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.

Sega Dreamcast Motherboard

Solder wires directly to the following points on the Dreamcast motherboard:

High resolution image.

Signal	Function	Pad Label	Color Code
MISO	Data In	R115	Red
MOSI	Data Out	R122	Blue
CLK	Clock	R140	Purple
CS	Chip Select	RA101	Yellow

On the same resistor array as the CS line (RA101), you can also solder to the other pins of this array to access additional SH-4 GPIO lines: GPIO0, GPIO5 and GPIO6, GPIO7 used as CS. This is useful for expanding control or using additional chip-select lines for multiple devices in future.

Sega NAOMI CN1 Connector

SCI (and also SCIF) signals are available on the CN1 connector without any soldering:

Signal	CN1 Pin	SH-4 Function
MISO	B45	SH4_RXD
MOSI	A46	SH4_MDI7/TXD
CLK	A47	SH4_MDO7/SCK
CS	B50	SH4_MD8/RTS2

SD Card pinout

Wiring & Notes

• 3.3V logic level only — use level shifters if necessary.
• For power, it's better to use a 5V to 3.3V DC-DC converter for the SD card, since both Dreamcast and NAOMI systems place the highest current load on the 3.3V rail.
• The length of the wires should not exceed 15-20 cm.
• For NAOMI, the CS line uses the RTS signal from SCIF, since no GPIO pins are available on the CN1 connector.

Example

The MOSI/MISO wires are mixed up in this photo, sorry =).
Additional wire is GPIO 0