LM1236 National Semiconductor Corporation, LM1236 Datasheet - Page 41

• Manufacturer Part Number: LM1236
• Description: 150 Mhz I2c Compatible Rgb Preamplifier With Internal 254 Character Osd Rom, 512 Character Ram And 4 Dacs
• Manufacturer: National Semiconductor Corporation

Attribute Table and Enhanced
Features
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 —
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:
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.
• overwriting the pixels in the top line of the character
• overwriting the pixels in the top line of the skipped lines
• No box may use the left most display character in the
• At least one skip line must be used beneath characters
• Skipped lines cannot be used within a box covering sev-
• Irregular shaped boxes, (i.e., other than rectangular),
below the character being enclosed by the box, or
below, in the case where skip lines are present below a
boxed character.
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.
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.
eral rows.
may have some missing edges.
(Continued)
41
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.
FIGURE 25. Operation of the Shadow Feature
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