LM1237BDKD/NA/NOPB National Semiconductor, LM1237BDKD/NA/NOPB Datasheet - Page 41

• Manufacturer Part Number: LM1237BDKD/NA/NOPB
• Manufacturer: National Semiconductor

Specifications of LM1237BDKD/NA/NOPB

• Operating Temperature (min): 0C
• Operating Temperature (max): 70C
• Operating Temperature Classification: Commercial
• Mounting: Through Hole
• Package Type: MDIP
• Lead Free Status / RoHS Status: Compliant

Attribute Table and Enhanced
Features
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
(Continued)
FIGURE 25. Character Font
41
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.
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