The r.Pi I/O pins are 3.3V only and can supply only 5 mA of current. You cannot connect
them directly to Rx,Tx, I2C, or SPI on 5V Arduino boards.
One of the books shows using a USB cable between r.Pi and Arduino as the easiest interface.
It says the r.Pi can supply at least enough current [50 mA] to run the Arduino, as long as
there are no significant loads on the Arduino pins. The r.Pi itself is a current hog, 700 mA or
so, and the on board power ckts cannot supply very much current to anything connected to it.