LM1237 datasheet(41/42 Pages) NSC

41 / 42 page

Attribute Table and Enhanced

Features (Continued)

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 must be substituted with 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, line 17, column 0 and column 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 LM1237, 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. Each character will be at least 18 lines high.

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.

Evaluation Character Fonts

The character font for evaluation of the LM1237 is shown in

Figure 25. The actual font will depend on customer customi-

zation requirements.

Note that the first two character codes of the two-color font

(0x00 and 0x01) are reserved for the Window End (WE) and

Row End (RE) codes respectively.

20023445

FIGURE 25. Character Font

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