Standard Television Interface Chip
# Standard Television Interface Chip

The Standard Television Interface Chip or STIC is a video encoder chip produced by General Instrument as AY-3-8900/AY-3-8900-1 and used on the Mattel Intellivision.

The chip provides all the display functions on the machine, and works as an internal timer. Resolution is 167 × 105 pixels in NTSC and 168 × 104 pixels in PAL, over which movable objects (MOBs) can be placed. These are restricted to a visible area of 159 × 96 pixels. Other objects, such as a 20 × 12 matrix of 8×8 background cards can be used to create scenery or provide game information. The STIC also computes collision information between the objects and screen borders. There are multiple display modes depending on how objects are handles, such as Color Stack, Colored Squares and Foreground/Background mode.

## Characteristics

* operates at 4 MHz or 3.579545 MHz (NTSC)
* 14-bit multiplexed data/address bus shared with CPU
* 20×12 tiled playfield, tiles are 8×8 pixels for a resolution of 159×96 (right pixel not displayed)
   * 16 color palette, two colors per tile
   * Foreground/Background mode; all 16 colors available for background and colors 1–8 available for foreground per tile; grom cards limited to the first 64
   * Color Stack mode; all 16 colors available for foreground per tile; background colour from a four colour rotating stack of any four colors, all 277 grom and gram cards available
   * Colored Squares mode allows each tile to have four different colored 4×4 blocks as in Snafu); first seven colors available for foreground blocks; background colour from the color stack
* 8 sprites (all visible on the same scanline). Hardware supports the following features per-sprite:
   * coordinate addressable off screen for smooth edge entries and exits
   * Size selection: 8×16 or 8 pixels wide by 8 half-pixels high
   * Stretching: horizontal (1× or 2×) and vertical (1×, 2×, 4× or 8×)
   * Mirroring: horizontal and vertical
   * Collision detection: sprite to sprite, sprite to background, and sprite to screen border
   * Priority: selects whether sprite appears in front of or behind background.
* fine horizontal and vertical pixel scrolling
* all STIC attributes and GRAM re-programmable at VBLANK, 60 times a second

## Color Palette

A 16 color palette is available, divided into two sets.

Primary Color Set

| Black | Blue | Red | Tan | Dark Green | Green | Yellow | White |
| - | - | - | - | - | - | - | - |

Pastel Color Set

| Gray | Cyan | Orange | Brown | Pink | Light Blue | Yellow Green | Purple |
| - | - | - | - | - | - | - | - |

Note: The displayed colors are approximate. Actual tones varied according to the analog television standard and quality of the CRT display.

