Auto animation
Some information from Charles MacDonald.
Auto-animation is a tile attribute for sprites and a special feature of the video controller, used to animate simple loops with no CPU usage.
An internal 8-bit timer is ticked each frame. When it underflows, two things happen:
- The timer 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.
- An internal 3bit counter is incremented. Those 3 bits can only be read in bits 2~0 of REG_LSPCMODE.
When setting the auto-animation speed via REG_LSPCMODE, the new value is only taken into account the next time the timer underflows (reloads), not immediately after the write.
When auto-animation is disabled (bit 3 of REG_LSPCMODE set), the timer and animation counter continue to function. However, all 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 | 3-bit auto-anim | 2-bit auto-anim | Vertical flip | Horizontal flip | ||||||||||
Effects:
| Bit 3 | Bit 2 | Effect |
|---|---|---|
| 0 | 0 | No auto-animation for this tile. Tile shown is the tile number. |
| 0 | 1 | Auto-animation over 4 tiles. |
| 1 | 0 | Auto-animation over 8 tiles. |
| 1 | 1 | Auto-animation over 8 tiles (bit 2 is ignored). |
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. Therefore, the tiles must be carefully aligned to the right positions in the C ROMs.