LM1253AN National Semiconductor, LM1253AN Datasheet - Page 49

• Manufacturer Part Number: LM1253AN
• Description: Monolithic Triple 180 MHz I2C CRT Pre-amp With Integrated Analog On Screen Display (OSD) Generator
• Manufacturer: National Semiconductor

Control Register Definitions
2. overwriting the pixels in the top line of the skipped lines below, in the case where skip lines are present below a boxed char-
acter.
Characters should be designed so that button boxes will not interfere with the character.
Some minor limitations result from the above box formation methodology:
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 override’ 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 cre-
ated.
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.
Operation of the Bordering Feature
Borders are created in a similar manner to shadowing, using the pixel override system to over write 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 ex-
amined.
The shadow feature is created as follows: as each 12 bit line in the character is called from ROM, the character line immediately
above and the line immediately below are also called. A ‘Pixel Override’ output mask is then created by looking at all pixels sur-
rounding 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 op-
erate correctly for pixels in line 0, line 17, and 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 vertical image 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 con-
stant image height. To prevent this, the OSD generator repeats some of the lines in the OSD character in order to maintain a con-
stant height.
• No box may use the left most display character in the Dis-
• At least one skip line must be used beneath characters
play Window, or it will have no left side of the Box. To cre-
ate 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.
on the bottom row, if a box is required around any charac-
ters on this row in order to accommodate the bottom
edge of the box.
FIGURE 44. Operation of the Shadow Feature
(Continued)
49
• Skipped lines cannot be used within a box covering sev-
• Irregular shaped boxes, (i.e., other than rectangular),
eral rows
may have some missing edges.
DS101265-57
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