Sega Dreamcast & NAOMI SCI-SPI Mod

[SWAT]

This guide explains how to access the SH-4 SCI port on Sega Dreamcast and NAOMI systems for both UART/SPI communication modes.
This is can be used for alternative SD card modding or debugging.
In SPI-like mode, SCI can reach transfer speeds of up to 1.5 MB/s, which is x2 faster than mod using the SCIF port for bit-bang SPI.

http://www.dc-swat.ru/blog/hardware/1143.html