Mixed voltage designs – interfacing components
To interface a 5V output to a 3.3V input on a slow signal, use a simple voltage divider. Note, however, that these added resistors will slow the rise time of the signal.
For a faster signal, run the signal through a buffer in the VHC logic family. These parts have 5V tolerant I/O even when powered by 3.3V.
To interface a 3.3V output to a 5V input, run the signal through a buffer in the HCT product family. These parts have a TTL input stage with a Vih spec of 2V.