Watchdog

The watchdog is an anti lock-up/protection feature in AES/MVS system: A resettable down counter located in the NEO-B1 chip 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 register REG_DIPSW ($300001). Games VBlank routines often take care of this task.

Constantly resetting systems produce a typical clicking sound (due to ?), giving a clue for solving boot issues.