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().

• https://www.gammon.com.au/forum/?id=10892