MEGA CD Preservation

Introduction

Sega MegaCD's drive is the easiest part to broke and it's becoming more and more difficult to find a replacement part.

For homebrew developper, it's also very difficult (and costly!) to burn CD-R at low speed for test, debug or release. (there is no more 1x CD burner on the market!)

There is so a real need to for an alternative.

This alternative already exist (while it's mainly for piracy reason) on newer systems : WiiLoader, X360Dock, WODE, ...

To develop an optical drive emulator for the MegaCD, you first should know what you have to emulate.

While newer systems use custom version of IDE or others classic connection, the MegaCD uses its own connection, its own protocal and its own driver.

This page aims to list all the informations needed to emulate the MegaCD drive at the hardware level.

Another alternative will be at the software level (BIOS) which won't be explained here.

Disclaimer

This page is awlays in WIP mode.
A lot of informations are still missing or incomplete, so use them as a basis not as official documentation !

Thanks

This page wouldn't exist without the help of some great people

• Stef (CDC reverse engineering)
• Eke (more CDC reverse engineering)
• l_oliveira (hint on I2S protocol, ...)
• Nemesis (LC8951 doc)
• omp (service manual funds)
• others members involved in the related post

thanks guys, it's what make me so proud to host SpritesMind

Hardware

The MegaCD 1 (front loading) is made of 3 boards

1. Main board (logic and sound process)
2. Sub board (power supply, sound)
3. CD board, attached to the CD drive (CD controller)

INSERT  INSIDE PICTURE OF MCD

Note : the MegaCD 2 is made of 1 board, which make it harder to mod

Since we want to emulate the CD controller, we'll focus on the connection between main board and cd board.

This connection is made throught a 25 Pin XX inch FCC.

Thanks to the MegaCD1 and MegaCD2 service manuals, we know the name and (almost) goal of each of the 25 pins

INSERT ZOOM OF page 99 MCD1

the 25 pins connector on MCD1

INSERT schema 5.3 page 9 MCD2

the connection pins on MCD2

Main board

On the main board, we'll focus on  these ICs

• 68000 (main CPU)
• Sega 515-5548 (ASIC)
• LC 8951 (CD ROM processing)
• LC 7883 (CD DA processing)

INSERT  INSIDE PICTURE OF MCD, FOCUSED ON MAIN BOARD

While 68000 is the main CPU, the brain of the MegaCD is the Sega 515-5548.

This ASIC drives every component of the MegaCD : memory, data, sound, CD...All of them take receive command from and report to 515-5548.

LC8951 and LC7883 are no exception : according the kind of data it receives from the CD board, 515-5548 forwards the data to one of them.

Since all of commands and reports are digital data, EVEN FOR AUDIO, it's a real good news !

What is happening on the main board ?

1. 68000 sends command to 515-5548
2. 515-5548 translates this command to identify IC required (memory, sound, video, data ...)
3. For CD related command, 515-5548 sends command to CD board (or simply ask for status)
4. 515-5548 receives status from CD board
5. if not problem, 515-5548 receives data from CD board
6. 515-5548 forward the data to LC 7883 if it's music or LC 8951 if it's not

CD board

On the cd board side, we'll focus on

• CXP2500AQ (CD  DSP)
• CXP5084 (4bit micro computer)
• CXA1372AQ (RF Amplifier)
• CD drive

INSERT  INSIDE PICTURE OF MCD, FOCUSED ON CD BOARD

What is happening on the CD board ?

1. CXP5084 informs main board it's ready for a new cycle
2. CXP5084 sends status of previous command
3. CXP5084 receive a new command from main board (515-5548)
4. CXP5084 translate this command to command ofor the CXD2500AQ and/or CXA1372AQ
5. CXA1372AQ sets up CD drive
6. CD drive flows analog (?) data to CXA1372AQ
7. CXA1372AQ applies error correction on data
8. CXA1372AQ sends fixed data to CXD2500AQ
9. CXD2500AQ informs CXP5084 if  data is music or others
10. CXD2500AQ informs CXP5084 of its status
11. CXD2500AQ sends serialized data to main board (515-5548)
12. CXP5084 stores status and .... loop

Process is missing more details, contact me if you could help !

Software

• commands sent to CXP5084
• status received from CXP5084
• logic data received from CXD2500AQ

This workflow is more detailled on MegaCD2 service manual

INSERT schema 5.3 page 9 MCD2

To emulate the CD controller means to be able to handle

• correct communication with main board
• CD data communication
• subcode data communication

Communication with main board

Sega 515-5548 sends commands to and receive status  from CXP5084.

But how, when and what exactly ?

You could find the details on page 77 of MegaCD 1 manual

INSERT translation of page 77 of MCD1

Basically, CXP5084 latches an IRQ to inform 515-5448 of the start of a new communication frame.

This communication frame is 1/75seconds long....which happens to be the time of a CD Track's sector.

Within this frame, CXP5084 will send 8bytes (10x 4bit nibble) of status to 515-5448 THEN receive 8bytes (10x 4bit nibble) of command from 515-5448.

Nibbles are 4 bits, which means they could be sent in parallel on the databus.

For command nibbles, nibble0 (TC0) must be shown as the command ID, while the 9 others like (optional) parameters.

While the need for 2 clocks rathen than 1 is still a mystery for me, CPX5084 sends its status nibbles in sync with its CDClock (CDCK) while 515-5448 sent its command in sync with its HostClock (HOCK).

The 2 clocks aren't exactly in sync, there is a delay of 33µs between then, HOCK in first.

Since it's the CXP5084 which drives the communication, CXP5084 latches IRQ every 1/75s, sends back last command status and gets an optional new command.

?? Unfortunatly, the service manual is not readable enougth to know if the 16 bytes takes the full frame or not.

?? It's so not clear if the task, result from the command, have to be handled in this frame or if task could take several frames (due to CD lags, it must be the valid possibility), CPX5084 sending a kind-of "working" status while waiting for results.

• Flux analysis
• Wonder libray analysis
• X eyes and WonderMega bios analysis
• sniffing commands sent to CXD2500AQ, according commands sent to CXP5084 (see CXD2500AQ pins DATA, ClK, and XlAT)

• a bit clock (BLCK) 
• a frame clock (LRCK), to drive left (HIGH) then right (LOW) channel
• serial data (SDATA)
• error flag (C2PO)