LM2467 datasheet(41/43 Pages) NSC

41 / 43 page

Attribute Table and Enhanced

Features (Continued)

BUTTON BOX FORMATION

The value of the most significant Enhanced Feature Bit

(EFB3) determines when to draw the left, right, bottom and

top sides of a Box. EFB1 denotes whether a box is raised or

depressed, and EFB2 denotes whether the box is normal or

“heavy”. For normal boxes, the lowlight color is determined

by the color code stored in the register EF2. For the heavy

box feature, the lowlight is determined by the color code

stored in register EF3. Boxes are created by a “pixel over-

ride” system that overwrites character cell pixel information

with either the highlight color (EF1) or low light shadow (EF2

or EF3) of the box. Only the top pixel line of the character

and the right edge of the character can be overwritten by the

pixel override system.

To form a complete box, the left hand edge of a box is

created by overwriting the pixels in the right most column of

the preceding character to one being enclosed by the box.

The bottom edge of a box is created by either —

•

overwriting the pixels in the top line of the character

below the character being enclosed by the box, or

•

overwriting the pixels in the top line of the skipped lines

below, in the case where skip lines are present below a

boxed character.

Characters should be designed so that button boxes will not

interfere with the character.

These are the limitations resulting from the button box for-

mation methodology:

•

No box may use the left most display character in the

Display Window, or it will have no left side of the Box. To

create a box around the left most displayed character, a

transparent “blank” character must be used in the first

character position. This character will not be visible on

the screen, but allows the formation of the box.

•

At least one skip line must be used beneath characters

on the bottom row, if a box is required around any char-

acters on this row in order to accommodate the bottom

edge of the box.

•

Skipped lines cannot be used within a box covering sev-

eral rows.

•

Irregular shaped boxes, (i.e., other than rectangular),

may have some missing edges.

Operation of the Shadow Feature

The shadow feature is created as follows: As each 12-bit line

in the character is called from ROM, the line immediately

preceding it is also called and used to create a “pixel over-

ride” mask. Bits 11 through 1 of the preceding line are

compared to bits 10 through 0 of the current character line.

Each bit X in the current line is compared to bit X+1 in the

preceding line (i.e., the pixel above and to the left of the

current pixel). Note that bit 11 of the current line cannot be

shadowed. A pixel override output mask is then created.

When a pixel override output is 1 for a given pixel position,

the color of that pixel must be substituted with the color code

stored in the register EF3. Please see Figure 25 for an

example.

Operation of the Bordering Feature

Borders are created in a similar manner to the shadows,

using the pixel override system to overwrite pixel data with a

pixel color set by EF3. However, instead of comparing just

the previous line to the current line, all pixels surrounding a

given pixel are examined.

The pixel override is created as follows: As each 12-bit line in

the character is called from ROM, the character line imme-

diately above and the line immediately below are also called.

A “Pixel Override” output mask is then created by looking at

all pixels surrounding the pixel. When a black override output

is 1 for a given pixel position, X, the color of that pixel

changed to the color code stored in the register EF3.

Because the shadowing relies upon information about the

pixels surrounding any given pixel, the bordering system

may not operate correctly for pixels in the perimeter of the

character (line 0 and 17, columns 0 and 11).

Constant Character Height Mechanism

The CRT monitor scan circuits ensure that the height of the

displayed image remains constant so the physical height of a

single displayed pixel row will decrease as the total number

of image scan lines increases. As the OSD character matrix

has a fixed number of lines, C, (where C = 18), then the

character height will reduce as the number of scan lines

increase, assuming a constant image height. To prevent this,

the OSD generator repeats some of the lines in the OSD

character in order to maintain a constant height percentage

of the vertical image size.

In the LM1247, an approximation method is used to deter-

mine which lines are repeated, and how many times each

line is repeated. The constant character height mechanism

will not decrease the OSD character matrix to less than 18

lines.

Display Window 1 to Display Window 2 Spacing

There is no required vertical spacing between Display Win-

dow 1 and Display Window 2, but they should not overlap.

There must be a two-character horizontal space between

Display Window 1 and Display Window 2 for proper opera-

tion of both windows or undefined results may occur.

20075743

FIGURE 25. Operation of the Shadow Feature

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