![](http://datasheet.mmic.net.cn/40000/LM1253AAE-NA_datasheet_1645330/LM1253AAE-NA_25.png)
OSD Generator Operation (Continued)
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
attribute 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
contains the coding information which defines which color is
represented 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
selections or ‘palettes’. As the look up table can be dynami-
cally coded by the micro-controller over the I2C interface,
each color can be assigned to any one of 2
9 (i.e. 512)
choices. This allows a maximum of 64 different colors to be
used within one page using the 4-color characters, with up to
4 different 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.
1.
Button Boxes — The OSD generator examines the char-
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
I2C.The raised pixel color (‘highlight’) is defined by the
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.
2.
Heavy Button Boxes — When heavy button boxes are
selected, the color palette value stored in register EF3 is
used for the depressed (‘lowlight’) pixel color instead of
the value in register EF2.
3.
Shadowing — Shadowing can be added to two-color
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).
4.
Bordering — A border can be added to the two-color
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).
5.
Blinking — If blinking is enabled as an attribute, all colors
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
2C control.
LM1253A
www.national.com
25