Standard Television Interface Circuit

The AY-3-8900, also known as the Standard Television Interface Chip or STIC, is a video display controller (VDC) produced by General Instrument for use with their CP1600 CPU in games consoles.^[1] It is best known as the basis for the Mattel Intellivision.^[2][1][3][4]

The STIC is typical of VDCs of the era, using a grid of character-like cells to draw a background graphic and then using up to eight sprites they called "movable objects" (MOBs), to produce animation. The overall resolution is 167 × 105 pixels in NTSC (in the 8900-1 model) and 168 × 104 pixels in PAL (8900 model),^[1] but only visible in an area of 159 × 96 pixels. The extra pixels around the visible area allow sprites to be placed in those locations and then smoothly move on-screen. The background consists of a 20 × 12 grid of 8×8 patterns known as "cards", which can be used as characters or other shapes. The STIC also computes collision information between the objects and screen borders.

Characteristics

• able to operate up to 4 MHz, but is generally run at 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^[5] 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

The chip generates a sixteen color palette, based on four bit input and divided into two sets. The following table shows bit and YIQ values as presented in technical information:^[3][6][7]

AY-3-8900 color palette

Color Set	Color	Bits	Y	I	Q
Primary	Black	0000	0.000	0.000	0.000
	Blue	0001	0.330	-0.733	+0.660
	Red	0010	0.523	+0.666	+0.200
	Tan	0011	0.715	+0.266	-0.133
	Dark Green	0100	0.413	-0.133	-0.600
	Green	0101	0.577	-0.200	-0.533
	Yellow	0110	0.853	+0.533	-0.333
	White	0111	1.000	0.000	0.000
Pastel	Gray	1000	0.550	0.000	0.000
	Cyan	1001	0.660	-0.533	-0.266
	Orange	1010	0.687	+0.533	-0.066
	Brown	1011	0.330	+0.266	-0.266
	Magenta	1100	0.550	+0.400	+0.667
	Light Blue	1101	0.660	-0.400	+0.400
	Yellow-Green	1110	0.687	+0.066	-0.533
	Purple	1111	0.440	+0.133	+0.533

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

See also

• Thomson EF9345
• Motorola 6845
• TMS9918
• MOS Technology VIC-II