LM1237 National Semiconductor Corporation, LM1237 Datasheet - Page 15

• Manufacturer Part Number: LM1237
• Description: 150 Mhz I2c Compatible Rgb Preamplifier With Internal 254 Character Osd And 4 Dacs
• Manufacturer: National Semiconductor Corporation

OSD Generator Operation
PAGE OPERATION
Figure 12. shows the block diagram of the OSD generator.
Video information is created using any of the 256 predefined
characters stored in the mask programmed ROM. Each
character has a unique 8-bit code that is used as its address.
Consecutive rows of characters make up the displayed win-
dow. These characters can be stored in the page RAM,
written under I
crocontroller. Each row can contain any number of charac-
ters up to the limit of the displayable line length, although
some restrictions concerning the enhanced features apply
on character rows longer than 32 characters.
The number of characters across the width and height of the
page can be varied under I
total number of characters that can be stored and displayed
2
C compatible commands by the monitor mi-
2
C compatible control, but the
FIGURE 12. Block Diagram of the OSD Generator
15
on the screen is limited to 512 including any character row
end characters. The horizontal and vertical start position can
also be programmed through the I
OSD VIDEO DAC
The OSD DAC is controlled by the 9-bit (3x3 bits) OSD video
information coming from the pixel serializer look-up table.
The look-up table in the OSD palette is programmed to
select 4 color levels out of 8 linearly spaced levels per
channel. The OSD DAC is shown in Figure 13, where the
gain is programmable by the 2-bit OSD CONTRAST register,
in 4 stages to give the required OSD signal. The OSD DACs
use the reference voltage, V
REF
, to bias the OSD outputs.
2
C compatible bus.
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