When developing more complex application that require additional I/O, more computation capabilities, or communication protocols not supported by the 3pi (e.g. SPI, I2C), it is helpful to use the 3pi as a Serial Slave of another microcontroller. According to the documentation: "This slave program receives serial data on port PD0 (RX) of the 3pi and transmits responses (when necessary) on port PD1 (TX), using a 115.2 kbaud, TTL-level serial protocol. "
