LM1253AN National Semiconductor, LM1253AN Datasheet - Page 25

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

OSD Generator Operation
Attribute Tables
Each character has an attribute value assigned to it in the
page RAM. The attribute value is 4 bits wide, making each
character entry in the page RAM 12 bits wide in total. The at-
tribute value acts as an address which points to one of 16
entries in either the two-color attribute table RAM or the four-
color attribute table RAM. The attribute word in the table con-
tains the coding information which defines which color is rep-
resented by Color 1 and Color 2 in the two color attribute
table and Color 1, Color 2, Color 3, Color 4 in the four-color
attribute table. Each color is defined by a 9 bit value, with 3
bits assigned to each channel of RGB. A dynamic look up
table defines each of the 16 different color combination se-
lections or ‘palettes’. As the look up table can be dynamically
coded by the micro-controller over the I
color can be assigned to any one of 2
This allows a maximum of 64 different colors to be used
within one page using the 4-color characters, with up to 4 dif-
ferent colors within any one character and 32 different colors
using the 2-color characters, with 2 different colors within
any one character.
Transparent Disable
In addition to the 9 lines of video data, a tenth data line is
generated by the transparent disable bit. When this line is
activated, the black color code will be translated as ‘trans-
parent’ or invisible. This allows the video information from
the PC system to be visible on the screen when this is
present. Note that this feature is enabled on all black of the
first 8 attribute table entries.
Enhanced Features
In addition to the wide selection of colors for each character,
additional character features can be selected on a character
by character basis.
9
2
(i.e. 512) choices.
C interface, each
(Continued)
25
1. Button Boxes — The OSD generator examines the char-
2. Heavy Button Boxes — When heavy button boxes are
3. Shadowing — Shadowing can be added to two-color
4. Bordering — A border can be added to the two-color
5. Blinking — If blinking is enabled as an attribute, all colors
acter string being displayed and if the ‘button box’ at-
tributes have been set in the Enhanced feature byte,
then a box creator selectively substitutes the character
pixels in either or both the top and right most pixel line or
column with a button box pixel. The shade of the button
box pixel depends upon whether a ‘depressed’ or
‘raised’ box is required, and can be programmed by
I
value in the color palette register, EF1 (normally white).
The depressed pixel (‘lowlight’) color by the value in the
color palette register EF2 (normally gray). See Figure
36 .
selected, the color palette value stored in register EF3 is
used for the depressed (‘lowlight’) pixel color instead of
the value in register EF2.
characters by choosing the appropriate attribute value
for the character. When a character is shadowed, a
shadow pixel is added to the lower right edges of the
color 2 image, as shown in Figure 37 . The color of the
shadow is determined by the value in the color palette
register EF3 (normally black).
characters. When a character is bordered, a border pixel
is added at every horizontal, vertical or diagonal transi-
tion between Color 1 and Color 2. See Figure 38 . The
color of the border is determined by the value in the color
palette register EF3 (normally black).
within the character except the button box pixels which
have been over-written will alternately switch to Color 1
and then back to Color 2 at a rate determined by the
micro-controller under I
2
C.The raised pixel color (‘highlight’) is defined by the
2
C control.
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