Watchdog

The watchdog is an anti lock-up/protection feature consisting of a resettable down counter located in the NEO-B1 chip, that resets the system if it underflows.

It must be therefor "kicked" regularly to reset the counter to its highest value. This is done by writing any byte to any odd addresses in the $300000~$31FFFF range. Register REG_DIPSW ($300001) is the most often used. VBlank routines often take care of this task.

A constantly resetting system produces a typical clicking sound, giving a clue for solving boot issues. (Sound file ?)

The NeoGeo CD 2 also has a watchdog timer ?

Informations from MESS's source:

The watchdog timer will reset the system after ~0.13 seconds (Clocked by what signal ?).

Newer games force a reset using the following code (from KOF99): MOVE.L  #$30D40,D0 SUBQ.L  #1,D0 BCC.S   *-0x2

The watchdog is used as a form of protecetion on a number of games, previously this was implemented as a specific hack which locked a single address of SRAM. If the game doesn't find valid data in the backup ram it will initialize it, then sit in a loop. The watchdog should then reset the system. If the watchdog fails to reset the system the code will continue and set a value in backup ram to indiate that the protection check has failed.