So, I'm getting ready to program an ATMega6450 for the first time and notice all these options for the clock source (below). I'm using an external 32kHz oscillator (the application is an actual clock, so I'm using a precision time base). I can tell I don't want an internal oscillator or an external xtal, but what's the difference between an EXTCLK and EXTXOSC? And where did you find that info? I can't find any of these prefixes in the datasheet, though I know the CKs and MSs refer to startup and wait times. Thanks.