Getting Up and Blinking with the Arduino - Exploring Arduino: Tools and Techniques for Engineering Wizardry - page 10

The Mega 2560 (see Figure 1-5) employs an ATMega 2560 as the main MCU,
which has 54 general I/Os to enable you to interface with many more devices.
The Mega also has more ADC channels, and has four hardware serial interfaces
(unlike the one serial interface found on the Uno).
Figure 1-5: The Arduino Mega 2560
Unlike all the other Arduino variants, which use 8-bit AVR MCUs, the Due
(see Figure 1-6) uses a 32-bit ARM Cortex M3 SAM3X MCU. The Due offers
higher-precision ADCs, selectable resolution pulse-width modulation (PWM),
Digital-to-Analog Converters (DACs), a USB host connector, and an 84 MHz
clock speed.
Figure 1-6: The Arduino Due