LM2467 datasheet(21/43 Pages) NSC

21 / 43 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 13 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 color 0 and color 1 in the two color attribute

table and color 0, color 1, color 2, color 3 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 ’palettes’. As the

look-up table can be dynamically coded by the microcontrol-

ler over the I

2C compatible interface, each color can be

assigned to any one of 2

9 (i.e. 254) 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 any of the first 8 attribute table entries.

HALF TONE TRANSPARENCY

In addition to the transparent disable bit, there is a half tone

disable bit. When the transparency is already in effect and

the half tone feature is activated, the contrast of the PC video

that is visible in the “transparent area” is reduced to 50%,

providing a semi-transparent effect. Just as in the conven-

tional transparency mode, half tone transparency is effective

on backgrounds or foregrounds with black color codes from

only the first 8 attribute table entries. This feature is con-

trolled by bit 7 of the frame control register, 0x8400, and is

only available when the transparency mode is already en-

abled.

20075733

FIGURE 18. Displaying Each Character Individually

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