# Communicating through SPI

Many very useful device are able to communicate through the SPI
protocol, for example, SD cards and AVR MCUs. In many cases,
however, CPUs can't "speak SPI" because of their inability to
bit-bang.

In most cases, we need an extra peripheral, which we can build
ourselves, to interface with devices that "speak SPI". We call
this peripheral a SPI relay.

The design of those relays depend on the CPU architecture. See
spi.txt in arch-specific folders for more information.