Put a new prototype board today (SMD) with the aim of using an external oscillator (4 pin variety) connected to XTAL1. (5v@20Mhz) It didn't work. I replaced it with an external crystal and it did. I'm presuming it's because I had the wrong fuse set.
I have two questions:
1. Are brand new IC's set to the internal MHz clock? If so, I presume I can then set the chip to use external clock?
2. There are 3 fuse options for LOW.SUT.CKSEL that mention Ext.Clock. The only difference seems to be startup speed. I'm guessing the slowest is a safe bet. But how to I get the chip to run at 20MHz?