My AVR is running very slowly, and UART transmission gives strange characters. Why?
AVR devices come with Internal RC set as clock source by default. The speed varies from 0.5-4MHz depending on which AVR device it is. To get the AVR to run from a crystal on the board, you need to change Clock Source fuses (CLKSEL) to External crystal. You may also need to switch off Clock divide by 8 fuse (CLKDIV8).