What happens when a Raspberry Pi meets an Arduino?
A simple example would be, when building a mobile robot, we use the Raspberry Pi to extend its vision and get a nice tiny monitor. Meanwhile, Arduino nicely handles the motor driving part and delivers fast responses like obstacle avoidance. Because, without the OS, the response time of rapid changes on the Arduino is much shorter. Also the high performance Pi can deal more easily with wireless communications, imaging and running complex algorithms.
Therefore, with the Arduino and Pi together, you get a powerful "brain" and "cerebellum" for your project, at one time.
Arduino Expansion Shield for Raspberry Pi Wikidocs Page
System Voltage: 5v
Arduino Digital I/O: 20
Arduino Analog I/O: 6
Raspberry Pi B+ GPIO: 16
Raspberry Pi B+ I2C: 1
Raspberry Pi B+ ID_I2C: 1
Raspberry Pi B+ SPI: 1
Raspberry Pi B+ TTL UART: 1
Dimension: 88mm x 56mm x 26mm(3.46" x 2.20" x 1.02")