Sega Mega Drive USB Link

Fun and profit sending bits over USB to the Mega EverDrive X7 and PRO cart

Getting Started

This program has three modes as defined by the code in src/main.c. The mode can be changed by modifying the source and re-compiling the ROM:

#define MODE_PING 0
#define MODE_RATE_TEST 1
#define MODE_PCM 2
#define MODE_PRO 3

static u16 default_mode = MODE_PRO;

PRO is for testing the Mega EverDrive PRO's USB functionality.

PING (X7 only) is a method of calculating round-trip latency from a PC to a Mega Drive by having the PC send a character to the Mega Drive, which upon receipt will immediately respond with a character back. The many_ping script can be used to find the average time to send & receive ping requests & responses.

RATE_TEST (X7 only) is a method of calculating throughput in bytes per second, by simply counting bytes received over the USB. Piping /dev/zero into the USB serial device works well!

PCM (X7 only) allows you to stream PCM audio through the USB serial interface direct to the DAC of the YM2612 FM chip. For example:

ffmpeg -y -i input.mp3 -ar 32000 -acodec pcm_u8 -f u8 -ac 1 - > /dev/cu.usbserial-A105J2JO

A sample rate of 32,000 Hz seems to work fine with the program as-is.

Results

The following metrics were calculated using the tooling described above:

Maximum throughput/rate = 50,000 bytes/sec.
Mean latency = ~2.1 ms.

The cart seems to have a 128 byte FIFO buffer which blocks if it becomes full.

Compiling ROM

Docker:

./docker-make all

Linux (requires cmake):

make all

Running Unit Tests

Unit tests built with cmocka and are compiled to x86. Mega Drive interfaces are mocked out.