The Bluefruit LE SPI Friend makes it easy to add Bluetooth Low Energy connectivity to anything with 4 or 5 GPIO pins. With SPI, you don't have to worry about baud rates, flow control, or giving up a hardware UART port. Connect to your Arduino or other microcontroller using the common four-pin SPI interface (MISO, MOSI, SCK and CS) plus a 5th GPIO pin for interrupts (to let the Arduino know when data or a response is ready).