ps that 20mhz even with externak xtal is only possible with vcc at 5v. if u have 3.3v you wilonly get to 8 or 10...cant remember whic, you need to check the datasheet. anyway, 1mhz is plenty for anyone unless u are building a space shuttle, life support system or massive hadron collider. try everything at 1mhz till you get more of a handle on things or jntil you have circuit that needs some sort of respone in less than a millionth of a second.....none of mine does!