Auto animation

2bit (4 tiles) example from Mutation Nation

(Information from Charles MacDonald)

Auto-animation is a tile attribute for sprites and a special feature of the GPU used to animate loops without CPU usage.

An internal write-only 8bit counter is automatically decremented each frame. When it underflows, it is reloaded with the last value written to it, which comes from the 8 highest bits of REG_LSPCMODE. This sets the speed of the animation.

Each time that counter underflows, another internal 3bit counter is incremented. Those 3 bits can only be read in bits 2~0 of REG_LSPCMODE.

When auto-animation is disabled (bit 3 of REG_LSPCMODE set), the frame counter and animation counter continue to function. However, tiles are shown as if their animation mode select bits were reset (so the tile number specified is the tile shown).

These bits can be set in each tile's attribute word (odd bytes of SCB1):

Bit	15	14	13	12	11	10	9	8	7	6	5	4	3	2	1	0
Def	Palette								Tile number MSBs				3bit auto-anim	2bit auto-anim	Vertical flip	Horizontal flip

Bit 3 has priority.

Depending on the animation period (4 or 8 frames), the tile number specified in SCB1 has the lower 2 or 3 bits replaced with those of the animation counter.

Note that the bits are replaced, not added. Therefor, the consecutive tiles used for animation must be aligned on 4 or 8.