External crystal:
Without an external crystal the microcontroller runs with 8 Mhz from an internal RC-combination (resistor-capacitor). This RC-combination is not very stable and does not allow exact timing measurments. As the Atmel is a real RISC controller, a doubled clock rate means a really doubled speed (other than at the usual Windows PC) . Especially with graphics display, this doubled speed is highly welcome as it is immediately visible. By the way: if you order with an external crystal, we will set all internal fuses to the correct settings so you do not have to bother with this sometimes dangerous settings.
16 Mhz crystal: Needed for CAN; for RS232/RS485 usually OK
14,7456 Mhz crystal: not recommended for CAN (odd divisor, thus not a 100% exact timing which is crucial for CAN); but: perfect for RS232 / RS485 (100% exact baud rates).