LM1237_04 datasheet(17/42 Pages) NSC

17 / 42 page

OSD Generator Operation (Continued)

ATTRIBUTE TABLES

Each character has an attribute value assigned to it in the

page RAM. The attribute value is 4 bits wide, making each

character entry in the page RAM 12 bits wide in total. The

attribute value acts as an address which points to one of 16

entries in either the two-color attribute table RAM or the

four-color attribute table RAM. The attribute word in the table

contains the coding information which defines which color is

represented by color1 and color2 in the two color attribute

table and color1, color2, color3, color4 in the four-color

attribute table. Each color is defined by a 9-bit value, with 3

bits assigned to each channel of RGB. A dynamic look-up

table defines each of the 16 different color combination

selections or ‘palettes’. As the look-up table can be dynami-

cally coded by the microcontroller over the I

2C compatible

interface, each color can be assigned to any one of 2

9 (i.e.

512) choices. This allows a maximum of 64 different colors to

be used within one page using the 4-color characters, with

up to 4 different colors within any one character and 32

different colors using the 2-color characters, with 2 different

colors within any one character.

TRANSPARENT DISABLE

In addition to the 9 lines of video data, a tenth data line is

generated by the transparent disable bit. When this line is

activated, the black color code will be translated as ‘trans-

parent’ or invisible. This allows the video information from

the PC system to be visible on the screen when this is

present. Note that this feature is enabled on any black color

in of the first 8 attribute table entries.

12 columns

18 rows

COLOR 1

COLOR 2

20023421

FIGURE 14. A Two-Color Character

Plane 1

Plane 2

Composite

20023422

FIGURE 15. A Four-Color Character

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