LM1246 datasheet(21/51 Pages) NSC

21 / 51 page

OSD Generator Operation (Continued)

CHARACTER CELL

Each character is defined as a 12 column by 18 row matrix of

picture elements, or “pixels”. The character font is shown in

Figure 17 through Figure 24. There are two types of charac-

ters defined in the character ROM:

Two-color: There are a total of 384 two-color characters

in 6 banks (banks 0, 1, 2, 4, 5 and 6). Each pixel of these

characters is defined by a single bit value. If the bit value

is 0, then the color is defined as “Color 0” or the “back-

ground” color. If the bit value is 1, then the color is

defined as “Color 1”, or the “foreground” color. An ex-

ample of a character is shown in Figure 17. The grid

lines are shown for clarity to delineate individual pixels

and are not part of the actual displayed character.

Four-color: There are a total of 128 four-color charac-

ters, in two banks of 64 (banks 3 and 7). Each pixel of

the four-color character is defined by two bits of infor-

mation, and thus can define four different colors, Color

0, Color 1, Color 2 and Color 3. Color 0 is defined as the

“background” color. All other colors are considered “fore-

ground” colors, although for most purposes, any of the

four colors may be used in any way. Because each

four-color character has two bits, the LM1247 internally

has a matrix of two planes of ROM as shown in Figure

18. In that figure, dark pixels indicate a logic “1” and light

pixels which indicate a logic “0”. The left side shows

plane 0 which is the least significant bit and the middle

figure shows plane 1 which is the most significant bit.

The right side composite character formed when each

pixel is represented by its two bits formed from the two

planes. The color palette used in this example is “00” for

white, “01” for black, “10” for blue and “11” for red.

By appropriately selecting the color attributes, it is pos-

sible to have two 2-color characters in one four color

ROM location. If the required number of four color char-

acters is less than 128, the remaining characters can be

used to increase the number of two color characters

from 384 to 384 + 2*N, where N is the number of unused

four color characters. This is explained in the next

section.

FOUR COLOR FONT AS TWO 2-COLOR

Using a 4 color character as two 2 color characters is

achieved by careful assignment of the four colors. When two

2 color characters are combined, there will be four pixel

colors:

Color 0: Those

that

are

background

pixels

for

both

characters,

Color 1: Those that are foreground pixels in character one

20068530

FIGURE 17. Two-Color Character

20068531

FIGURE 18. Four-Color Character

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