Every AVR microcontroller, from the ATtiny in your thermostat to the ATMega in your Arduino, stores its configuration in a series of fuse bits. These fuse bits control settings such as the ...

AVR chips can use external clocks like a crystal ... which spans pages 133 and 134 we can find the answer. Diagram 15-5 shows a clock settings table. In our case we need to set CS10 and CS11 ...