This week's Tip of the Week:Originally posted in cdnusers.org by wtan
When is the best time to insert isolation cells and level shifters? The earlier the better! If you are using a synthesis engine that understands isolation cells and level shifters, it's much better to insert them during synthesis, rather than waiting until the implementation stage. This way, the synthesis engine has knjowledge about the level shifters and isolation cells, and can optimize accordingly.
Level shifters, and level shifter/isolation combo cells have significant cell delay - on the order of hundreds of picoseconds.