CD drive control

Communication between the CD Drive (CDD) and the CD Controller (CDC) is done via a special 4-bit protocol handled by the system ROM.

The NEO-MGA chip provides the I/Os through registers REG_CDDCTRL, REG_CDDINPUT, and REG_CDDOUTPUT.

• The CDD regularly (~64Hz not 75) sends a low pulse on its /IRQ output to initiate a data exchange.
• HOCK goes low, indicating that the CDC aknowledges the request. The 4-bit bus direction is set to CDD -> CDC.
• CDCK goes low, indicating that the CDD is starting to send its status data (A).
• HOCK goes high and the CDC reads the status data (A).
• CDCK returns high.
• HOCK returns low.
• CDCK goes low again, the B word is ready.
• The cycle continues on for 10 words...

(The delay here is not necessary, it is seen on the NeoGeo CD because of the system ROM preparing stuff).

• HOCK goes low and the 4-bit bus direction is reversed (CDC -> CDD). The first command word is set (K).
• CDCK goes low, indicating that the CDD has read the data (A).
• HOCK returns high.
• CDCK returns high.
• HOCK goes low again, the B word is ready.
• The cycle continues on for 10 words...

Status data (green)

• A: Status
  • 0=Stop mode
  • 1=Playing data or music
  • 2=Seeking
  • 4=Paused
  • 5=Tray opened
  • 6=Comm. checksum error
  • 7=Command error
  • 8=Function error (mech/laser ?)
  • 9=Reading TOC
  • 10=Sled moving
  • 11=Couldn't focus, no disk
  • 12=Sled at edge, end of disk
  • 14=Tray moving
  • 15=In test mode
• B: Status
• C: Minutes high digit
• D: Minutes low digit
• E: Seconds high digit
• F: Seconds low digit
• G: Frames high digit
• H: Frames low digit
• I: Extension
• J: Checksum (see below)

Command data (red)

• K: Command
• L~S: Depends on command (see below)
• T: Checksum (see below)

Checksum

The checksum digit is the sum of all 9 previous digits, + 5, XOR 15, AND 15: ((sum + 5) ^ 0xF) & 0xF.

Commands

See CD drive commands.