LM1247 datasheet(19/48 Pages) NSC

19 / 48 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 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. 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.

ENHANCED FEATURES

In addition to the wide selection of colors for each character,

additional character features can be selected on a character

by character basis. There are 3 Enhanced Feature Regis-

ters, EF0, EF1 and EF2.

Button Boxes — The OSD generator examines the char-

acter string being displayed and if the “button box” at-

tributes have been set in the Enhanced feature byte,

then a box creator selectively substitutes the character

pixels in either or both the top and left most pixel line or

column with a button box pixel. The shade of the button

box pixel depends upon whether a “depressed” or

“raised” box is required, and can be programmed

through the I

2C compatible interface. The raised pixel

color (“highlight”) is defined by the value in the color

palette register, EF1 (0x8405 - 0x8406), which is nor-

mally set to white. The depressed pixel (“lowlight”) color

by the value in the color palette register, EF2 (0x8407 -

0x8408), which is normally set to gray. See Figure 19 for

detail and Figure 20 for the on-screen effect.

Heavy Button Boxes — When heavy button boxes are

selected, the color palette value stored in register EF3

(0x8409 - 0x840A) is used for the depressed (“lowlight”)

pixel color instead of the value in register EF2.

Shadowing — Shadowing can be added to two-color

characters by choosing the appropriate attribute value

for the character. When a character is shadowed, a

shadow pixel is added to the lower right edges of the

color 1 image, as shown in Figure 21. The color of the

shadow is determined by the value in the color palette

Bordering — A border can be added to the two-color

characters. When a character is bordered, a border pixel

is added at every horizontal, vertical or diagonal transi-

tion between color 0 and color 1. See Figure 22. The

color of the border is determined by the value in the color

palette register EF3 (normally black).

Blinking — If blinking is enabled as an attribute, all colors

within the character except the button box pixels which

have been overwritten will alternately switch to color 0

and then back to the correct color at a rate determined

by the microcontroller through the I

2C compatible

interface.

20048433

FIGURE 18. Displaying Each Character Individually

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