SPI
SPI requires the following pins:
- SCK, to synchronize devices
- MOSI/COPI, for data transfer from the master to the slave
- MISO/CIPO, for data transfer from the slave to the master
- SS/CS (one for each slave), for selecting a slave to communicate with
Though it varies by chip manufacturer, the general process is to pull the desired CS pin low, perform a transfer, and set CS high again. In the Arduino SPI library, these steps should be surrounded by SPI.beginTransaction() and SPI.endTransaction().