LM1247 National Semiconductor, LM1247 Datasheet - Page 43

• Manufacturer Part Number: LM1247
• Description: 150 MHz I2C Compatible RGB Preamplifier with Internal 512 Character OSD ROM, 512 Character RAM and 4 DACs
• Manufacturer: National Semiconductor

Attribute Table and Enhanced
Features
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.
(Continued)
FIGURE 29. ROM Bank 0 Two Color Character Font
43
Evaluation Character Fonts
The character font for evaluation of the LM1247 is shown in
Figure 29 through Figure 36, where each represents one of
the 8 available ROM banks. Each bank is shown with in-
creasing character address going from upper left to low right.
The actual font will depend on customer customization re-
quirements.
Note that the first two character codes of the two-color font in
ROM bank 4 (0x00 and 0x01) are carried over from the
LM1237 ROM where they were reserved for the End-of-
Screen (EOS) and End-of-Line (EOL) codes respectively.
In the case of the LM1247, these two locations can be used
for displayable characters as long as they are not needed
when this bank is addressed from Bank Select Register 0. If
it is addressed from Bank Select Registers 1, 2 or 3 then
these two lower characters will be usable. Please see the
section “END-OF-LINE AND END-OF-SCREEN CODES”.
Similarly, the first two characters in any bank which is ad-
dressed from Bank Select Register 0 will not be usable since
those addresses will be interpreted as the EOL and EOS
codes.
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