MEGA CD Preservation

Timed communication with Game Processor

Game Processor receives status and sends commands from/to MiCom.
The process is available on page 78 of MegaCD 1 manual and page 21 of XEye technical manual

Within the communication frame, MiCom will

1. latches an IRQ when ready
2. sends 5bytes (10x 4bit nibbles) of status to Game Processor
3. receives 5bytes (10x 4bit nibbles) of command from Game Processor
4. process command (see next chapter)

TBCWhich commands should be proceeded within the time frame vs commands should be proceeded within several time frames/sectors ?

IRQ is latched by the MiCom, when ready, and reset after ?? usec.
An IRQ is latched every 1/75s (13.3ms) while data transfer is on progress , time for a full CD Track's sector,
and every 15.8ms if not. TBC

Game Process acknowledge the start of a new communication frame starting its HostClock (HOCK).
HOCK starts 8usec after IRQ latch.
??frequency unknown (4Khz?)

MiCom starts its MiComlock (CDCK) 33usec after HOCK start, acknowledging Host comm.
It sends its status nibbles in sync with CDCK.
??frequency unknown (4Khz?)

After the 10 nibbles of MiCom, Game Processor send its command nibbles in sync with HOCK.
??frequency unknown (4Khz?)

After the 10 nibbles of Game Process, HOCK and CDCK are stopped. TBC
MiCom and Game Process handle their own received nibbles and launch related tasks.

Nibbles are 4 bits sent in parallel on the databus (D0-D4).
At ?? Hz, sending 20 nibbles take ?? ms.
It means MiCom and Game Process have at least ?? ms to process received command/status before next IRQ.

Warning:
It's the Game Processor who drives the communication with its clock.
IRQ latch is a "I'm ready" signal, not a "Be ready" signal.
and MiCom answers to Host clock (not reverse).

Process command and update status

MiCom "translates" the received command to the CD Driver, the CD DSP or the drive itself.
It could so update its status for next frame and optionnaly the D/M signal.
Data are not handled by the MiCom itself, but by the CD DSP

Commands

Commands were not documented because Sega prohibited developpers to direct access the MiCom (see page 33 of MegaCD Hardware manual)
Hopefully, Stef, Michel Gerritse and Eke did a really great job to discover commands / status through BIOS analysis and hardware testing, to improve MegaCD emulation.
Years later, the JVC XEyes technical manual was found which include commands but also errors code (but not status!).

The 9 commands values (TCx) are nibble so between 0x00 and 0x0F

TC0: command ID (see MiCom Commands))
TC1: 0
TC2: MSN of Minute (BCD)
TC3: LSN of Minute (BCD) => TC2-TC3 = MM
TC4: MSN of Second (BCD)
TC5: LSN of Second (BCD) => TC4-TC5 = SS
TC6: MSN of Frame (BCD)
TC7: LSN of Frame (BCD) => TC6-TC7 = FF
TC8: 0
TC9: checksum <= NOT(TC0-8) & 0xF

MiCom Commands

TC0	Name	Parameters	Notes
0x00	NOP	-
0x01	STOP	-	Stop drive and lazer current position is 00:00:00
0x02	REQUEST	TC3 : request code TC6-7: if TC3 = 5	status according request
0x03	READ/PLAY	TC1-TC9 : start position	SEEK to start position THEN Play music / Read data ??seek time to be defined
0x04	SEEK	TC1-TC9 : start position	Move to position and wait ??seek time to be defined
0x05	unused
0x06	PAUSE	-
0x07	RESUME	-	Resume playing after one of the previous command (0x01 to 0x06) no SEEK needed
0x08	SCAN-FF	-	Fast forward/td>
0x09	SCAN-FR	-	Fast rewind/td>
0x0A/0x0B	TRACK ACCESS	TC3: direction (forward/rewind) TC4-7: seek length (*)	Doc mismatch between 0xA and 0xB, to confirm
0x0C	CLOSE	-	Close tray and STOP Only valid on MCD1 ?
0x0D	OPEN	-	STOP and Open tray Only valid on MCD1 ?
0x0E	MISC	????	WonderMega exclusive ? to confirm
0x0F	unused

 

Request code

Code	Description	Parameters	Status	Notes
0x00	get absolute time	-	absolute time on RS2-7
0x01	get present time	-	present time on RS2-7
0x02	get current track indedx	-	track index on RS2-3 (BCD), 1-based defaut:0x0A0A (see Eke)
0x03	get end time (TOC end)	-	total CD duration on RS2-7
0x04	get first and last track indexes	-	first track index on RS2-3 (BCD), so 1 last track index on RS4-5 (BCD) frame is 00
0x05	track start time	Track index in TC6-7 (BCD)	starttime on RS2-7 track index on RS8
0x06	get last error code	-	last error code on RS2-3	see below
0x0F	host is not ready	-	update status like with 0x00	it won't process data send nor acknowledge IRQ ?

 

While these commands are probably enough to handle most games, I won't be surprised to find more with

• Flux analysis
• Wonder libray analysis
• X eyes and WonderMega bios analysis (especially the 0xE command)

 

Status

The 9 status values (RSx) are nibble so between 0x00 and 0x0F

RS0: MSN of status (see Code Status)
RS1: LSN of status : 0 or, if command 0x2, TC3 back
RS2: MSN of Minute (BCD) or see command 0x2 (TC3 = 0x3) and 0x3
RS3: LSN of Minute (BCD) or see command 0x2 (TC3 = 0x3) and 0x3
RS4: MSN of Second (BCD) or see command 0x2 (TC3 = 0x3)
RS5: LSN of Second (BCD) or see command 0x2 (TC3 = 0x3)
RS6: MSN of Frame (BCD)
RS7: LSN of Frame (BCD)
RS8: 0 or see command 0x3 (TC3 = 0x5)
RS9: checksum <= NOT(RS0-8) & 0xF
??according Eke, RS8 (bit0 = mute status, bit1: pre-emphasis status, bit2: track type), ie Q Control bits ?

Status code

Code	Name	Description	Notes
0x00	STOP	stopped
0x01	PLAY	Playing music or loading music
0x02	SEEK	Moving to requested time
0x03	SCAN	scan in progress
0x04	PAUSE	paused	not stopped (0x00)
0x05	OPEN	tray opened	there is no closing status
0x06	SUM_E	Error communication checksum	RS8-9 / TC8-9 error
0x07	COMMAND_E	Missed the last command	too busy to handle it ? request host to send it again ?
0x08	ERROR	send error code (see below)	wait for the get last error command or not ?
0x09	TOC	Readind TOC
0x0A	TRK_MOVE	moving to requested track
0x0B	DISC_E	No disc
0x0C	END_DISC	drive reach lead-out
0x0D	not used
0x0E	TRAY	Manual action on tray
0x0F	TEST	Test mode	??? which test mode

 

Error code

Code	Name	Description	Notes
0x00	NO-ERROR	-
0x03	E-FOCUS	Focus down for more than 100msec	will retry until ok or issue an error 0x08
0x04	E-TRK	Returned to the normal state	?? whatever it means !!!
0x05	E-GFS1	No sync
0x06	E-GFS2	No sync	diff wih 0x05 ?
0x07	E-ACCESS	Timeout accessing CD
0x08	E-FOCUS2	Unable to focus

Serialization of CD data to Game Processor or LC8951

Whatever the data is (Audio or others), its flows on the SDATA line, serialized.

Thanks to l_oliveira, we know it uses the I²S protocol, with

• a bit clock (BLCK), so send each bit
• a frame clock (LRCK), to drive left (HIGH) then right (LOW) channel
• serial data (SDATA)
• + error flag (C2PO)
• + mute flag(D/M)

According I2S specification, BLCK is sample rate x number of channels x bits per channel.
Note: CXD2500AQ has a 48bits (24 bits per channel) precision. So we need to send 16bit + two's complement 8 bits first

LRCLK is sample rate

SDATA will so send its bits at audio speed, even if it's data

C2 Parity Output will raise when CD DSP was unable to fix error

D/M will raise if data should not be forwarded to LC7883

So, in fine, we have

• BCLK = 2.1168MHz (16MHz/8)
• LRCK = 44.1 KHz
• SDATA = channel L then channel R
• C2PO = 1 if error (0 if not)
• D/M = 1 if data, 0 if audio

Serialization of CD subcodes to Game Processor

Subcodes are send throught dedicated pins

• frame clock (WFCK) : so 1/75/98 (98 channel frames per sector) -> 7.35KHz TBC
• block clock (SCOR) : raise when sync word are detected
• host clock (EXCK) : init by host (10us after WFCK fall) to receive bit, at 1MHz
• serial data  (SBSO) : 8 bits per sector, 1bit x 8 subcode

SBSO updates its bit's value after EXCK raise, so it means 9 update for 8 bits.
The last bit is probably a false one (ie not read by Game Processor).
On every datasheet I read, they ask it to the same than first bit.
CD DSP probably roll the subcode byte, not shift it.

SCOR is set when SYNC word is detected.
So basically, on 2nd frame when CD DSP decoded S0 and S1 bytes.
SCOR is NOT a frame start indicator since first frame (with SO byte) doesnt rise it.

If you want to know more, the "Description of the prior art" of this patent is propably the easiest document to read I found

CD drive operations

a CD drive is able to

1. open/close tray or door, if available
2. detect valid CD
3. access to lead-in (TOC) and lead-out
4. jump/seek to a specific frame
5. transmit/read a frame to DSP
6. stop/resume

Nothing new here.