Hardware Details - arthurbenemann/ardupilot GitHub Wiki

http://ardupilot-mega.googlecode.com/svn/ArduPilotMegaImages/IMU1.jpg

!ArduPilot Mega is an Arduino Mega-compatible autopilot. In consists of two boards:

1. A main processor board with either an Atmega1280 or an Atmega2560 processor, and an Atmega328 co-processors used as an RC interface. Details are here.
2. A "shield" or second board, which fits atop the first, which has a range of sensors, from gyros and accelerometers to pressure and voltage sensors. It serves as an Inertial Measurement Unit (IMU). Details are here.