I hope this would be useful for you .
http://www.philipslogic.com/products/...m=74HC4514
then you can imagine how to start stop your servo formidably . This datasheet also contain devicespec .

If you want to transmit command via serial port ,it take cpu time only when 1 data byte complete .It's up to you how hard -real time you need and how your software scheduling.

For other question please refer to data sheet.