LM1236 National Semiconductor Corporation, LM1236 Datasheet - Page 21

• 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

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 13 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 0 and color 1 in the two color attribute
table and color 0, color 1, color 2, color 3 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 ’palettes’. As the
look-up table can be dynamically coded by the microcontrol-
ler over the I
assigned to any one of 2
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-
2
C compatible interface, each color can be
9
(i.e. 254) choices. This allows a
FIGURE 18. Displaying Each Character Individually
(Continued)
21
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 any black color
in any of the first 8 attribute table entries.
HALF TONE TRANSPARENCY
In addition to the transparent disable bit, there is a half tone
disable bit. When the transparency is already in effect and
the half tone feature is activated, the contrast of the PC video
that is visible in the “transparent area” is reduced to 50%,
providing a semi-transparent effect. Just as in the conven-
tional transparency mode, half tone transparency is effective
on backgrounds or foregrounds with black color codes from
only the first 8 attribute table entries. This feature is con-
trolled by bit 7 of the frame control register, 0x8400, and is
only available when the transparency mode is already en-
abled.
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