3 volts then program the Cygnal outputs to use totem pole output (i.e. to use active pullup to Cygnal 3V Vcc). If the 5V logic has a Vih higher then 3 volts then use the Cygnal outputs in the open drain mode and add external pullup resistors to +5 volts.

That in a nutshell is what you have to do. I include it here for our readers to see how easy it is. Rob's reference to the App Note is the best complete discussion.

Good Luck
Michael Karas